More AVAudioPlayerNode with Swift and CompletionHandlers

Continuing on the subject of my previous posts looking at the new AVFoundation Audio classes in OSX10.10/iOS8 with Swift I finally found the error and a relatively obvious one at that. It wasn't closures specifically, but reading about them extended my knowledge and helped finding the cause of the problem.

I had the type-alias for the AVAudioNodeCompletionHandler all wrong. Not sure where I got that definition from, but my newness to the terminology of the error report put me off the scent. Xcode was quite clearly saying what the problem was:

Taking out atTime and options brings the function down to the simpler case of:

func scheduleBuffer(_ buffer: AVAudioPCMBuffer!,
  completionHandler completionHandler: AVAudioNodeCompletionHandler!)

What the error message is saying (when I finally understood it) was that the completion handler Tuple  - e.g. parameter types were not correct - the correct one is () and I had used (AVAudioPCMBuffer!, AVAudioTime!). It helped reading about closures to understand this, although that wasn't the cause of the problem. It does help understanding the syntax and concepts of Swift in a good deal more detail though.

The type alias of AVAudioNodeCompletionHandler is far much simpler, and for completeness is described below:

typealias AVAudioNodeCompletionHandler = @objc_block () -> Void

Putting this into the code (again in a Playground this is too slow), you get something like this:

fun handler() -> Void

{

    // do some audio work

}

player.scheduleBuffer(buffer,atTime:nil, options:nil, completionHandler: handler)

Or, now with my new found understanding of closures, like this:

player.scheduleBuffer(buffer,atTime:nil, options:nil,

    completionHandler: { () -> Void in

        // do some audio work

    })

Trying this again with the completion handler trick works nicely this time, but still annoyingly beats, so there is some other effect here that isn't working

//

//  main.swift

//  Audio

//

//  Created by hondrou on 11/09/2014.

//  Copyright (c) 2014 hondrou. All rights reserved.

//

import Foundation

import AVFoundation

let twopi:Float = 2.0 * 3.14159

var freq:Float = 440.00

var sampleRate:Float = 44100.00

var engine = AVAudioEngine()

var player:AVAudioPlayerNode = AVAudioPlayerNode()

var mixer = engine.mainMixerNode

var length = 4000

var buffer = AVAudioPCMBuffer(PCMFormat: player.outputFormatForBus(0),frameCapacity:AVAudioFrameCount(length))

buffer.frameLength = AVAudioFrameCount(length)

engine.attachNode(player)

engine.connect(player,to:mixer,format:mixer.outputFormatForBus(0))

var error:NSErrorPointer = nil

engine.startAndReturnError(error)

var j:Int=0;

func handler() -> Void

{

    for (var i=0; i<length; i++)

    {

        var val:Float = 5.0 * sin(Float(j)*twopi*freq/sampleRate)

        buffer.floatChannelData.memory[i] = val

        j++

    }

    

    player.scheduleBuffer(buffer,atTime:nil,options:.InterruptsAtLoop,completionHandler:handler)

}

handler()

player.play()

while (true)

{

    NSThread.sleepForTimeInterval(2)

    

    freq += 10

}

Hmph.... more still to get sorted


Now, just in case you wondered (I did) where I cooked up the handler function parameters, it was all down to mixing up two function type aliases that I'd been looking at. The previous incorrect handler function for the completionHandler below:

func handler(buffer:AVAudioPCMBuffer!,time:AVAudioTime!) -> Void

{

}

Is completely the proper type of function if you are installing an audio node tap block:

typealias AVAudioNodeTapBlock = (AVAudioPCMBuffer!, AVAudioTime!) -> Void

Which I'd also been thinking about at the time (and we will most likely be coming to next in our investigations).

For completeness, this is used in the AudioNode function installTapOnBus below:

func installTapOnBus(_ bus: AVAudioNodeBus,
          bufferSize bufferSize: AVAudioFrameCount,
              format format: AVAudioFormat!,
               block tapBlock: AVAudioNodeTapBlock!)