Recursion in async callback (Swift)

Question

Here's some code that it intended to make the phone vibrate (async), await the completion of this task, and then make it vibrate again immediately after:

func vibrate() {
    AudioServicesPlayAlertSound(SystemSoundID(kSystemSoundID_Vibrate))
    AudioServicesAddSystemSoundCompletion(SystemSoundID(kSystemSoundID_Vibrate), nil, nil, { (soundId, clientData) -> Void in
        self.vibrate()
    }, nil)
}

Xcode gives the following error:

A C function pointer cannot be formed from a closure that captures context

How I can recurse from inside of this async function?

Compare https://stackoverflow.com/questions/33260808/how-to-use-instance-method-as-callback-for-function-which-takes-only-func-or-lit/33262376#33262376 — Martin R, Oct 16 '18 at 04:54

score 1 · Accepted Answer · answered Oct 16 '18 at 03:56

1

I was trying hard to find a solution to your question, and I stumbled upon this thread: https://forums.swift.org/t/a-c-function-pointer-cannot-be-formed-from-a-local-function-that-captures-context-on-swift-package/9388/6

I did encapsulate the vibrate() method into a new struct, like so:

import AudioToolbox
import CoreAudioKit

struct Vibrator {
    static func vibrate() {
        AudioServicesPlayAlertSound(SystemSoundID(kSystemSoundID_Vibrate))
        AudioServicesAddSystemSoundCompletion(SystemSoundID(kSystemSoundID_Vibrate), nil, nil, { (soundId, clientData) in
            Vibrator.vibrate()
        }, nil)
    }
}

and call it of course like so: Vibrator.vibrate(). Voila!

I hope this helps!

answered Oct 16 '18 at 03:56

Glenn Posadas

12,555
6
54
95

You can also use this way to call any function within class, (i tried to convert code part separate, until now no success.) ` AudioServicesPlayAlertSound(SystemSoundID(kSystemSoundID_Vibrate)) let myData = unsafeBitCast(self, to: UnsafeMutableRawPointer.self) AudioServicesAddSystemSoundCompletion(SystemSoundID(kSystemSoundID_Vibrate), CFRunLoopGetMain(), nil ,{ (soundId, clientData) in let currentSelf = unsafeBitCast(clientData, to: SelectGenreViewController.self) currentSelf.vibrate() }, myData)` – M Abubaker Majeed Oct 16 '18 at 04:28
Interesting. Post that as an answer for people to see the code formatted properly. – Glenn Posadas Oct 16 '18 at 04:39

score 0 · Answer 2 · answered Oct 16 '18 at 04:48

You can also use this way to call any function within class;

AudioServicesPlayAlertSound(SystemSoundID(kSystemSoundID_Vibrate)) 
let myData = unsafeBitCast(self, to: UnsafeMutableRawPointer.self) 
AudioServicesAddSystemSoundCompletion(SystemSoundID(kSystemSoundID_Vibrate), 
CFRunLoopGetMain(), nil ,{ (soundId, clientData) in 
let currentSelf = unsafeBitCast(clientData, to: YOUR_CLASS_NAME.self) 
currentSelf.vibrate() // call any funcation from current Controller
}, myData)

score 0 · Answer 3 · answered Oct 16 '18 at 15:00

Here's the solution I eventually found. Turns out, there is a built-in function, AudioServicesPlaySystemSoundWithCompletion, that takes a callback as an argument:

func vibrate() {
    AudioServicesPlaySystemSoundWithCompletion(SystemSoundID(kSystemSoundID_Vibrate)) {
        self.vibrate()
    }
}

Recursion in async callback (Swift)

3 Answers3