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Add PromiseKit dependency

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- Added PromiseKit dependency
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James Griffin
2018-11-15 22:08:00 -04:00
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# Common Misusage
## Doubling up Promises
Don’t do this:
```swift
func toggleNetworkSpinnerWithPromise<T>(funcToCall: () -> Promise<T>) -> Promise<T> {
return Promise { seal in
firstly {
setNetworkActivityIndicatorVisible(true)
return funcToCall()
}.then { result in
seal.fulfill(result)
}.always {
setNetworkActivityIndicatorVisible(false)
}.catch { err in
seal.reject(err)
}
}
}
```
Do this:
```swift
func toggleNetworkSpinnerWithPromise<T>(funcToCall: () -> Promise<T>) -> Promise<T> {
return firstly {
setNetworkActivityIndicatorVisible(true)
return funcToCall()
}.always {
setNetworkActivityIndicatorVisible(false)
}
}
```
You already *had* a promise, you don’t need to wrap it in another promise.
## Optionals in Promises
When we see `Promise<Item?>`, it usually implies a misuse of promises. For
example:
```swift
return firstly {
getItems()
}.then { items -> Promise<[Item]?> in
guard !items.isEmpty else {
return .value(nil)
}
return Promise(value: items)
}
```
The second `then` chooses to return `nil` in some circumstances. This choice
imposes the need to check for `nil` on the consumer of the promise.
It's usually better to shunt these sorts of exceptions away from the
happy path and onto the error path. In this case, we can create a specific
error type for this condition:
```swift
return firstly {
getItems()
}.map { items -> [Item]> in
guard !items.isEmpty else {
throw MyError.emptyItems
}
return items
}
```
> *Note*: Use `compactMap` when an API outside your control returns an Optional and you want to generate an error instead of propagating `nil`.
# Tips n’ Tricks
## Background-Loaded Member Variables
```swift
class MyViewController: UIViewController {
private let ambience: Promise<AVAudioPlayer> = DispatchQueue.global().async(.promise) {
guard let asset = NSDataAsset(name: "CreepyPad") else { throw PMKError.badInput }
let player = try AVAudioPlayer(data: asset.data)
player.prepareToPlay()
return player
}
}
```
## Chaining Animations
```swift
firstly {
UIView.animate(.promise, duration: 0.3) {
self.button1.alpha = 0
}
}.then {
UIView.animate(.promise, duration: 0.3) {
self.button2.alpha = 1
}
}.then {
UIView.animate(.promise, duration: 0.3) {
adjustConstraints()
self.view.layoutIfNeeded()
}
}
```
## Voiding Promises
It is often convenient to erase the type of a promise to facilitate chaining.
For example, `UIView.animate(.promise)` returns `Guarantee<Bool>` because UIKit’s
completion API supplies a `Bool`. However, we usually don’t need this value and
can chain more simply if it is discarded (that is, converted to `Void`). We can use
`asVoid()` to achieve this conversion:
```swift
UIView.animate(.promise, duration: 0.3) {
self.button1.alpha = 0
}.asVoid().done(self.nextStep)
```
For situations in which we are combining many promises into a `when`, `asVoid()`
becomes essential:
```swift
let p1 = foo()
let p2 = bar()
let p3 = baz()
//…
let p10 = fluff()
when(fulfilled: p1.asVoid(), p2.asVoid(), /*…*/, p10.asVoid()).then {
let value1 = p1.value! // safe bang since all the promises fulfilled
// …
let value10 = p10.value!
}.catch {
//…
}
```
You normally don't have to do this explicitly because `when` does it for you
for up to 5 parameters.
## Blocking (Await)
Sometimes you have to block the main thread to await completion of an asynchronous task.
In these cases, you can (with caution) use `wait`:
```swift
public extension UNUserNotificationCenter {
var wasPushRequested: Bool {
let settings = Guarantee(resolver: getNotificationSettings).wait()
return settings != .notDetermined
}
}
```
The task under the promise **must not** call back onto the current thread or it
will deadlock.
## Starting a Chain on a Background Queue/Thread
`firstly` deliberately does not take a queue. A detailed rationale for this choice
can be found in the ticket tracker.
So, if you want to start a chain by dispatching to the background, you have to use
`DispatchQueue.async`:
```swift
DispatchQueue.global().async(.promise) {
return value
}.done { value in
//…
}
```
However, this function cannot return a promise because of Swift compiler ambiguity
issues. Thus, if you must start a promise on a background queue, you need to
do something like this:
```swift
Promise { seal in
DispatchQueue.global().async {
seal(value)
}
}.done { value in
//…
}
```
Or more simply (though with caveats; see the documentation for `wait`):
```swift
DispatchQueue.global().async(.promise) {
return try fetch().wait()
}.done { value in
//…
}
```
However, you shouldn't need to do this often. If you find yourself wanting to use
this technique, perhaps you should instead modify the code for `fetch` to make it do
its work on a background thread.
Promises abstract asynchronicity, so exploit and support that model. Design your
APIs so that consumers don’t have to care what queue your functions run on.

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# Common Patterns
One feature of promises that makes them particularly useful is that they are composable.
This fact enables complex, yet safe asynchronous patterns that would otherwise be quite
intimidating when implemented with traditional methods.
## Chaining
The most common pattern is chaining:
```swift
firstly {
fetch()
}.then {
map($0)
}.then {
set($0)
return animate()
}.ensure {
cleanup()
}.catch {
handle(error: $0)
}
```
If you return a promise in a `then`, the next `then` *waits* on that promise
before continuing. This is the essence of promises.
Promises are easy to compose, so they encourage you to develop highly asynchronous
apps without fear of the spaghetti code (and associated refactoring pains) of
asynchronous systems that use completion handlers.
## APIs That Use Promises
Promises are composable, so return them instead of accepting completion blocks:
```swift
class MyRestAPI {
func user() -> Promise<User> {
return firstly {
URLSession.shared.dataTask(.promise, with: url)
}.compactMap {
try JSONSerialization.jsonObject(with: $0.data) as? [String: Any]
}.map { dict in
User(dict: dict)
}
}
func avatar() -> Promise<UIImage> {
return user().then { user in
URLSession.shared.dataTask(.promise, with: user.imageUrl)
}.compactMap {
UIImage(data: $0.data)
}
}
}
```
This way, asynchronous chains can cleanly and seamlessly incorporate code from all over
your app without violating architectural boundaries.
> *Note*: We provide [promises for Alamofire](https://github.com/PromiseKit/Alamofire-) too!
## Background Work
```swift
class MyRestAPI {
func avatar() -> Promise<UIImage> {
let bgq = DispatchQueue.global(qos: .userInitiated)
return firstly {
user()
}.then(on: bgq) { user in
URLSession.shared.dataTask(.promise, with: user.imageUrl)
}.compactMap(on: bgq) {
UIImage(data: $0)
}
}
}
```
All PromiseKit handlers take an `on` parameter that lets you designate the dispatch queue
on which to run the handler. The default is always the main queue.
PromiseKit is *entirely* thread safe.
> *Tip*: With caution, you can have all `then`, `map`, `compactMap`, etc., run on
a background queue. See `PromiseKit.conf`. Note that we suggest only changing
the queue for the `map` suite of functions, so `done` and `catch` will
continue to run on the main queue, which is *usually* what you want.
## Failing Chains
If an error occurs mid-chain, simply throw an error:
```swift
firstly {
foo()
}.then { baz in
bar(baz)
}.then { result in
guard !result.isBad else { throw MyError.myIssue }
//…
return doOtherThing()
}
```
The error will surface at the next `catch` handler.
Since promises handle thrown errors, you don't have to wrap calls to throwing functions
in a `do` block unless you really want to handle the errors locally:
```swift
foo().then { baz in
bar(baz)
}.then { result in
try doOtherThing()
}.catch { error in
// if doOtherThing() throws, we end up here
}
```
> *Tip*: Swift lets you define an inline `enum Error` inside the function you
are working on. This isn’t *great* coding practice, but it's better than
avoiding throwing an error because you couldn't be bothered to define a good global
`Error` `enum`.
