Qfyilyi

Im totally helpless right now, i'm trying to send an eMail for data upload in an iOS Swift 4 App. I found the Obj-C Mailcore library which works in normal code, for example in a button press callback. But if i'm trying to use the same code in my AppDelegate fetch func it doesn't return from the completionHandler.

The Mailcore send operation is async so i used a semaphore wrapper around it, but the completion handler is never called.

Edit:

Apparently the code does not work in the button with the Semaphore. Only if i remove the

let _ = semaphore.wait(timeout: DispatchTime.distantFuture)

part.

So is there another way to wait for the completion of the async completionBlock?

Yes, look at my answer :-)

My code:

let smtpSession = MCOSMTPSession()

[...]

let sendOperation = smtpSession.sendOperation(with: rfc822Data!)

[...]

let semaphore = DispatchSemaphore(value: 0)

if (sendOperation != nil) {

    print("Starting sendOperation...")

    sendOperation?.start { (error) -> Void in

        if (error != nil) {

            sendingError = true

            print("Error sending email: (error)")

        } else {

            print("Successfully sent email!")

        }

        sendingDone = true

        semaphore.signal()

    }

    let _ = semaphore.wait(timeout: DispatchTime.distantFuture)

} else {

    print("Cant init sendOperation")

    sendingError = true

}

The completion is either coming on the same thread or runs something in that thread.

The normal thing to do is to not wait, but instead do whatever you need to do after it completes in the completion itself.

If you must do it with a semaphore, move the start call to another thread.

So i found a solution:

SyncManager.swift

import Foundation



/// A generic Cocoa-Style completion handler

typealias CompletionHandler = (Error?) -> Void



/// Provides syncronous access to results returned by

/// asynchronous processes with completion handlers

class SyncMaker {

    var result: Error? = nil



    /// Generates a synchronous-compliant completion handler

    func completion() -> CompletionHandler{

        return {

            (error: Error?) in



            // Store result, return control

            self.result = error

            CFRunLoopStop(CFRunLoopGetCurrent())

        }

    }



    // Perform task (that must use custom completion handler) and wait

    func run(_ task: @escaping () -> Void) -> Error? {

        task()

        CFRunLoopRun()

        return result

    }

}

AppDelegate.swift (partly)

[...]

let syncMaker = SyncMaker()

let result = syncMaker.run {

    sendOperation?.start(

        syncMaker.completion())

}

if (result != nil) {

    print("Error sending email: (result)")

} else {

    sendingError = false

    print("Successfully sent email!")

}

[...]