【AVFoundation】AVAssetWriter

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AVFoundation 是Apple iOS和OS X系统中用于处理基于时间的媒体数据的高级框架，通过开发所需的工具提供了强大的功能集，让开发者能够基于苹果平台创建当下最先进的媒体应用程序，其针对64位处理器设计，充分利用了多核硬件优势，会自动提供硬件加速操作，确保大部分设备能以最佳性能运行，是iOS开发接触音视频开发必学的框架之一。

参与掘金日新计划，持续记录AVFoundation学习，Demo学习地址，里面封装了一些工具类，可以直接使用，这篇文章主要讲述AVAssetWriter将捕获到的Buffer封装成视频文件，其他类的相关用法可查看我的其他文章。

AVAssetWriter

当我们给捕捉会话配置了AVCaptureVideoDataOutput、AVCaptureAudioDataOutput，就无法使用AVCaptureMovieFileOutput来记录输出的视频了，这时可以使用AVAssetWriter，将输出的视频流音频流Buffer封装成一个视频文件保存

利用AVAssetWriter封装工具

• AVAssetWriter通过多个(音频、视频等)AVAssetWriterInput对象配置。AVAssetWriterInput通过mediaType和outputSettings来初始化，我们可以在outputSettings中进行视频比特率、视频宽高、关键帧间隔等细致的配置，这也是AVAssetWrite相比AVAssetExportSession明显的优势。AVAssetWriterInput在附加数据后会在最终输出时生成一个独立的AVAssetTrack.
• AssetWriterInput都期望接收CMSampelBufferRef格式的数据，如果是CVPixelBuffer格式的数据，就需要通过adaptor来格式化后再写入。
• 一定要注意kCVPixelBufferPixelFormatTypeKey的格式，输出的视频流是什么这里就用什么，我Demo里是kCVPixelFormatType_32BGRA

protocol MovieWriterDelegate {
    func didWriteMovieSuccess(at url: URL)
    func didWriteMovieFailed()
}

extension MovieWriterDelegate {
    func didWriteMovieSuccess(at url: URL) {}
    func didWriteMovieFailed() {}
}

final class MovieWriter {
    /// 代理回调
    var delegate: MovieWriterDelegate?
    /// 是否正在写入标识
    private(set) var isWritingFlag: Bool = false
    /// coreImage上下文
    var ciContext: CIContext = {
        let eaglContext = EAGLContext(api: .openGLES2)!
        let ciContext = CIContext(eaglContext: eaglContext, options: [CIContextOption.workingColorSpace: nil])
        return ciContext
    }()
    
    private let videoSettings: [String: Any]?
    private let audioSettings: [String: Any]?
    
    private let inputPixelBufferAdaptor: AVAssetWriterInputPixelBufferAdaptor
    private let assetWriter: AVAssetWriter!
    private let videoInput: AVAssetWriterInput!
    private let audioInput: AVAssetWriterInput!
    
    /// 色彩空间
    private let colorSpace: CGColorSpace = CGColorSpaceCreateDeviceRGB()
    /// 是否是第一帧标识
    private var isFirstSampleFlag: Bool = true
    
    init(videoSettings: [String: Any]?, audioSettings: [String: Any]?) {
        self.videoSettings = videoSettings
        self.audioSettings = audioSettings
        
        /**
         AVAssetWriter通过多个(音频、视频等)AVAssetWriterInput对象配置。AVAssetWriterInput通过mediaType和outputSettings来初始化，我们可以在outputSettings中进行视频比特率、视频宽高、关键帧间隔等细致的配置，这也是AVAssetWrite相比AVAssetExportSession明显的优势。AVAssetWriterInput在附加数据后会在最终输出时生成一个独立的AVAssetTrack.

         此处用到了PixelBufferAdaptor来附加CVPixelBuffer类型的数据，它在附加CVPixelBuffer对象的视频样本时能提供最优性能。
         */
        
        // assetWriter
        assetWriter = try! AVAssetWriter(url: WriteUtil.outputURL(), fileType: .mov)
        
        // 添加videoInput
        // 每个AssetWriterInput都期望接收CMSampelBufferRef格式的数据，如果是CVPixelBuffer格式的数据，就需要通过adaptor来格式化后再写入
        videoInput = AVAssetWriterInput(mediaType: .video, outputSettings: videoSettings)
        // 针对实时性进行优化
        videoInput.expectsMediaDataInRealTime = true
        videoInput.transform = WriteUtil.writeTransform(for: UIDevice.current.orientation)
        if (assetWriter.canAdd(videoInput) == true) {
            assetWriter.add(videoInput)
        } else {
            print("MovieWriter-无法添加视频输入.")
        }
        
        // 处理inputPixelBufferAdaptor，一定注意kCVPixelFormatType_32BGRA
        var videoAttributes: [String: Any] = [kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA,
                                         kCVPixelFormatOpenGLESCompatibility as String: true]
        if let videoWidthKey = videoSettings?[AVVideoWidthKey] {
            videoAttributes[kCVPixelBufferWidthKey as String] = videoWidthKey
        }
        if let videoHeightKey = videoSettings?[AVVideoHeightKey] {
            videoAttributes[kCVPixelBufferHeightKey as String] = videoHeightKey
        }
        inputPixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: videoInput, sourcePixelBufferAttributes: videoAttributes)
        