## Abstracting Away Asychronicity
```swift
var fetch = API.fetch()
override func viewDidAppear() {
fetch.then { items in
//…
}
}
func buttonPressed() {
fetch.then { items in
//…
}
}
func refresh() {
// ensure only one fetch operation happens at a time
if fetch.isResolved {
startSpinner()
fetch = API.fetch().ensure {
stopSpinner()
}
}
return fetch
}
```
With promises, you don’t need to worry about *when* your asynchronous operation
finishes. Just act like it already has.
Above, we see that you can call `then` as many times on a promise as you
like. All the blocks will be executed in the order they were added.
## Chaining Sequences
When you have a series of tasks to perform on an array of data:
```swift
// fade all visible table cells one by one in a “cascading” effect
let fade = Guarantee()
for cell in tableView.visibleCells {
fade = fade.then {
UIView.animate(.promise, duration: 0.1) {
cell.alpha = 0
}
}
}
fade.done {
// finish
}
```
> *Note*: You *usually* you want `when()`, since `when` executes all of its
component promises in parallel and so completes much faster. Use the pattern
shown above in situations where tasks *must* be run sequentially; animation
is a good example.
We also provide `when(concurrently:)`, which lets you schedule more than
one promise at a time if you need to.
## Timeout
```swift
let fetches: [Promise<T>] = makeFetches()
let timeout = after(seconds: 4)
race(when(fulfilled: fetches).asVoid(), timeout).then {
//…
}
```
`race` continues as soon as one of the promises it is watching finishes.
Make sure the promises you pass to `race` are all of the same type. The easiest way
to ensure this is to use `asVoid()`.
Note that if any component promise rejects, the `race` will reject, too.
# Minimum Duration
Sometimes you need a task to take *at least* a certain amount of time. (For example,
you want to show a progress spinner, but if it shows for less than 0.3 seconds, the UI
appears broken to the user.)
```swift
let waitAtLeast = after(seconds: 0.3)
firstly {
foo()
}.then {
waitAtLeast
}.done {
//…
}
```
The code above works because we create the delay *before* we do work in `foo()`. By the
time we get to waiting on that promise, either it will have already timed out or we will wait
for whatever remains of the 0.3 seconds before continuing the chain.
## Cancellation
Promises don’t have a `cancel` function, but they do support cancellation through a
special error type that conforms to the `CancellableError` protocol.
```swift
func foo() -> (Promise<Void>, cancel: () -> Void) {
let task = Task(…)
var cancelme = false
let promise = Promise<Void> { seal in
task.completion = { value in
guard !cancelme else { return reject(PMKError.cancelled) }
seal.fulfill(value)
}
task.start()
}
let cancel = {
cancelme = true
task.cancel()
}
return (promise, cancel)
}
```
Promises don’t have a `cancel` function because you don’t want code outside of
your control to be able to cancel your operations--*unless*, of course, you explicitly
want to enable that behavior. In cases where you do want cancellation, the exact way
that it should work will vary depending on how the underlying task supports cancellation.
PromiseKit provides cancellation primitives but no concrete API.
Cancelled chains do not call `catch` handlers by default. However you can
intercept cancellation if you like:
```swift
foo.then {
//…
}.catch(policy: .allErrors) {
// cancelled errors are handled *as well*
}
```
**Important**: Canceling a promise chain is *not* the same as canceling the underlying
asynchronous task. Promises are wrappers around asynchronicity, but they have no
control over the underlying tasks. If you need to cancel an underlying task, you
need to cancel the underlying task!
> The library [CancellablePromiseKit](https://github.com/johannesd/CancellablePromiseKit) extends the concept of Promises to fully cover cancellable tasks.
## Retry / Polling
```swift
func attempt<T>(maximumRetryCount: Int = 3, delayBeforeRetry: DispatchTimeInterval = .seconds(2), _ body: @escaping () -> Promise<T>) -> Promise<T> {
var attempts = 0
func attempt() -> Promise<T> {
attempts += 1
return body().recover { error -> Promise<T> in
guard attempts < maximumRetryCount else { throw error }
return after(delayBeforeRetry).then(on: nil, attempt)
}
}
return attempt()
}
attempt(maximumRetryCount: 3) {
flakeyTask(parameters: foo)
}.then {
//…
}.catch { _ in
// we attempted three times but still failed
}
```
In most cases, you should probably supplement the code above so that it re-attempts only for
specific error conditions.
## Wrapping Delegate Systems
Be careful with Promises and delegate systems, as they are not always compatible.
Promises complete *once*, whereas most delegate systems may notify their delegate many
times. This is why, for example, there is no PromiseKit extension for a
`UIButton`.
A good example of an appropriate time to wrap delegation is when you need a
single `CLLocation` lookup:
```swift
extension CLLocationManager {
static func promise() -> Promise<CLLocation> {
return PMKCLLocationManagerProxy().promise
}
}
class PMKCLLocationManagerProxy: NSObject, CLLocationManagerDelegate {
private let (promise, seal) = Promise<[CLLocation]>.pending()
private var retainCycle: PMKCLLocationManagerProxy?
private let manager = CLLocationManager()
init() {
super.init()
retainCycle = self
manager.delegate = self // does not retain hence the `retainCycle` property
promise.ensure {
// ensure we break the retain cycle
self.retainCycle = nil
}
}
@objc fileprivate func locationManager(_: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
seal.fulfill(locations)
}
@objc func locationManager(_: CLLocationManager, didFailWithError error: Error) {
seal.reject(error)
}
}
// use:
CLLocationManager.promise().then { locations in
//…
}.catch { error in
//…
}
```
> Please note: we provide this promise with our CoreLocation extensions at
> https://github.com/PromiseKit/CoreLocation
## Recovery
Sometimes you don’t want an error to cascade. Instead, you want to supply a default result:
```swift
CLLocationManager.requestLocation().recover { error -> Promise<CLLocation> in
guard error == MyError.airplaneMode else {
throw error
}
return .value(CLLocation.savannah)
}.done { location in
//…
}
```
Be careful not to ignore all errors, though! Recover only those errors that make sense to recover.
## Promises for Modal View Controllers
```swift
class ViewController: UIViewController {
private let (promise, seal) = Guarantee<…>.pending() // use Promise if your flow can fail
func show(in: UIViewController) -> Promise<…> {
in.show(self, sender: in)
return promise
}
func done() {
dismiss(animated: true)
seal.fulfill(…)
}
}
// use:
ViewController().show(in: self).done {
//…
}.catch { error in
//…
}
```
This is the best approach we have found, which is a pity as it requires the
presentee to control the presentation and requires the presentee to dismiss itself
explicitly.
Nothing seems to beat storyboard segues for decoupling an app's controllers.
## Saving Previous Results
Let’s say you have:
```swift
login().then { username in
fetch(avatar: username)
}.done { image in
//…
}
```
What if you want access to both `username` and `image` in your `done`?
The most obvious way is to use nesting:
```swift
login().then { username in
fetch(avatar: username).done { image in
// we have access to both `image` and `username`
}
}.done {
// the chain still continues as you'd expect
}
```
However, such nesting reduces the clarity of the chain. Instead, we could use Swift
tuples:
```swift
login().then { username in
fetch(avatar: username).map { ($0, username) }
}.then { image, username in
//…
}
```
The code above simply maps `Promise<String>` into `Promise<(UIImage, String)>`.
## Waiting on Multiple Promises, Whatever Their Result
Use `when(resolved:)`:
```swift
when(resolved: a, b).done { (results: [Result<T>]) in
// `Result` is an enum of `.fulfilled` or `.rejected`
}
// ^^ cannot call `catch` as `when(resolved:)` returns a `Guarantee`
```
Generally, you don't want this! People ask for it a lot, but usually because
they are trying to ignore errors. What they really need is to use `recover` on one of the
promises. Errors happen, so they should be handled; you usually don't want to ignore them.

