使用 AVAssetWriter 录制小视频

前言

最近在开发一个小项目，要用到 AVFoundation 来录制小视频，在开发的过程中遇到不少的坑，希望记录下来。

整个 AVFoundation 框架真是相当复杂而有序的（这里不得不佩服 Apple 的工程师），要使用 AVFoundation 录制小视频，并且保存成文件，你需要用到以下的类。

• AVCaptureSession
• AVCaptureDeviceInput
• AVCaptureVideoDataOutput
• AVCaptureAudioDataOutput
• AVCaptureConnection
• AVAssetWriter
• AVAssetWriterInput

类的定义

建立一个 Session 类，先定义好必须的属性

class VideoSessionEntity: NSObject {
    
    let session = AVCaptureSession()
    var tmpFileURL: NSURL?
    
    var videoInput: AVCaptureDeviceInput?
    var videoOutput: AVCaptureVideoDataOutput = AVCaptureVideoDataOutput()
    var videoConnection: AVCaptureConnection?
    
    var audioInput: AVCaptureDeviceInput?
    var audioOutput: AVCaptureAudioDataOutput = AVCaptureAudioDataOutput()
    var audioConnection: AVCaptureConnection?
    
    var assetWriter: AVAssetWriter?
    var videoWriterInput: AVAssetWriterInput?
    var audioWriterInput: AVAssetWriterInput?

}

我们需要这么多的属性去完成我们的工作，这里面包括视频、音频的相关属性。

• session 是整个录制工作的控制中心
• videoInput 是视频源的入口
• videoOutput 是视频源的出口
• videoConnection 是视频源的控制中心
• audioInput 是音频源的入口
• audioOutput 是音频源的出口
• audioConnection 是音频源的控制中心

创建回调

其中 videoOutput 和 audioOutput 是需要设置 delegate 的，并且，他们的 delegate 方法是同一个的，它们各需要一个 dispatch_queue_t 进行 GCD 异步回调，这样子创建就可以。

let videoDataOutputQueue = dispatch_queue_create("com.firefly.videoDataOutputQueue", DISPATCH_QUEUE_SERIAL)
let audioDataOutputQueue = dispatch_queue_create("com.firefly.audioDataOutputQueue", DISPATCH_QUEUE_SERIAL)

初始化

在初始化这些属性前，先写两个方法，获取设备。

func deviceForVideo() -> AVCaptureDevice? {
    let devices = AVCaptureDevice.devicesWithMediaType(AVMediaTypeVideo)
    for device in devices {
        if let device = device as? AVCaptureDevice {
            if device.position == self.videoSource { //在这里选择前后摄像头
                return device
            }
        }
    }
    return nil
}

func deviceForAudio() -> AVCaptureDevice? {
    let devices = AVCaptureDevice.devicesWithMediaType(AVMediaTypeAudio)
    for device in devices {
        if let device = device as? AVCaptureDevice {
            return device
        }
    }
    return nil
}

然后，我们可以初始化这些属性了，其实都是简单的操作，就是比较繁琐。

func configureVideo() {
    if let device = FireFlyCore.sharedCore.post.videoManager.deviceForVideo() {
        do {
            try! device.lockForConfiguration()
            device.activeVideoMaxFrameDuration = CMTimeMake(1, 24)
            device.unlockForConfiguration()
        }
        do {
            videoInput = try AVCaptureDeviceInput(device: device)
            videoOutput.setSampleBufferDelegate(self, queue: videoDataOutputQueue)
            if session.canAddInput(videoInput) {
                session.addInput(videoInput)
            }
            if session.canAddOutput(videoOutput) {
                session.addOutput(videoOutput)
            }
            if session.canSetSessionPreset(AVCaptureSessionPreset352x288) {
                session.sessionPreset = AVCaptureSessionPreset352x288
            }
            videoConnection = videoOutput.connectionWithMediaType(AVMediaTypeVideo)
        }
        catch _ {
            
        }
    }
}

func configureAudio() {
    if let device = FireFlyCore.sharedCore.post.videoManager.deviceForAudio() {
        do {
            audioInput = try AVCaptureDeviceInput(device: device)
            audioOutput.setSampleBufferDelegate(self, queue: audioDataOutputQueue)
            if session.canAddInput(audioInput) {
                session.addInput(audioInput)
            }
            if session.canAddOutput(audioOutput) {
                session.addOutput(audioOutput)
            }
            audioConnection = audioOutput.connectionWithMediaType(AVMediaTypeAudio)
        }
        catch _ {
            
