iOS：音频单元、OpenAL 与 Core Audio

iOS音频开发可以通过多种方式来实现，其中包括使用音频单元、OpenAL和Core Audio。本文将介绍这三种方法，并提供相应的案例代码。

swift
import UIKit
import AVFoundation
class ViewController: UIViewController {
    var audioUnit: AUAudioUnit?
    var audioFile: AVAudioFile?
    override func viewDidLoad() {
        super.viewDidLoad()
        guard let audioFileURL = Bundle.main.url(forResource: "audio", withExtension: "wav") else {
            return
        }
        do {
            audioFile = try AVAudioFile(forReading: audioFileURL)
            let audioUnitDescription = AudioComponentDescription(componentType: kAudioUnitType_Generator,
                                                                componentSubType: kAudioUnitSubType_AudioFilePlayer,
                                                                componentManufacturer: kAudioUnitManufacturer_Apple,
                                                                componentFlags: 0,
                                                                componentFlagsMask: 0)
            try audioUnit = AUAudioUnit(componentDescription: audioUnitDescription, options: [])
        } catch {
            print("Failed to create audio unit: \(error)")
        }
    }
    @IBAction func playButtonTapped(_ sender: UIButton) {
        if let audioUnit = audioUnit {
            do {
                try AVAudioSession.sharedInstance().setCategory(.playback, mode: .default)
                try AVAudioSession.sharedInstance().setActive(true)
                let audioUnitOutput = audioUnit.outputBusses[0]
                try audioUnitOutput.setFormat(audioFile!.processingFormat)
                let audioUnitPlayer = try AVAudioPlayerNode()
                audioUnit.auAudioUnit = audioUnit
                let audioEngine = AVAudioEngine()
                audioEngine.attach(audioUnitPlayer)
                audioEngine.connect(audioUnitPlayer, to: audioEngine.mainMixerNode, format: audioFile!.processingFormat)
                audioUnitPlayer.scheduleFile(audioFile!, at: nil, completionHandler: nil)
                try audioEngine.start()
                audioUnitPlayer.play()
            } catch {
                print("Failed to play audio: \(error)")
            }
        }
    }
}

swift
import UIKit
import OpenAL
class ViewController: UIViewController {
    var device: OALDevice?
    var context: OALContext?
    var source: OALSource?
    var buffer: OALBuffer?
    override func viewDidLoad() {
        super.viewDidLoad()
        guard let audioFileURL = Bundle.main.url(forResource: "audio", withExtension: "wav") else {
            return
        }
        device = OALDevice()
        context = OALContext(device: device!)
        do {
            try context?.makeCurrent()
            buffer = OALBuffer(url: audioFileURL)
            source = OALSource()
            source?.buffer = buffer
        } catch {
            print("Failed to create OpenAL context: \(error)")
        }
    }
    @IBAction func playButtonTapped(_ sender: UIButton) {
        if let source = source {
            source.play()
        }
    }
}

swift
import UIKit
import AudioToolbox
class ViewController: UIViewController {
    var audioQueue: AudioQueueRef?
    var audioFile: AudioFileID?
    var buffer: AudioQueueBufferRef?
    override func viewDidLoad() {
        super.viewDidLoad()
        guard let audioFileURL = Bundle.main.url(forResource: "audio", withExtension: "wav") else {
            return
        }
        AudioFileOpenURL(audioFileURL as CFURL, .readPermission, 0, &audioFile)
        var audioFormat = AudioStreamBasicDescription()
        var audioFormatSize = UInt32(MemoryLayout.size(ofValue: audioFormat))
        AudioFileGetProperty(audioFile!, kAudioFilePropertyDataFormat, &audioFormatSize, &audioFormat)
        var audioQueueBufferByteSize: UInt32 = 0
        var packetCount: UInt32 = 0
        var maxPacketSize: UInt32 = 0
        var dataFormatSize = UInt32(MemoryLayout.size(ofValue: audioFormat))
        AudioFileGetProperty(audioFile!, kAudioFilePropertyPacketSizeUpperBound, &dataFormatSize, &maxPacketSize)
        AudioFileGetProperty(audioFile!, kAudioFilePropertyEstimatedDuration, &dataFormatSize, &packetCount)
        audioQueueBufferByteSize = 2048 * maxPacketSize
        AudioQueueNewOutput(&audioFormat, outputCallback, nil, nil, nil, 0, &audioQueue)
        AudioQueueAllocateBuffer(audioQueue!, audioQueueBufferByteSize, &buffer)
    }
    @IBAction func playButtonTapped(_ sender: UIButton) {
        if let audioQueue = audioQueue, let buffer = buffer {
            AudioQueueEnqueueBuffer(audioQueue, buffer, 0, nil)
            AudioQueueStart(audioQueue, nil)
        }
    }
    let outputCallback: AudioQueueOutputCallback = { (inUserData, inAQ, inBuffer) in
        let viewController = Unmanaged.fromOpaque(inUserData!).takeUnretainedValue()
        var numBytes: UInt32 = 0
        var numPackets: UInt32 = viewController.packetCount
        AudioFileReadPacketData(viewController.audioFile!, false, &numBytes, nil, 0, &numPackets, inBuffer.pointee.mAudioData)
        if numPackets > 0 {
            inBuffer.pointee.mAudioDataByteSize = numBytes
            AudioQueueEnqueueBuffer(inAQ, inBuffer, 0, nil)
        } else {
            AudioQueueStop(inAQ, false)
        }
    }
}