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# Image Cache with Promises
Here is an example of a simple image cache that uses promises to simplify the
state machine:
```swift
import Foundation
import PromiseKit
/**
* Small (10 images)
* Thread-safe
* Consolidates multiple requests to the same URLs
* Removes stale entries (FIXME well, strictly we may delete while fetching from cache, but this is unlikely and non-fatal)
* Completely _ignores_ server caching headers!
*/
private let q = DispatchQueue(label: "org.promisekit.cache.image")
private var active: [URL: Promise<Data>] = [:]
private var cleanup = Promise()
public func fetch(image url: URL) -> Promise<Data> {
var promise: Promise<Data>?
q.sync {
promise = active[url]
}
if let promise = promise {
return promise
}
q.sync(flags: .barrier) {
promise = Promise(.start) {
let dst = try url.cacheDestination()
guard !FileManager.default.isReadableFile(atPath: dst.path) else {
return Promise(dst)
}
return Promise { seal in
URLSession.shared.downloadTask(with: url) { tmpurl, _, error in
do {
guard let tmpurl = tmpurl else { throw error ?? E.unexpectedError }
try FileManager.default.moveItem(at: tmpurl, to: dst)
seal.fulfill(dst)
} catch {
seal.reject(error)
}
}.resume()
}
}.then(on: .global(QoS: .userInitiated)) {
try Data(contentsOf: $0)
}
active[url] = promise
if cleanup.isFulfilled {
cleanup = promise!.asVoid().then(on: .global(QoS: .utility), execute: docleanup)
}
}
return promise!
}
public func cached(image url: URL) -> Data? {
guard let dst = try? url.cacheDestination() else {
return nil
}
return try? Data(contentsOf: dst)
}
public func cache(destination remoteUrl: URL) throws -> URL {
return try remoteUrl.cacheDestination()
}
private func cache() throws -> URL {
guard let dst = FileManager.default.docs?
.appendingPathComponent("Library")
.appendingPathComponent("Caches")
.appendingPathComponent("cache.img")
else {
throw E.unexpectedError
}
try FileManager.default.createDirectory(at: dst, withIntermediateDirectories: true, attributes: [:])
return dst
}
private extension URL {
func cacheDestination() throws -> URL {
var fn = String(hashValue)
let ext = pathExtension
// many of Apple's functions don’t recognize file type
// unless we preserve the file extension
if !ext.isEmpty {
fn += ".\(ext)"
}
return try cache().appendingPathComponent(fn)
}
}
enum E: Error {
case unexpectedError
case noCreationTime
}
private func docleanup() throws {
var contents = try FileManager.default
.contentsOfDirectory(at: try cache(), includingPropertiesForKeys: [.creationDateKey])
.map { url -> (Date, URL) in
guard let date = try url.resourceValues(forKeys: [.creationDateKey]).creationDate else {
throw E.noCreationTime
}
return (date, url)
}.sorted(by: {
$0.0 > $1.0
})
while contents.count > 10 {
let rm = contents.popLast()!.1
try FileManager.default.removeItem(at: rm)
}
}
````

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Promise(.pending) { seal in
URLSession.shared.dataTask(with: rq, completionHandler: { data, rsp, error in
if let data = data {
seal.fulfill(data)
} else if let error = error {
if case URLError.badServerResponse = error, let rsp = rsp as? HTTPURLResponse {
seal.reject(Error.badResponse(rsp.statusCode))
} else {
seal.reject(error)
}
} else {
seal.reject(PMKError.invalidCallingConvention)
}
})
}
enum Error: Swift.Error {
case badUrl
case badResponse(Int)
}

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# FAQ
## Why should I use PromiseKit over X-Promises-Foo?
* PromiseKit has a heavy focus on **developer experience**. You’re a developer; do you care about your experience? Yes? Then pick PromiseKit.
* Do you care about having any bugs you find fixed? Then pick PromiseKit.
* Do you care about having your input heard and reacted to in a fast fashion? Then pick PromiseKit.
* Do you want a library that has been maintained continuously and passionately for 6 years? Then pick PromiseKit.
* Do you want a library that the community has chosen to be their №1 Promises/Futures library? Then pick PromiseKit.
* Do you want to be able to use Promises with Apple’s SDKs rather than having to do all the work of writing the Promise implementations yourself? Then pick PromiseKit.
* Do you want to be able to use Promises with Swift 3.x, Swift 4.x, ObjC, iOS, tvOS, watchOS, macOS, Android & Linux? Then pick PromiseKit.
* PromiseKit verifies its correctness by testing against the entire [Promises/A+ test suite](https://github.com/promises-aplus/promises-tests).
## How do I create a fulfilled `Void` promise?
```swift
let foo = Promise()
// or:
let bar = Promise.value(())
```
## How do I “early `return`”?
```swift
func foo() -> Promise<Void> {
guard thingy else {
return Promise()
}
//…
}
func bar() -> Promise<SomethingNotVoid> {
guard thingy else {
return .value(instanceOfSomethingNotVoid)
}
//…
}
```
## Do I need to worry about retain cycles?
Generally, no. Once a promise completes, all handlers are released and so
any references to `self` are also released.
However, if your chain contains side effects that you would typically
not want to happen after, say, a view controller is popped, then you should still
use `weak self` (and check for `self == nil`) to prevent any such side effects.
*However*, in our experience most things that developers consider side effects that
should be protected against are in fact *not* side effects.
Side effects include changes to global application state. They *do not* include
changing the display state of a viewController. So, protect against setting UserDefaults or
modifying the application database, and don't bother protecting against changing
the text in a `UILabel`.
[This stackoverflow question](https://stackoverflow.com/questions/39281214/should-i-use-weak-self-in-promisekit-blocks)
has some good discussion on this topic.
## Do I need to retain my promises?
No. Every promise handler retains its promise until the handler is executed. Once
all handlers have been executed, the promise is deallocated. So you only need to retain
the promise if you need to refer to its final value after its chain has completed.
## Where should I put my `catch`?
`catch` deliberately terminates the chain. You should put it low in your promise
hierarchy at a point as close to the root as possible. Typically, this would be
somewhere such as a view controller, where your `catch` can then display a message
to the user.
This means you should be writing one catch for many `then`s and returning
promises that do not have internal `catch` handlers of their own.
This is obviously a guideline; do what is necessary.
## How do branched chains work?
Suppose you have a promise:
```
let promise = foo()
```
And you call `then` twice:
```
promise.then {
// branch A
}
promise.then {
// branch B
}
```
You now have a branched chain. When `promise` resolves, both chains receive its
value. However, the two chains are entirely separate and Swift will prompt you
to ensure that both have `catch` handlers.
You can most likely ignore the `catch` for one of these branches, but be careful:
in these situations, Swift cannot help you ensure that your chains are error-handled.
```
promise.then {
// branch A
}.catch { error in
//…
}
_ = promise.then {
print("foo")
// ignoring errors here as print cannot error and we handle errors above
}
```
It may be safer to recombine the two branches into a single chain again:
```
let p1 = promise.then {
// branch A
}
let p2 = promise.then {
// branch B
}
when(fulfilled: p1, p2).catch { error in
//…
}
```
> It's worth noting that you can add multiple `catch` handlers to a promise, too.
> And indeed, both will be called if the chain is rejected.
## Is PromiseKit “heavy”?
No. PromiseKit contains hardly any source code. In fact, it is quite lightweight. Any
“weight” relative to other promise implementations derives from 6 years of bug fixes
and tuning, from the fact that we have *stellar* Objective-C-to-Swift bridging and
from important things such as [Zalgo prevention](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony)
that hobby-project implementations don’t consider.
## Why is debugging hard?
Because promises always execute via dispatch, the backtrace you see at the point of
an error has less information than is usually required to trace the path of execution.
One solution is to turn off dispatch during debugging:
```swift
// Swift
DispatchQueue.default = zalgo
//ObjC
PMKSetDefaultDispatchQueue(zalgo)
```
Don’t leave this on. In normal use, we always dispatch to avoid you accidentally writing
a common bug pattern. See [this blog post](http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony).