        }
    }
}

如果没有任何问题，你可以调用 session.startRuning() 方法，这个时候，func captureOutput(captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!) 代理方法应该可以收到好多的数据回调， 这代表工作正常。

写入文件

我们需要初始化 AssetWriter，初始化 AssetWriter 需要一些参数，它们代表视频、音频的压缩参数。

let videoSetting: [String : AnyObject] = [
    AVVideoCodecKey: AVVideoCodecH264,
    AVVideoWidthKey: 320,
    AVVideoHeightKey: 240,
    AVVideoCompressionPropertiesKey: [
        AVVideoPixelAspectRatioKey: [
            AVVideoPixelAspectRatioHorizontalSpacingKey: 1,
            AVVideoPixelAspectRatioVerticalSpacingKey: 1
        ],
        AVVideoMaxKeyFrameIntervalKey: 1,
        AVVideoAverageBitRateKey: 1280000
    ]
]

let audioSetting: [String: AnyObject] = [
    AVFormatIDKey: NSNumber(unsignedInt: kAudioFormatMPEG4AAC),
    AVNumberOfChannelsKey: 1,
    AVSampleRateKey: 22050
]

然后开始初始化 AssetWriter。

• 注意，这里只是初始化，不要在这里调用startWriting() 方法，如果在这此处调用这个方法，你就没有办法设置 startSessionAtSourceTime() 了。
• 这里还有一个坑，.expectsMediaDataInRealTime = true 必须设为 true，否则，视频会丢帧。

func startRecording() {
    tmpFileURL = NSURL.fileURLWithPath("\(NSTemporaryDirectory())tmp\(arc4random()).mp4")
    do {
        assetWriter = try AVAssetWriter(URL: tmpFileURL!, fileType: AVFileTypeMPEG4)
        videoWriterInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: videoSetting)
        videoWriterInput?.expectsMediaDataInRealTime = true
        videoWriterInput?.transform = CGAffineTransformMakeRotation(CGFloat(M_PI / 2))
        if assetWriter!.canAddInput(videoWriterInput!) {
            assetWriter!.addInput(videoWriterInput!)
        }
        audioWriterInput = AVAssetWriterInput(mediaType: AVMediaTypeAudio, outputSettings: audioSetting)
        audioWriterInput?.expectsMediaDataInRealTime = true
        if assetWriter!.canAddInput(audioWriterInput!) {
            assetWriter!.addInput(audioWriterInput!)
        }
    }
    catch _ {
        
    }
}

func endRecording() {
    if let assetWriter = assetWriter {
        if let videoWriterInput = videoWriterInput {
            videoWriterInput.markAsFinished()
        }
        if let audioWriterInput = audioWriterInput {
            audioWriterInput.markAsFinished()
        }
        assetWriter.finishWritingWithCompletionHandler({ () -> Void in
            
        })
    }
}

写入文件

回调方法的可以参考这样的写法，如果需要为视频添加滤镜可以在这里实现。

func captureOutput(captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, fromConnection connection: AVCaptureConnection!) {
    objc_sync_enter(self)
    if let assetWriter = assetWriter {
        if assetWriter.status != .Writing && assetWriter.status != .Unknown {
            return
        }
    }
    if let assetWriter = assetWriter where assetWriter.status == AVAssetWriterStatus.Unknown {
        assetWriter.startWriting()
        assetWriter.startSessionAtSourceTime(CMSampleBufferGetPresentationTimeStamp(sampleBuffer))
    }
    if connection == self.videoConnection {
        dispatch_async(videoDataOutputQueue, { () -> Void in
            if let videoWriterInput = self.videoWriterInput where videoWriterInput.readyForMoreMediaData {
                videoWriterInput.appendSampleBuffer(sampleBuffer)
            }
        })
    }
    else if connection == self.audioConnection {
        dispatch_async(audioDataOutputQueue, { () -> Void in
            if let audioWriterInput = self.audioWriterInput where audioWriterInput.readyForMoreMediaData {
                audioWriterInput.appendSampleBuffer(sampleBuffer)
            }
        })
    }
    objc_sync_exit(self)
}