        // 添加audioInput
        self.audioInput = AVAssetWriterInput(mediaType: .audio, outputSettings: audioSettings)
        // 针对实时性进行优化
        audioInput.expectsMediaDataInRealTime = true
        if (assetWriter.canAdd(audioInput) == true) {
            assetWriter.add(audioInput)
        } else {
            print("MovieWriter-无法添加音频输入.")
        }
    }
    
    deinit {
        CQLog("MovieWriter-deinit")
    }

}

开始写入结束写入

• 写入结束调用finishWriting即可封装成视频文件

// MARK: - Public Func
extension MovieWriter {
    func startWriting() {
        isWritingFlag = true
        isFirstSampleFlag = true
    }
    
    func stopWriting() {
        isWritingFlag = false;
        assetWriter.finishWriting {
            if self.assetWriter.status == .completed {
                self.delegate?.didWriteMovieSuccess(at: WriteUtil.outputURL())
            } else {
                self.delegate?.didWriteMovieFailed()
                print("MovieWriter-写入视频失败.")
            }
        }
    }
}

写入

• 在视频流音频流的回调函数中调用这里将Buffer持续写入封装
• 这里要求传入CIImage是因为我Demo利用CIFilter做了滤镜，想要将保存的视频也有预览的滤镜效果，如果是用OpenGL做滤镜直接要求传入CVPixelBuffer即可
• 使用AVAssetWriter实时写入将加工后的媒体资源进行编码并写入到容器文件中。
• AVAssetWriterInput期望接收到的数据是CMSampleBuffer格式，但也可以通过pixelBufferAdaptor适配成它所期望的数据，这里改为了写入CVPixelBuffer。

func process(image: CIImage, atTime time: CMTime) {
        guard isWritingFlag == true else { return }
        if isFirstSampleFlag == true {
            if assetWriter.startWriting() == true {
                assetWriter.startSession(atSourceTime: time)
            } else {
                print("MovieWriter-开始写入失败.")
            }
            isFirstSampleFlag = false
        }
        
        guard let pixelBufferPool: CVPixelBufferPool = inputPixelBufferAdaptor.pixelBufferPool else {
            print("MovieWriter-缓冲池为空.")
            return
        }
        // 从池中创建一个新的PixelBuffer对象。
        var outputRenderBuffer: CVPixelBuffer?
        let createResult = CVPixelBufferPoolCreatePixelBuffer(nil, pixelBufferPool, &outputRenderBuffer)
        if createResult != kCVReturnSuccess {
            print("MovieWriter-无法从池中获取像素缓冲区.")
            return
        }
        
        // 将CIImage渲染到CVPixelBuffer，再进行写入
        ciContext.render(image, to: outputRenderBuffer!, bounds: image.extent, colorSpace: colorSpace)
        if videoInput.isReadyForMoreMediaData == true {
            let result = inputPixelBufferAdaptor.append(outputRenderBuffer!, withPresentationTime: time)
            if result == false {
                print("MovieWriter-附加像素缓冲区错误.")
            }
        }
    }
    
    func process(audioBuffer: CMSampleBuffer) {
        guard isWritingFlag == true else { return }
        guard isFirstSampleFlag == false else { return }
        if audioInput.isReadyForMoreMediaData == true {
            let result = audioInput.append(audioBuffer)
            if result == false {
                print("MovieWriter-附加音频样本缓冲区错误.")
            }
        }
    }

struct WriteUtil {
    /**
     根据原始图像Rect和预览Rect裁剪
     - parameter sourceRect: 原始图像Rect
     - parameter previewRect: 预览Rect
     - returns: 裁剪后Rect
     */
    static func centerCropImageRect(sourceRect: CGRect, previewRect: CGRect) -> CGRect {
        let sourceAspectRatio: CGFloat = sourceRect.size.width/sourceRect.size.height
        let previewAspectRatio: CGFloat = previewRect.size.width/previewRect.size.height
        // 想要保持屏幕大小，所以裁剪视频图像
        var drawRect = sourceRect
        if sourceAspectRatio>previewAspectRatio {
            // 使用视频图像的全部高度，并中心裁剪宽度
            let scaledHeight = drawRect.size.height*previewAspectRatio
            drawRect.origin.x += (drawRect.self.width-scaledHeight)/2
            drawRect.size.width = scaledHeight
        } else {
            // 使用视频图像的全宽度，并中心裁剪高度
            drawRect.origin.y += (drawRect.size.height-drawRect.size.width/previewAspectRatio)/2
            drawRect.size.height = drawRect.size.width/previewAspectRatio
        }
        return drawRect
    }

    /**
     根据设备方向计算写入Transform
     - parameter deviceOrientation: 设备方向
     - returns: 写入Transform
     */
    static func writeTransform(for deviceOrientation: UIDeviceOrientation) -> CGAffineTransform {
        let result: CGAffineTransform
        switch deviceOrientation {
        case .landscapeRight:
            result = CGAffineTransform(rotationAngle: Double.pi)
        case .portraitUpsideDown:
            result = CGAffineTransform(rotationAngle: Double.pi/2*3)
        case .portrait, .faceUp, .faceDown, .unknown:
            result = CGAffineTransform(rotationAngle: Double.pi/2)
        case .landscapeLeft:
            result = CGAffineTransform.identity
        @unknown default:
            result = CGAffineTransform.identity
        }
        return result
    }

    static func outputURL() -> URL {
        let filePath = NSTemporaryDirectory() + "AVAssetWriter_movie.mov"
        let filerUrl = URL(fileURLWithPath: filePath)
        if FileManager.default.fileExists(atPath: filePath) {
            try? FileManager.default.removeItem(atPath: filePath)
        }
        return filerUrl
    }
}