## Where is `all()`?
Some promise libraries provide `all` for awaiting multiple results. We call this function
`when`, but it is the same thing. We chose `when` because it's the more common term and
because we think it reads better in code.
## How can I test APIs that return promises?
You need to use `XCTestExpectation`.
We also define `wait()` and `hang()`. Use them if you must, but be careful because they
block the current thread!
## Is PromiseKit thread-safe?
Yes, entirely.
However the code *you* write in your `then`s might not be!
Just make sure you don’t access state outside the chain from concurrent queues.
By default, PromiseKit handlers run on the `main` thread, which is serial, so
you typically won't have to worry about this.
## Why are there separate classes for Objective-C and Swift?
`Promise<T>` is generic and and so cannot be represented by Objective-C.
## Does PromiseKit conform to Promises/A+?
Yes. We have tests that prove this.
## How do PromiseKit and RxSwift/ReactiveSwift differ?
PromiseKit is a lot simpler.
The top-level difference between PromiseKit and RxSwift is that RxSwift `Observable`s (roughly
analogous to PromiseKit `Promise`s) do not necessarily return a single result: they may emit
zero, one, or an infinite stream of values. This small conceptual change leads to an API
that's both surprisingly powerful and surprisingly complex.
RxSwift requires commitment to a paradigm shift in how you program. It proposes that you
restructure your code as a matrix of interacting value pipelines. When applied properly
to a suitable problem, RxSwift can yield great benefits in robustness and simplicity.
But not all applications are suitable for RxSwift.
By contrast, PromiseKit selectively applies the best parts of reactive programming
to the hardest part of pure Swift development, the management of asynchrony. It's a broadly
applicable tool. Most asynchronous code can be clarified, simplified and made more robust
just by converting it to use promises. (And the conversion process is easy.)
Promises make for code that is clear to most developers. RxSwift, perhaps not. Take a look at this
[signup panel](https://github.com/ReactiveX/RxSwift/tree/master/RxExample/RxExample/Examples/GitHubSignup)
implemented in RxSwift and see what you think. (Note that this is one of RxSwift's own examples.)
Even where PromiseKit and RxSwift are broadly similar, there are many differences in implementation:
* RxSwift has a separate API for chain-terminating elements ("subscribers") versus interior
elements. In PromiseKit, all elements of a chain use roughly the same code pattern.
* The RxSwift API to define an interior element of a chain (an "operator") is hair-raisingly complex.
So, RxSwift tries hard to supply every operator you might ever want to use right off the shelf. There are
hundreds. PromiseKit supplies a few utilities to help with specific scenarios, but because it's trivial
to write your own chain elements, there's no need for all this extra code in the library.
* PromiseKit dispatches the execution of every block. RxSwift dispatches only when told to do so. Moreover, the
current dispatching state is an attribute of the chain, not the specific block, as it is in PromiseKit.
The RxSwift system is more powerful but more complex. PromiseKit is simple, predictable and safe.
* In PromiseKit, both sides of a branched chain refer back to their shared common ancestors. In RxSwift,
branching normally creates a duplicate parallel chain that reruns the code at the head of the chain...except
when it doesn't. The rules for determining what will actually happen are complex, and given
a chain created by another chunk of code, you can't really tell what the behavior will be.
* Because RxSwift chains don't necessarily terminate on their own, RxSwift needs you to take on some
explicit garbage collection duties to ensure that pipelines that are no longer needed are properly
deallocated. All promises yield a single value, terminate and then automatically deallocate themselves.
You can find some additional discussion in [this ticket](https://github.com/mxcl/PromiseKit/issues/484).
## Why can’t I return from a catch like I can in Javascript?
Swift demands that functions have one purpose. Thus, we have two error handlers:
* `catch`: ends the chain and handles errors
* `recover`: attempts to recover from errors in a chain
You want `recover`.
## When do promises “start”?
Often people are confused about when Promises “start”. Is it immediately? Is it
later? Is it when you call then?
The answer is: promises do not choose when the underlying task they represent
starts. That is up to that task. For example here is the code for a simple
promise that wraps Alamofire:
```swift
func foo() -> Promise<Any>
return Promise { seal in
Alamofire.request(rq).responseJSON { rsp in
seal.resolve(rsp.value, rsp.error)
}
}
}
```
Who chooses when this promise starts? The answer is: Alamofire does, and in this
case, it “starts” immediately when `foo()` is called.
## What is a good way to use Firebase with PromiseKit
There is no good way to use Firebase with PromiseKit. See the next question for
a more detailed rationale.
The best option is to embed your chain in your Firebase handler:
```
foo.observe(.value) { snapshot in
firstly {
bar(with: snapshot)
}.then {
baz()
}.then {
baffle()
}.catch {
//…
}
}
```
## I need my `then` to fire multiple times
Then we’re afraid you cannot use PromiseKit for that event. Promises only
resolve *once*. This is the fundamental nature of promises and it is considered a
feature because it gives you guarantees about the flow of your chains.
## How do I change the default queues that handlers run on?
You can change the values of `PromiseKit.conf.Q`. There are two variables that
change the default queues that the two kinds of handler run on. A typical
pattern is to change all your `then`-type handlers to run on a background queue
and to have all your “finalizers” run on the main queue:
```
PromiseKit.conf.Q.map = .global()
PromiseKit.conf.Q.return = .main //NOTE this is the default
```
Be very careful about setting either of these queues to `nil`. It has the
effect of running *immediately*, and this is not what you usually want to do in
your application. This is, however, useful when you are running specs and want
your promises to resolve immediately. (This is basically the same idea as "stubbing"
an HTTP request.)
```swift
// in your test suite setup code
PromiseKit.conf.Q.map = nil
PromiseKit.conf.Q.return = nil
```
## How do I use PromiseKit on the server side?
If your server framework requires that the main queue remain unused (e.g., Kitura),
then you must use PromiseKit 6 and you must tell PromiseKit not to dispatch to the
main queue by default. This is easy enough:
```swift
PromiseKit.conf.Q = (map: DispatchQueue.global(), return: DispatchQueue.global())
```
> Note, we recommend using your own queue rather than `.global()`, we've seen better performance this way.
Here’s a more complete example:
```swift
import Foundation
import HeliumLogger
import Kitura
import LoggerAPI
import PromiseKit
HeliumLogger.use(.info)
let pmkQ = DispatchQueue(label: "pmkQ", qos: .default, attributes: .concurrent, autoreleaseFrequency: .workItem)
PromiseKit.conf.Q = (map: pmkQ, return: pmkQ)
let router = Router()
router.get("/") { _, response, next in
Log.info("Request received")
after(seconds: 1.0).done {
Log.info("Sending response")
response.send("OK")
next()
}
}
Log.info("Starting server")
Kitura.addHTTPServer(onPort: 8888, with: router)
Kitura.run()
```
## My question was not answered
[Please open a ticket](https://github.com/mxcl/PromiseKit/issues/new).

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# `then` and `done`
Here is a typical promise chain:
```swift
firstly {
login()
}.then { creds in
fetch(avatar: creds.user)
}.done { image in
self.imageView = image
}
```
If this code used completion handlers, it would look like this:
```swift
login { creds, error in
if let creds = creds {
fetch(avatar: creds.user) { image, error in
if let image = image {
self.imageView = image
}
}
}
}
```
`then` *is* just another way to structure completion handlers, but it is also quite a
bit more. At this initial stage of our understanding, it mostly helps
readability. The promise chain above is easy to scan and understand: one asynchronous
operation leads into the other, line by line. It's as close to
procedural code as we can easily come given the current state of Swift.
`done` is the same as `then` but you cannot return a promise. It is
typically the end of the “success” part of the chain. Above, you can see that we
receive the final image in our `done` and use it to set up the UI.
Let’s compare the signatures of the two login methods:
```swift
func login() -> Promise<Creds>
// Compared with:
func login(completion: (Creds?, Error?) -> Void)
// ^^ ugh. Optionals. Double optionals.
```
The distinction is that with promises, your functions return *promises* instead
of accepting and running callbacks. Each handler in a chain returns a promise.
