issues finding a replacement for "pointee" during converting Swift 2 to Swift 3

Question

I've been working on trying to grasp the idea behind AUAudioUnits and wrote down the sample code in Xcode given in the presentation video from Apple's WWDC 2016 which introduces the topic. It turns out that this code was written for Swift 2, and Swift 3 introduced a new way to do pointers (as seen here and here). Now I'm fairly new to programming with Swift and not familiar with some of it's concepts, and I could not figure out how to perform the conversion from Swift 2 to Swift 3 manually. Even using the Build Setting

Use Legacy Swift Language Version = yes

I was not able to get it running.

Here is the code for Swift 2, which is exactly the code from the video:

import Foundation
import AVFoundation


class SquareWaveGenerator {
    let sampleRate: Double
    let frequency: Double
    let amplitude: Float

    var counter: Double = 0.0

    init(sampleRate: Double, frequency: Double, amplitude: Float) {
        self.sampleRate = sampleRate
        self.frequency = frequency
        self.amplitude = amplitude
    }

    func render(buffer: AudioBuffer) {
        let nframes = Int(buffer.mDataByteSize) / sizeof(Float)
        var ptr = UnsafeMutablePointer<Float>(buffer.mData)

        var j = self.counter
        let cycleLength = self.sampleRate / self.frequency
        let halfCycleLength = cycleLength / 2

        let amp = self.amplitude, minusAmp = -amp

        for _ in 0..<nframes {
            if j < halfCycleLength {
                ptr.pointee = amp
            } else {
                ptr.pointee = minusAmp
            }
            ptr = ptr.successor()
            j += 1.0
            if (j > cycleLength) {
                j -= cycleLength
            }
        }

        self.counter = j
    }
}

func main() {
    //Create an AudioComponentDescription for the input/output unit we want to use.
#if os(iOS)
    let kOutputUnitSubType = kAudioUnitSubType_RemoteIO
#else
    let kOutputUnitSubType = kAudioUnitSubType_HALOutput
#endif

    let ioUnitDesc = AudioComponentDescription(
        componentType: kAudioUnitType_Output,
        componentSubType: kOutputUnitSubType,
        componentManufacturer: kAudioUnitManufacturer_Apple,
        componentFlags: 0,
        componentFlagsMask: 0)

    let ioUnit = try! AUAudioUnit(componentDescription: ioUnitDesc, options: AudioComponentInstantiationOptions())

    /*
        Set things up to render at the same sample rate as the hardware,
        up to 2 channels. Note that the hardware format may not be a standard
        format, so we make a separate render format with the same sample rate
        and the desired channel count.
    */
    let hardwareFormat = ioUnit.outputBusses[0].format
    let renderFormat = AVAudioFormat(standardFormatWithSampleRate: hardwareFormat.sampleRate, channels: min(2,hardwareFormat.channelCount))

    try! ioUnit.inputBusses[0].setFormat(renderFormat)

    // Create square wave generators.
    let generatorLeft = SquareWaveGenerator(sampleRate: renderFormat.sampleRate, frequency: 440.0, amplitude: 0.1)
    let generatorRight = SquareWaveGenerator(sampleRate: renderFormat.sampleRate, frequency: 440.0, amplitude: 0.1)

    // Install a block which will be called to render.
    ioUnit.outputProvider = { (actionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>, timestamp: UnsafePointer<AudioTimeStamp>, frameCount: AUAudioFrameCount, busIndex: Int, rawBufferList: UnsafeMutablePointer<AudioBufferList>) -> AUAudioUnitStatus in

    let bufferList = UnsafeMutableAudioBufferListPointer(rawBufferList)
        if bufferList.count > 0 {
            generatorLeft.render(bufferList[0])
            if bufferList.count > 1 {
                generatorRight.render(bufferList[1])
            }
        }

        return noErr

    }

    // Allocate render resources, then start the audio hardware.
    try! ioUnit.allocateRenderResources()

    try! ioUnit.startHardware()

    sleep(3)
    ioUnit.stopHardware()
}

main()

This code:

ptr.pointee = amp
[...]
ptr.pointee = minusAmp

Throws the following error:

Value of type 'UnsafeMutablePointer' has no member 'pointee'

As I was unable to resolve this issue manually, I tried to manually convert the code to Swift 3, hoping that the issue would be resolved then. Here it is:

import Foundation
import AVFoundation


class SquareWaveGenerator {
    let sampleRate: Double
    let frequency: Double
    let amplitude: Float

    var counter: Double = 0.0

    init(sampleRate: Double, frequency: Double, amplitude: Float) {
        self.sampleRate = sampleRate
        self.frequency = frequency
        self.amplitude = amplitude
    }

    func render(buffer: AudioBuffer) {
        let nframes = Int(buffer.mDataByteSize) / MemoryLayout<Float>.size
        var ptr = buffer.mData

        var j = self.counter
        let cycleLength = self.sampleRate / self.frequency
        let halfCycleLength = cycleLength / 2

