Change Promise properties: isPending, isFulfilled, isRejected from In…

[ghost]

Change Promise properties: isPending, isFulfilled, isRejected from Internal to Public

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[shoumikhin]

Hi @oxview, could you provide any more info on why you believe exposing those properties is necessary?

[ghost]

Hi @shoumikhin, I have something like this:

var promise: Promise<Void>

func connect() -> Promise<Void> {
    self.promise = Promise<Void>.pending()
    
    // connect to host ...
    
    return self.promise
}

func socket(didConnectToHost host: String) {
    self.promise.fulfill(())
}

func socket(didDisconnectFromHost host: String) {
    if self.promise.isPending {
        self.promise.reject("some error")
        
        // we are sure that we was not connected, self.promise was not fullfiled/rejected
        // do 1 logic ...
    } else {
        // do 2 logic ...
    }
}

[ghost]

@shoumikhin Moving from other Promises someone throw exception if promise is already NOT in pending state and you try to fulfill or reject it, in google/promises it will be ignored, I understand that doing this now will break API, but because human factor are not excluded throwing exception on fulfill/reject if not in pending state sometime help to prevent wrong logic (helpful for beginners), but this is just a suggestion.

[shoumikhin]

Thanks you for the details!

Are you using some kind of a networking library that provides a delegate to notify about connections? Wonder if there's any API which provides a completion handler instead, which you could then wrap with resolve?

If no, and being a delegate is the only way, how about a nullable Promise?

var promise: Promise<Void>?

func connect() -> Promise<Void> {
    if promise != nil {
        promise?.reject(MyError.abandoned)
        promise = nil
    }
    let newPromise = Promise<Void>.pending()
    promise = newPromise

    // connect to host ...
    
    return newPromise
}

func socket(didConnectToHost host: String) {
    promise?.fulfill(())
    promise = nil
}

func socket(didDisconnectFromHost host: String) {
    if promise != nil {
        promise?.reject(MyError.disconnected)
        promise = nil

        // we are sure that we was not connected, self.promise was not fullfiled/rejected
        // do 1 logic ...
    } else {
        // do 2 logic ...
    }
}

Just make sure the users of such promise won't introduce retain cycles with something like:

object.connect().then { _ in
  object.doSomethingElse() 
  // object is now captured by the block which is captured by the promise returned from connect(), which in turn is owned by the object
  // although, the retain cycle will get broken once the promise gets resolved in either delegate call and nullified
}

Anyhow, checking the state of a promise with isPending() for logical branching is potentially error-prone, because the promise can get resolved on a background thread immediately after such check. Those methods are available for testing purposes only, so you need to @testable import Promises to get them.

[ghost]

Thank you for you advise and I am absolutely agree with checking the state of a promise with isPending() for logical branching is potentially error-prone I will rewrite my code. We can close this