`Promise` objects define the `then` method, which waits for the completion of the
promise before continuing the chain. Chains resolve procedurally, one promise
at a time.
A `Promise` represents the future value of an asynchronous task. It has a type
that represents the type of object it wraps. For example, in the example above,
`login` is a function that returns a `Promise` that *will* represent an instance
of `Creds`.
> *Note*: `done` is new to PromiseKit 5. We previously defined a variant of `then` that
did not require you to return a promise. Unfortunately, this convention often confused
Swift and led to odd and hard-to-debug error messages. It also made using PromiseKit
more painful. The introduction of `done` lets you type out promise chains that
compile without additional qualification to help the compiler figure out type information.
---
You may notice that unlike the completion pattern, the promise chain appears to
ignore errors. This is not the case! In fact, it has the opposite effect: the promise
chain makes error handling more accessible and makes errors harder to ignore.
# `catch`
With promises, errors cascade along the promise chain, ensuring that your apps are
robust and your code is clear:
```swift
firstly {
login()
}.then { creds in
fetch(avatar: creds.user)
}.done { image in
self.imageView = image
}.catch {
// any errors in the whole chain land here
}
```
> Swift emits a warning if you forget to `catch` a chain. But we'll
> talk about that in more detail later.
Each promise is an object that represents an individual, asychnronous task.
If a task fails, its promise becomes *rejected*. Chains that contain rejected
promises skip all subsequent `then`s. Instead, the next `catch` is executed.
(Strictly speaking, *all* subsequent `catch` handlers are executed.)
For fun, let’s compare this pattern with its completion handler equivalent:
```swift
func handle(error: Error) {
//…
}
login { creds, error in
guard let creds = creds else { return handle(error: error!) }
fetch(avatar: creds.user) { image, error in
guard let image = image else { return handle(error: error!) }
self.imageView.image = image
}
}
```
The use of `guard` and a consolidated error handler help, but the promise chain’s
readability speaks for itself.
# `ensure`
We have learned to compose asynchronicity. Next let’s extend our primitives:
```swift
firstly {
UIApplication.shared.isNetworkActivityIndicatorVisible = true
return login()
}.then {
fetch(avatar: $0.user)
}.done {
self.imageView = $0
}.ensure {
UIApplication.shared.isNetworkActivityIndicatorVisible = false
}.catch {
//…
}
```
No matter the outcome of your chain—-failure or success—-your `ensure`
handler is always called.
Let’s compare this pattern with its completion handler equivalent:
```swift
UIApplication.shared.isNetworkActivityIndicatorVisible = true
func handle(error: Error) {
UIApplication.shared.isNetworkActivityIndicatorVisible = false
//…
}
login { creds, error in
guard let creds = creds else { return handle(error: error!) }
fetch(avatar: creds.user) { image, error in
guard let image = image else { return handle(error: error!) }
self.imageView.image = image
UIApplication.shared.isNetworkActivityIndicatorVisible = false
}
}
```
It would be very easy for someone to amend this code and forget to unset
the activity indicator, leading to a bug. With promises, this type of error is
almost impossible: the Swift compiler resists your supplementing the chain without
using promises. You almost won’t need to review the pull requests.
> *Note*: PromiseKit has perhaps capriciously switched between the names `always`
and `ensure` for this function several times in the past. Sorry about this. We suck.
You can also use `finally` as an `ensure` that terminates the promise chain and does not return a value:
```
spinner(visible: true)
firstly {
foo()
}.done {
//…
}.catch {
//…
}.finally {
self.spinner(visible: false)
}
```
# `when`
With completion handlers, reacting to multiple asynchronous operations is either
slow or hard. Slow means doing it serially:
```swift
operation1 { result1 in
operation2 { result2 in
finish(result1, result2)
}
}
```
The fast (*parallel*) path code makes the code less clear:
```swift
var result1: …!
var result2: …!
let group = DispatchGroup()
group.enter()
group.enter()
operation1 {
result1 = $0
group.leave()
}
operation2 {
result2 = $0
group.leave()
}
group.notify(queue: .main) {
finish(result1, result2)
}
```
Promises are easier:
```swift
firstly {
when(fulfilled: operation1(), operation2())
}.done { result1, result2 in
//…
}
```
`when` takes promises, waits for them to resolve and returns a promise containing the results.
As with any promise chain, if any of the component promises fail, the chain calls the next `catch`.
# PromiseKit Extensions
When we made PromiseKit, we understood that we wanted to use *only* promises to implement
asynchronous behavior. So wherever possible, we offer extensions to Apple’s APIs that reframe
the API in terms of promises. For example:
```swift
firstly {
CLLocationManager.promise()
}.then { location in
CLGeocoder.reverseGeocode(location)
}.done { placemarks in
self.placemark.text = "\(placemarks.first)"
}
```
To use these extensions, you need to specify subspecs:
```ruby
pod "PromiseKit"
pod "PromiseKit/CoreLocation"
pod "PromiseKit/MapKit"
```
All of these extensions are available at the [PromiseKit organization](https://github.com/PromiseKit).
Go there to see what's available and to read the source code and documentation. Every file and function
has been copiously documented.
> We also provide extensions for common libraries such as [Alamofire](https://github.com/PromiseKit/Alamofire-).
# Making Promises
The standard extensions will take you a long way, but sometimes you'll still need to start chains
of your own. Maybe you're using a third party API that doesn’t provide promises, or perhaps you wrote
your own asynchronous system. Either way, it's easy to add promises. If you look at the code of the
standard extensions, you'll see that it uses the same approach described below.
Let’s say we have the following method:
```swift
func fetch(completion: (String?, Error?) -> Void)
```
How do we convert this to a promise? Well, it's easy:
```swift
func fetch() -> Promise<String> {
return Promise { fetch(completion: $0.resolve) }
}
```
You may find the expanded version more readable:
```swift
func fetch() -> Promise<String> {
return Promise { seal in
fetch { result, error in
seal.resolve(result, error)
}
}
}
```
The `seal` object that the `Promise` initializer provides to you defines
many methods for handling garden-variety completion handlers. It even
covers a variety of rarer situations, thus making it easy for you to add
promises to an existing codebase.
> *Note*: We tried to make it so that you could just do `Promise(fetch)`, but we
were not able to make this simpler pattern work universally without requiring
extra disambiguation for the Swift compiler. Sorry; we tried.
> *Note*: In PMK 4, this initializer provided two parameters to your closure:
`fulfill` and `reject`. PMK 5 and 6 give you an object that has both `fulfill` and
`reject` methods, but also many variants of the method `resolve`. You can
typically just pass completion handler parameters to `resolve` and let Swift figure
out which variant to apply to your particular case (as shown in the example above).
> *Note* `Guarantees` (below) have a slightly different initializer (since they
cannot error) so the parameter to the initializer closure is just a closure. Not
a `Resolver` object. Thus do `seal(value)` rather than `seal.fulfill(value)`. This
is because there is no variations in what guarantees can be sealed with, they can
*only* fulfill.
# `Guarantee<T>`
Since PromiseKit 5, we have provided `Guarantee` as a supplementary class to
`Promise`. We do this to complement Swift’s strong error handling system.
Guarantees *never* fail, so they cannot be rejected. A good example is `after`:
```
firstly {
after(seconds: 0.1)
}.done {
// there is no way to add a `catch` because after cannot fail.
}
```
Swift warns you if you don’t terminate a regular `Promise` chain (i.e., not
a `Guarantee` chain). You're expected to silence this warning by supplying
either a `catch` or a `return`. (In the latter case, you will then have to `catch`
at the point where you receive that promise.)
Use `Guarantee`s wherever possible so that your code has error handling where
it's required and no error handling where it's not required.
In general, you should be able to use `Guarantee`s and `Promise`s interchangeably,
We have gone to great lengths to try and ensure this, so please open a ticket
if you find an issue.