        let amp = self.amplitude, minusAmp = -amp

        for _ in 0..<nframes {
            if j < halfCycleLength {
                ptr?.pointee = amp
            } else {
                ptr?.pointee = minusAmp
            }
            ptr = ptr?.advanced(by: 1)
            j += 1.0
            if (j > cycleLength) {
                j -= cycleLength
            }
        }

        self.counter = j
    }
}

func main() {
    //Create an AudioComponentDescription for the input/output unit we want to use.
#if os(iOS)
    let kOutputUnitSubType = kAudioUnitSubType_RemoteIO
#else
    let kOutputUnitSubType = kAudioUnitSubType_HALOutput
#endif

    let ioUnitDesc = AudioComponentDescription(
        componentType: kAudioUnitType_Output,
        componentSubType: kOutputUnitSubType,
        componentManufacturer: kAudioUnitManufacturer_Apple,
        componentFlags: 0,
        componentFlagsMask: 0)

    let ioUnit = try! AUAudioUnit(componentDescription: ioUnitDesc, options: AudioComponentInstantiationOptions())

    /*
        Set things up to render at the same sample rate as the hardware,
        up to 2 channels. Note that the hardware format may not be a standard
        format, so we make a separate render format with the same sample rate
        and the desired channel count.
    */
    let hardwareFormat = ioUnit.outputBusses[0].format
    let renderFormat = AVAudioFormat(standardFormatWithSampleRate: hardwareFormat.sampleRate, channels: min(2,hardwareFormat.channelCount))

    try! ioUnit.inputBusses[0].setFormat(renderFormat)

    // Create square wave generators.
    let generatorLeft = SquareWaveGenerator(sampleRate: renderFormat.sampleRate, frequency: 440.0, amplitude: 0.1)
    let generatorRight = SquareWaveGenerator(sampleRate: renderFormat.sampleRate, frequency: 440.0, amplitude: 0.1)

    // Install a block which will be called to render.
    ioUnit.outputProvider = { (actionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>, timestamp: UnsafePointer<AudioTimeStamp>, frameCount: AUAudioFrameCount, busIndex: Int, rawBufferList: UnsafeMutablePointer<AudioBufferList>) -> AUAudioUnitStatus in

    let bufferList = UnsafeMutableAudioBufferListPointer(rawBufferList)
        if bufferList.count > 0 {
            generatorLeft.render(buffer: bufferList[0])
            if bufferList.count > 1 {
                generatorRight.render(buffer: bufferList[1])
            }
        }

        return noErr

    }

    // Allocate render resources, then start the audio hardware.
    try! ioUnit.allocateRenderResources()

    try! ioUnit.startHardware()

    sleep(3)
    ioUnit.stopHardware()
}

main()

Where again I run into above error

Value of type 'UnsafeMutablePointer' has no member 'pointee'

In the end, I figured that something like

ptr?.storeBytes(of: T, as: T.Type)

should be able to replace the "pointee" construction. If I understood it correctly, "T" is the value which I'd like to store at the position of the pointer. In my case, that would be "amp". "amp" is of type Float.

But no matter what I did, I could not get the code to run. It just would not accept anything like

ptr?.storeBytes(of: amp, as: Float())

throwing

Cannot convert value of type 'Float' to expected argument type 'T.Type'

or

ptr?.storeBytes(of: amp, as: Float.self)

throwing no immediate error anymore and compiling properly, but when running, getting the lldb error message

fatal error: storeBytes to misaligned raw pointer

In essence, I have no idea what I am doing anymore, not understanding the concept of 'T.Type' in this context, and I am stuck. So I have two questions:

1) How do I solve this issue and get the code running?

2) Where can I learn more about these types of constructions à la Type which will help me understand what they are and what they mean?

Answer 1

What you're running into here is that while AudioBuffer.mData used to be an UnsafeMutablePointer, it's now an UnsafeMutableRawPointer, which is new in Swift 3. To work with that data the way you were before, you can bind the referenced memory to the Float type, like this:

guard let mData = buffer.mData 
    else { return /* or error */ }
let nframes = Int(buffer.mDataByteSize) / MemoryLayout<Float>.size
var ptr = mData.bindMemory(to: Float.self, capacity: nframes)

Now ptr is an UnsafeMutablePointer<Float>, which is what you were working with before, and you should be able to access its pointee property without trouble.

Note: Whenever you see T.Type in a function declaration, it's asking for a type itself, not an instance of a type. In this case, you want to pass the type of Float, which is Float.self. Calling Float(), on the other hand, creates a new Float instance.

Finally, instead of continuing to work directly with ptr, I would create a buffer, which will at least give you bounds checking in debug mode and a nicer interface:

let buffer = UnsafeMutableBufferPointer<Float>(start: ptr, count: frames)
// ...
for i in 0..<nframes {
    if j < halfCycleLength {
        buffer[i] = amp
    } else {
        buffer[i] = minusAmp
    }
    j += 1.0
    // ...
}