FAQ.md

FAQ

Do I need to worry about retain cycles?

Generally no, provided the promise completes then all handlers are released thus any references to self are also released.

This does mean that 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.

This stackoverflow question has some good discussion on the topic.

Where should I put my catch?

catch deliberately terminates the chain, you should place low in your promise heirarchy: at as root a point as possible. Typically this would be your view controllers where your catch can then display a message to the user.

This means you should be writing one catch for many thens and be returning promises without there being catch handlers.

This is obviously a guideline, do what is necessary.

How do branched chains work?

If 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 both have catch handlers.

Probably however you can ignore the catch for one, but be careful in these situations as Swift cannot help you ensure 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 is hardly any sources in fact, it’s actually pretty light. Any weight relative to other promise implementations is 6 years of bug fixes.

Why is debugging hard?

Because promises always execute via dispatch the backtraces you get have less information than is often required to trace the path of execution.

One solution is (during debugging) to turn off the dispatch:

// Swift
DispatchQueue.default = zalgo

//ObjC
PMKSetDefaultDispatchQueue(zalgo)

Don’t leave this is on, we always dispatch to avoid you accidentally writing a common bug pattern: http://blog.izs.me/post/59142742143/designing-apis-for-asynchrony

Where is all()?

Some promise libraries provide all, we provide when, it is the same. when was chosen as it is the more common choice which we also think reads better.

How can I test APIs that return promises?

You need to use XCTestExpectation.

Is PromiseKit thread-safe?

Yes, entirely.

However the code you write in your thens 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 typically you won't have to worry about this.

Why are there separate classes for Objective-C and Swift?

Promise<T> is generic and and thus cannot be represented by Objective-C.

Does PromiseKit conform to Promises/A+?

Yes, we have tests that prove this.

How do PromiseKit and RxSwift differ?

#484

Why can’t I return from a catch like I can in Javascript?

Swift demands functions with 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:

func foo() -> Promise<Any>
    return Promise { fulfill, reject in
        Alamofire.request(rq).responseJSON { rsp in
            if let error = rsp.error {
                reject(error)
            } else {
                fulfill(rsp.value)
            }
        }
    }
}

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 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 thgat you cannot use PromiseKit for that event. Promises only resolve once, this is the fundamental nature of promises and is considered a feature since it gives you guarantees about the flow of your chains.

My question was not answered

Please open a ticket.