---
If you are creating your own guarantees the syntax is simpler than that of promises;
```swift
func fetch() -> Promise<String> {
return Guarantee { seal in
fetch { result in
seal(result)
}
}
}
```
Which could be reduced to:
```swift
func fetch() -> Promise<String> {
return Guarantee(resolver: fetch)
}
```
# `map`, `compactMap`, etc.
`then` provides you with the result of the previous promise and requires you to return
another promise.
`map` provides you with the result of the previous promise and requires you to return
an object or value type.
`compactMap` provides you with the result of the previous promise and requires you
to return an `Optional`. If you return `nil`, the chain fails with
`PMKError.compactMap`.
> *Rationale*: Before PromiseKit 4, `then` handled all these cases, and it was
painful. We hoped the pain would disappear with new Swift versions. However,
it has become clear that the various pain points are here to stay. In fact, we
as library authors are expected to disambiguate at the naming level of our API.
Therefore, we have split the three main kinds of `then` into `then`, `map` and
`done`. After using these new functions, we realized this is much nicer in practice,
so we added `compactMap` as well (modeled on `Optional.compactMap`).
`compactMap` facilitates quick composition of promise chains. For example:
```swift
firstly {
URLSession.shared.dataTask(.promise, with: rq)
}.compactMap {
try JSONSerialization.jsonObject($0.data) as? [String]
}.done { arrayOfStrings in
//…
}.catch { error in
// Foundation.JSONError if JSON was badly formed
// PMKError.compactMap if JSON was of different type
}
```
> *Tip*: We also provide most of the functional methods you would expect for sequences,
e.g., `map`, `thenMap`, `compactMapValues`, `firstValue`, etc.
# `get`
We provide `get` as a `done` that returns the value fed to `get`.
```swift
firstly {
foo()
}.get { foo in
//…
}.done { foo in
// same foo!
}
```
# `tap`
We provide `tap` for debugging. It's the same as `get` but provides the
`Result<T>` of the `Promise` so you can inspect the value of the chain at this
point without causing any side effects:
```swift
firstly {
foo()
}.tap {
print($0)
}.done {
//…
}.catch {
//…
}
```
# Supplement
## `firstly`
We've used `firstly` several times on this page, but what is it, really? In fact,
it is just [syntactic sugar](https://en.wikipedia.org/wiki/Syntactic_sugar).
You don’t really need it, but it helps to make your chains more readable. Instead of:
```swift
firstly {
login()
}.then { creds in
//…
}
```
You could just do:
```swift
login().then { creds in
//…
}
```
Here is a key understanding: `login()` returns a `Promise`, and all `Promise`s have a `then` function. `firstly` returns a `Promise`, and `then` returns a `Promise`, too! But don’t worry too much about these details. Learn the *patterns* to start with. Then, when you are ready to advance, learn the underlying architecture.
## `when` Variants
`when` is one of PromiseKit’s more useful functions, and so we offer several variants.
* The default `when`, and the one you should typically use, is `when(fulfilled:)`. This variant
waits on all its component promises, but if any fail, `when` fails too, and thus the chain *rejects*.
It's important to note that all promises in the `when` *continue*. Promises have *no* control over
the tasks they represent. Promises are just wrappers around tasks.
* `when(resolved:)` waits even if one or more of its component promises fails. The value produced
by this variant of `when` is an array of `Result<T>`. Consequently, this variant requires all its
component promises to have the same generic type. See our advanced patterns guide for work-arounds
for this limitation.
* The `race` variant lets you *race* several promises. Whichever finishes first is the result. See the
advanced patterns guide for typical usage.
## Swift Closure Inference
Swift automatically infers returns and return types for one-line closures.
The following two forms are the same:
```swift
foo.then {
bar($0)
}
// is the same as:
foo.then { baz -> Promise<String> in
return bar(baz)
}
```
Our documentation often omits the `return` for clarity.
However, this shorthand is both a blessing and a curse. You may find that the Swift compiler
often fails to infer return types properly. See our [Troubleshooting Guide](Troubleshooting.md) if
you require further assistance.
> By adding `done` to PromiseKit 5, we have managed to avoid many of these common
pain points in using PromiseKit and Swift.
# Further Reading
The above information is the 90% you will use. We **strongly** suggest reading the
[API Reference].
There are numerous little
functions that may be useful to you, and the documentation for everything outlined above
is more thorough at the source.
In Xcode, don’t forget to option-click on PromiseKit functions to access this
documentation while you're coding.
Here are some recent articles that document PromiseKit 5+:
* [Using Promises - Agostini.tech](https://agostini.tech/2018/10/08/using-promisekit)
Careful with general online references, many of them refer to PMK < 5 which has a subtly
different API (sorry about that, but Swift has changed a lot over the years and thus
we had to too).
[API Reference]: https://promisekit.org/reference/

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# Xcode 8.3, 9.x or 10.x / Swift 3 or 4
We recommend Carthage over CocoaPods, but both installation methods are supported.
## CocoaPods
```ruby
use_frameworks!
target "Change Me!" do
pod "PromiseKit", "~> 6.0"
end
```
If the generated Xcode project gives you a warning that PromiseKit needs to be upgraded to
Swift 4.0 or Swift 4.2, then add the following:
```ruby
post_install do |installer|
installer.pods_project.targets.each do |target|
if target.name == 'PromiseKit'
target.build_configurations.each do |config|
config.build_settings['SWIFT_VERSION'] = '4.2'
end
end
end
end
```
Adjust the value for `SWIFT_VERSION` as needed.
CocoaPods are aware of this [issue](https://github.com/CocoaPods/CocoaPods/issues/7134).
## Carthage
```ruby
github "mxcl/PromiseKit" ~> 6.0
```
## SwiftPM
```ruby
package.dependencies.append(
.Package(url: "https://github.com/mxcl/PromiseKit", majorVersion: 6)
)
```
## Manually
You can just drop `PromiseKit.xcodeproj` into your project and then add
`PromiseKit.framework` to your app’s embedded frameworks.
# PromiseKit vs. Xcode
PromiseKit contains Swift, so there have been rev-lock issues with Xcode:
| PromiseKit | Swift | Xcode | CI Status | Release Notes |
| ---------- | ----------------------- | -------- | ------------ | ----------------- |
| 6 | 3.1, 3.2, 3.3, 4.x | 8.3, 9.x, 10.x | ![ci-master] | [2018/02][news-6] |
| 5 | 3.1, 3.2, 3.3, 4.x | 8.3, 9.x | *Deprecated* | *n/a* |
| 4† | 3.0, 3.1, 3.2, 3.3, 4.x | 8.x, 9.x | ![ci-master] | [2016/09][news-4] |
| 3 | 2.x | 7.x, 8.0 | ![ci-swift2] | [2015/10][news-3] |
| 2 | 1.x | 7.x | *Deprecated* | [2015/10][news-3] |
| 1‡ | *N/A* | * | ![ci-legacy] | – |
† Probably supports Xcode 10 and Swift 4.2, if not, PR welcome.
‡ PromiseKit 1 is pure Objective-C and thus can be used with any Xcode, it is
also your only choice if you need to support iOS 7 or below.
---
We also maintain a series of branches to aid migration for PromiseKit 2:
| Xcode | Swift | PromiseKit | Branch | CI Status |
| ----- | ----- | -----------| --------------------------- | --------- |
| 8.0 | 2.3 | 2 | [swift-2.3-minimal-changes] | ![ci-23] |
| 7.3 | 2.2 | 2 | [swift-2.2-minimal-changes] | ![ci-22] |
| 7.2 | 2.2 | 2 | [swift-2.2-minimal-changes] | ![ci-22] |
| 7.1 | 2.1 | 2 | [swift-2.0-minimal-changes] | ![ci-20] |
| 7.0 | 2.0 | 2 | [swift-2.0-minimal-changes] | ![ci-20] |
We do **not** usually backport fixes to these branches, but pull requests are welcome.
