Learn AudioUnit from live demo in WWDC 2016 session 507.

AudioUnitDemo

//
//  main.swift
//  SquareWaveGenerator
//
//  Created by dolphin on 2016/8/7.
//  Copyright © 2016年 dolphin. All rights reserved.
//

import Foundation
import AudioToolbox
import AVFoundation

class SquareWaveGenerator {
    // 一秒有幾個 frame
    let sampleRate: Double
    // 波形密度, 音高
    let frequency: Double
    // 波形高度, 音量
    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) / Int(MemoryLayout<Float>.size)
        var ptr = UnsafeMutablePointer<Float>.allocate(capacity: nFrames)
        
        var j = self.counter
        // 用一個 frame 裡會包含幾個波形來推算出一個波形的長度
        let cycleLength = self.sampleRate / self.frequency
        let halfCycleLength = cycleLength / 2
        
        let amp = self.amplitude
        let 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() {
    #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)
    // AUAudioUnit: 我們用來操作 AudioUnit 的介面
    let ioUnit = try! AUAudioUnit(componentDescription: ioUnitDesc, options: AudioComponentInstantiationOptions())
    let hardwareFormat = ioUnit.outputBusses[0].format
    let renderFormat = AVAudioFormat(standardFormatWithSampleRate: hardwareFormat.sampleRate, channels: min(2, hardwareFormat.channelCount))
    
    try! ioUnit.inputBusses[0].setFormat(renderFormat)
    
    let generatorLeft = SquareWaveGenerator(sampleRate: renderFormat.sampleRate, frequency: 440.0, amplitude: 0.1)
    let generatorRight = SquareWaveGenerator(sampleRate: renderFormat.sampleRate, frequency: 440.0, amplitude: 0.1)
    // The block that the output unit will call to get audio to send to the output.
    // 當 output unit 要丟音檔資料給 output 裝置時，會 call 這個 block
    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
    }
    
    try! ioUnit.allocateRenderResources()
    
    try! ioUnit.startHardware()
    // 播放 3 秒
    sleep(3)
    ioUnit.stopHardware()
}
main()

update for Swift 3