## Xcode 8 / Swift 2.3 or Xcode 7
```ruby
# CocoaPods
swift_version = "2.3"
pod "PromiseKit", "~> 3.5"
# Carthage
github "mxcl/PromiseKit" ~> 3.5
```
[travis]: https://travis-ci.org/mxcl/PromiseKit
[ci-master]: https://travis-ci.org/mxcl/PromiseKit.svg?branch=master
[ci-legacy]: https://travis-ci.org/mxcl/PromiseKit.svg?branch=legacy-1.x
[ci-swift2]: https://travis-ci.org/mxcl/PromiseKit.svg?branch=swift-2.x
[ci-23]: https://travis-ci.org/mxcl/PromiseKit.svg?branch=swift-2.3-minimal-changes
[ci-22]: https://travis-ci.org/mxcl/PromiseKit.svg?branch=swift-2.2-minimal-changes
[ci-20]: https://travis-ci.org/mxcl/PromiseKit.svg?branch=swift-2.0-minimal-changes
[news-2]: http://promisekit.org/news/2015/05/PromiseKit-2.0-Released/
[news-3]: https://github.com/mxcl/PromiseKit/blob/master/CHANGELOG.markdown#300-oct-1st-2015
[news-4]: http://promisekit.org/news/2016/09/PromiseKit-4.0-Released/
[news-6]: http://promisekit.org/news/2018/02/PromiseKit-6.0-Released/
[swift-2.3-minimal-changes]: https://github.com/mxcl/PromiseKit/tree/swift-2.3-minimal-changes
[swift-2.2-minimal-changes]: https://github.com/mxcl/PromiseKit/tree/swift-2.2-minimal-changes
[swift-2.0-minimal-changes]: https://github.com/mxcl/PromiseKit/tree/swift-2.0-minimal-changes
# Using Git Submodules for PromiseKit’s Extensions
> *Note*: This is a more advanced technique.
If you use CocoaPods and a few PromiseKit extensions, then importing PromiseKit
causes that module to import all the extension frameworks. Thus, if you have an
app and a few app extensions (e.g., iOS app, iOS watch extension, iOS Today
extension) then all your final products that use PromiseKit will have forced
dependencies on all the Apple frameworks that PromiseKit provides extensions
for.
This isn’t that bad, but every framework that loads entails overhead and
lengthens startup time.
It’s both better and worse with Carthage. We build individual micro-frameworks
for each PromiseKit extension, so your final products link
against only the Apple frameworks that they actually need. However, Apple has
advised that apps link only against “about 12” frameworks for performance
reasons. So with Carthage, we are worse off on this metric.
The solution is to instead import only CorePromise:
```ruby
# CocoaPods
pod "PromiseKit/CorePromise"
# Carthage
github "mxcl/PromiseKit"
# ^^ for Carthage *only* have this
```
And to use the extensions you need via `git submodules`:
```
git submodule init
git submodule add https://github.com/PromiseKit/UIKit Submodules/PMKUIKit
```
Then in Xcode you can add these sources to your targets on a per-target basis.
Then when you `pod update`, ensure that you also update your submodules:
pod update && git submodule update --recursive --remote
# Release History
## [6.0](https://github.com/mxcl/PromiseKit/releases/tag/6.0.0) Feb 13th, 2018
* [PromiseKit 6 announcement post][news-6].
## [4.0](https://github.com/mxcl/PromiseKit/releases/tag/4.0.0)
* [PromiseKit 4 announcement post][news-4].
## [3.0](https://github.com/mxcl/PromiseKit/releases/tag/3.0.0) Oct 1st, 2015
In Swift 2.0 `catch` and `defer` became reserved keywords mandating we rename
our functions with these names. This forced a major semantic version change on
PromiseKit and thus we took the opportunity to make other minor (source
compatibility breaking) improvements.
Thus if you cannot afford to adapt to PromiseKit 3 but still want to use
Xcode-7.0/Swift-2.0 we provide a [minimal changes branch] where `catch` and
`defer` are renamed `catch_` and `defer_` and all other changes are the bare
minimum to make PromiseKit 2 compile against Swift 2.
If you still are using Xcode 6 and Swift 1.2 then use PromiseKit 2.
[minimal changes branch]: https://github.com/mxcl/PromiseKit/tree/swift-2.0-minimal-changes
## [2.0](https://github.com/mxcl/PromiseKit/releases/tag/2.0.0) May 14th, 2015
[PromiseKit 2 announcement post](http://promisekit.org/news/2015/05/PromiseKit-2.0-Released/).
## [1.5](https://github.com/mxcl/PromiseKit/releases/tag/1.5.0)
Swift 1.2 support. Xcode 6.3 required.

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# Objective-C
PromiseKit has two promise classes:
* `Promise<T>` (Swift)
* `AnyPromise` (Objective-C)
Each is designed to be an appropriate promise implementation for the strong points of its language:
* `Promise<T>` is strict, defined and precise.
* `AnyPromise` is loose and dynamic.
Unlike most libraries, we have extensive bridging support, you can use PromiseKit
in mixed projects with mixed language targets and mixed language libraries.
# Using PromiseKit with Objective-C
`AnyPromise` is our promise class for Objective-C. It behaves almost identically to `Promise<T>`, our Swift promise class.
```objc
myPromise.then(^(NSString *bar){
return anotherPromise;
}).then(^{
//…
}).catch(^(NSError *error){
//…
});
```
You make new promises using `promiseWithResolverBlock`:
```objc
- (AnyPromise *)myPromise {
return [AnyPromise promiseWithResolverBlock:^(PMKResolver resolve){
resolve(foo); // if foo is an NSError, rejects, else, resolves
}];
}
```
---
You reject promises by throwing errors:
```objc
myPromise.then(^{
@throw [NSError errorWithDomain:domain code:code userInfo:nil];
}).catch(^(NSError *error){
//…
});
```
#### :warning: Caution:
ARC in Objective-C, unlike in Objective-C++, is not exception-safe by default.
So, throwing an error will result in keeping a strong reference to the closure
that contains the throw statement.
This pattern will consequently result in memory leaks if you're not careful.
> *Note:* Only having a strong reference to the closure would result in memory leaks.
> In our case, PromisKit automatically keeps a strong reference to the closure until it's released.
__Workarounds:__
1. Return a Promise with value NSError\
Instead of throwing a normal error, you can return a Promise with value NSError instead.
```objc
myPromise.then(^{
return [AnyPromise promiseWithValue:[NSError myCustomError]];
}).catch(^(NSError *error){
if ([error isEqual:[NSError myCustomError]]) {
// In case, same error as the one we thrown
return;
}
//…
});
```
2. Enable ARC for exceptions in Objective-C (not recomended)\
You can add this ```-fobjc-arc-exceptions to your``` to your compiler flags to enable ARC for exceptions.
This is not recommended unless you've read the Apple documentation and are comfortable with the caveats.
For more details on ARC and exceptions:
https://clang.llvm.org/docs/AutomaticReferenceCounting.html#exceptions
---
One important feature is the syntactic flexability of your handlers:
```objc
myPromise.then(^{
// no parameters is fine
});
myPromise.then(^(id foo){
// one parameter is fine
});
myPromise.then(^(id a, id b, id c){
// up to three parameter is fine, no crash!
});
myPromise.then(^{
return @1; // return anything or nothing, it's fine, no crash
});
```
We do runtime inspection of the block you pass to achieve this magic.
---
Another important distinction is that the equivalent function to Swift’s `recover` is combined with `AnyPromise`’s `catch`. This is typical to other “dynamic” promise implementations and thus achieves our goal that `AnyPromise` is loose and dynamic while `Promise<T>` is strict and specific.
A sometimes unexpected consequence of this fact is that returning nothing from a `catch` *resolves* the returned promise:
```objc
myPromise.catch(^{
[UIAlertView …];
}).then(^{
// always executes!
});
```
---
Another important distinction is that the `value` property returns even if the promise is rejected; in that case, it returns the `NSError` object with which the promise was rejected.
# Bridging Between Objective-C & Swift
Let’s say you have:
```objc
@interface Foo
- (AnyPromise *)myPromise;
@end
```
Ensure that this interface is included in your bridging header. You can now use the
following pattern in your Swift code:
```swift
let foo = Foo()
foo.myPromise.then { (obj: AnyObject?) -> Int in
// it is not necessary to specify the type of `obj`
// we just do that for demonstrative purposes
}
```
---
Let’s say you have:
```swift
@objc class Foo: NSObject {
func stringPromise() -> Promise<String>
func barPromise() -> Promise<Bar>
}
@objc class Bar: NSObject { /*…*/ }
```
Ensure that your project is generating a `…-Swift.h` header so that Objective-C can see your Swift code.
If you built this project and opened the `…-Swift.h` header, you would only see this:
```objc
@interface Foo
@end
@interface Bar
@end
```
That's because Objective-C cannot import Swift objects that are generic. So we need to write some stubs:
```swift
@objc class Foo: NSObject {
@objc func stringPromise() -> AnyPromise {
return AnyPromise(stringPromise())
}
@objc func barPromise() -> AnyPromise {
return AnyPromise(barPromise())
}
}
```
If we built this and opened our generated header, we would now see:
```objc
@interface Foo
- (AnyPromise *)stringPromise;
- (AnyPromise *)barPromise;
@end
@interface Bar
@end
```
Perfect.
Note that AnyPromise can only bridge objects that conform to `AnyObject` or derive from `NSObject`. This is a limitation of Objective-C.

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# Contents
* [README](/README.md)
* Handbook
* [Getting Started](GettingStarted.md)
* [Promises: Common Patterns](CommonPatterns.md)
* [Frequently Asked Questions](FAQ.md)
* Manual
* [Installation Guide](Installation.md)
* [Objective-C Guide](ObjectiveC.md)
* [Troubleshooting](Troubleshooting.md)
* [Appendix](Appendix.md)
* [API Reference](https://promisekit.org/reference/)

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# Troubleshooting
## Compilation errors
99% of compilation issues involving PromiseKit can be addressed or diagnosed by one of the fixes below.
### Check your handler
```swift
return firstly {
URLSession.shared.dataTask(.promise, with: url)
}.compactMap {
JSONSerialization.jsonObject(with: $0.data) as? [String: Any]
}.then { dict in
User(dict: dict)
}
```
Swift (unhelpfully) says:
> Cannot convert value of type '([String : Any]) -> User' to expected argument type '([String : Any]) -> _'
What’s the real problem? `then` *must* return a `Promise`, and you're trying to return something else. What you really want is `map`:
```swift
return firstly {
URLSession.shared.dataTask(.promise, with: url)
}.compactMap {
JSONSerialization.jsonObject(with: $0.data) as? [String: Any]
}.map { dict in
User(dict: dict)
}
```
### Specify closure parameters **and** return type
For example:
```swift
return firstly {
foo()
}.then { user in
//…
return bar()
}
```
This code may compile if you specify the type of `user`:
```swift
return firstly {
foo()
}.then { (user: User) in
//…
return bar()
}
```
If it still doesn't compile, perhaps you need to specify the return type, too:
```swift
return firstly {
foo()
}.then { (user: User) -> Promise<Bar> in
//…
return bar()
}
```
We have made great effort to reduce the need for explicit typing in PromiseKit 6,
but as with all Swift functions that return a generic type (e.g., `Array.map`),
you may need to explicitly tell Swift what a closure returns if the closure's body is
longer than one line.
> *Tip*: Sometimes you can force a one-liner by using semicolons.
### Acknowledge all incoming closure parameters
Swift does not permit you to silently ignore a closure's parameters. For example, this code:
```swift
func _() -> Promise<Void> {
return firstly {
proc.launch(.promise) // proc: Foundation.Process
}.then {
when(fulfilled: p1, p2) // both p1 & p2 are `Promise<Void>`
}
}
```
Fails to compile with the error:
Cannot invoke 'then' with an argument list of type '(() -> _)
What's the problem? Well, `Process.launch(.promise)` returns
`Promise<(String, String)>`, and we are ignoring this value in our `then` closure.
If we’d referenced `$0` or named the parameter, Swift would have been satisfied.
Assuming that we really do want to ignore the argument, the fix is to explicitly
acknowledge its existence by assigning it the name "_". That's Swift-ese for "I
know there's a value here, but I'm ignoring it."
```swift
func _() -> Promise<Void> {
return firstly {
proc.launch(.promise)
}.then { _ in
when(fulfilled: p1, p2)
}
}
```
In this situation, you won't always receive an error message that's as clear as the
one shown above. Sometimes, a missing closure parameter sends Swift scurrying off
into type inference limbo. When it finally concludes that there's no way for it to make
all the inferred types work together, it may end up assigning blame to some other
closure entirely and giving you an error message that makes no sense at all.
When faced with this kind of enigmatic complaint, a good rule of thumb is to
double-check your argument and return types carefully. If everything looks OK,
temporarily add explicit type information as shown above, just to rule
out mis-inference as a possible cause.
### Try moving code to a temporary inline function
Try taking the code out of a closure and putting it in a standalone function. Now Swift
will give you the *real* error message. For example:
```swift
func doStuff() {
firstly {
foo()
}.then {
let bar = bar()
let baz = baz()
when(fulfilled: bar, baz)
}
}
```
Becomes:
```swift
func doStuff() {
func fluff() -> Promise<…> {
let bar = bar()
let baz = baz()
when(fulfilled: bar, baz)
}
firstly {
foo()
}.then {
fluff()
}
}
```
An *inline* function like this is all you need. Here, the problem is that you
forgot to mark the last line of the closure with an explicit `return`. It's required
here because the closure is longer than one line.
## You copied code off the Internet that doesn’t work
Swift has changed a lot over the years and so PromiseKit has had to change to keep
up. The code you copied is probably for an older PromiseKit. *Read the definitions of the
functions.* It's easy to do this in Xcode by option-clicking or command-clicking function names.
All PromiseKit functions are documented and provide examples.
## "Context type for closure argument expects 1 argument, which cannot be implicitly ignored"
You have a `then`; you want a `done`.
## "Missing argument for parameter #1 in call"
This is part of Swift 4’s “tuplegate”.
You must specify your `Void` parameter:
```swift
seal.fulfill(())
```
Yes: we hope they revert this change in Swift 5 too.
## Other issues
### `Pending Promise Deallocated!`
If you see this warning, you have a path in your `Promise` initializer that allows
the promise to escape without being sealed:
```swift
Promise<String> { seal in
task { value, error in
if let value = value as? String {
seal.fulfill(value)
} else if let error = error {
seal.reject(error)
}
}
}
```
There are two missing paths here, and if either occurs, the promise will soon be
deallocated without resolving. This will manifest itself as a bug in your app,
probably the awful infinite spinner.
So let’s be thorough:
```swift
Promise<String> { seal in
task { value, error in
if let value = value as? String {
fulfill(value)
} else if let error = error {
reject(error)
} else if value != nil {
reject(MyError.valueNotString)
} else {
// should never happen, but we have an `PMKError` for task being called with `nil`, `nil`
reject(PMKError.invalidCallingConvention)
}
}
}
```
If this seems tedious, it shouldn’t. You would have to be this thorough without promises, too.
The difference is that without promises, you wouldn’t get a warning in the console notifying
you of your mistake!
### Slow compilation / compiler cannot solve in reasonable time
Add return types to your closures.
### My promise never resolves
Check to be sure that your asynchronous task even *starts*. You’d be surprised how
often this is the cause.
For example, if you are using `URLSession` without our extension (but
don’t do that; *use* our extension! we know all the pitfalls), did you forget
to call `resume` on the task? If so, the task never actually starts, and so of
course it never finishes, either.
### `Result of call to 'done(on:_:)' is unused`, `Result of call to 'then(on:_:)' is unused`
PromiseKit deliberately avoids the `@discardableResult` annotation because the
unused result warning is a hint that you have not handled the error in your
chain. So do one of these:
1. Add a `catch`
2. `return` the promise (thus punting the error handling to the caller)
3. Use `cauterize()` to silence the warning.
Obviously, do 1 or 2 in preference to 3.