我创建了三个类来导出带有过滤器的视频。这是实用程序类，它基本上只包含一个 export 函数并调用所有必需的代码

class VideoFilterExport{

    let asset: AVAsset
    init(asset: AVAsset){
        self.asset = asset
    }

    func export(toURL url: NSURL, callback: (url: NSURL?) -> Void){
        guard let track: AVAssetTrack = self.asset.tracksWithMediaType(AVMediaTypeVideo).first else{callback(url: nil); return}

        let composition = AVMutableComposition()
        let compositionTrack = composition.addMutableTrackWithMediaType(AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)

        do{
            try compositionTrack.insertTimeRange(track.timeRange, ofTrack: track, atTime: kCMTimeZero)
        }
        catch _{callback(url: nil); return}

        let videoComposition = AVMutableVideoComposition(propertiesOfAsset: composition)
        videoComposition.customVideoCompositorClass = VideoFilterCompositor.self
        videoComposition.frameDuration = CMTimeMake(1, 30)
        videoComposition.renderSize = compositionTrack.naturalSize

        let instruction = VideoFilterCompositionInstruction(trackID: compositionTrack.trackID)
        instruction.timeRange = CMTimeRangeMake(kCMTimeZero, self.asset.duration)
        videoComposition.instructions = [instruction]

        let session: AVAssetExportSession = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetMediumQuality)!
        session.videoComposition = videoComposition
        session.outputURL = url
        session.outputFileType = AVFileTypeMPEG4

        session.exportAsynchronouslyWithCompletionHandler(){
            callback(url: url)
        }
    }
}

这是另外两个类 - 我将把它们放在一个代码块中以缩短这篇文章

// Video Filter Composition Instruction Class - from what I gather,
// AVVideoCompositionInstruction is used only to pass values to
// the AVVideoCompositing class

class VideoFilterCompositionInstruction : AVMutableVideoCompositionInstruction{

    let trackID: CMPersistentTrackID
    let filters: ImageFilterGroup
    let context: CIContext


    // When I leave this line as-is, startVideoCompositionRequest: isn't called.
    // When commented out, startVideoCompositionRequest(request) is called, but there
    // are no valid CVPixelBuffers provided by request.sourceFrameByTrackID(below value)
    override var passthroughTrackID: CMPersistentTrackID{get{return self.trackID}}
    override var requiredSourceTrackIDs: [NSValue]{get{return []}}
    override var containsTweening: Bool{get{return false}}


    init(trackID: CMPersistentTrackID, filters: ImageFilterGroup, context: CIContext){
        self.trackID = trackID
        self.filters = filters
        self.context = context

        super.init()

        //self.timeRange = timeRange
        self.enablePostProcessing = true
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }

}


// My custom AVVideoCompositing class. This is where the problem lies -
// although I don't know if this is the root of the problem

class VideoFilterCompositor : NSObject, AVVideoCompositing{

    var requiredPixelBufferAttributesForRenderContext: [String : AnyObject] = [
        kCVPixelBufferPixelFormatTypeKey as String : NSNumber(unsignedInt: kCVPixelFormatType_32BGRA), // The video is in 32 BGRA
        kCVPixelBufferOpenGLESCompatibilityKey as String : NSNumber(bool: true),
        kCVPixelBufferOpenGLCompatibilityKey as String : NSNumber(bool: true)
    ]
    var sourcePixelBufferAttributes: [String : AnyObject]? = [
        kCVPixelBufferPixelFormatTypeKey as String : NSNumber(unsignedInt: kCVPixelFormatType_32BGRA),
        kCVPixelBufferOpenGLESCompatibilityKey as String : NSNumber(bool: true),
        kCVPixelBufferOpenGLCompatibilityKey as String : NSNumber(bool: true)
    ]

    let renderQueue = dispatch_queue_create("co.getblix.videofiltercompositor.renderingqueue", DISPATCH_QUEUE_SERIAL)

    override init(){
        super.init()
    }

    func startVideoCompositionRequest(request: AVAsynchronousVideoCompositionRequest){
       // This code block is never executed when the
       // passthroughTrackID variable is in the above class  

        autoreleasepool(){
            dispatch_async(self.renderQueue){
                guard let instruction = request.videoCompositionInstruction as? VideoFilterCompositionInstruction else{
                    request.finishWithError(NSError(domain: "getblix.co", code: 760, userInfo: nil))
                    return
                }
                guard let pixels = request.sourceFrameByTrackID(instruction.passthroughTrackID) else{
                    // This code block is executed when I comment out the
                    // passthroughTrackID variable in the above class            

                    request.finishWithError(NSError(domain: "getblix.co", code: 761, userInfo: nil))
                    return
                }
                // I have not been able to get the code to reach this point
                // This function is either not called, or the guard
                // statement above executes

                let image = CIImage(CVPixelBuffer: pixels)
                let filtered: CIImage = //apply the filter here

                let width = CVPixelBufferGetWidth(pixels)
                let height = CVPixelBufferGetHeight(pixels)
                let format = CVPixelBufferGetPixelFormatType(pixels)

                var newBuffer: CVPixelBuffer?
                CVPixelBufferCreate(kCFAllocatorDefault, width, height, format, nil, &newBuffer)

                if let buffer = newBuffer{
                    instruction.context.render(filtered, toCVPixelBuffer: buffer)
                    request.finishWithComposedVideoFrame(buffer)
                }
                else{
                    request.finishWithComposedVideoFrame(pixels)
                }
            }
        }
    }

    func renderContextChanged(newRenderContext: AVVideoCompositionRenderContext){
        // I don't have any code in this block
    }

    // This is interesting - this is called twice,
    // Once before startVideoCompositionRequest is called,
    // And once after. In the case when startVideoCompositionRequest
    // Is not called, this is simply called twice in a row
    func cancelAllPendingVideoCompositionRequests(){
        dispatch_barrier_async(self.renderQueue){
            print("Cancelled")
        }
    }
}

我一直在看Apple's AVCustomEdit sample project对此有很多指导，但我似乎无法在其中找到发生这种情况的任何原因。

我怎样才能得到 request.sourceFrameByTrackID:函数正确调用，并为每个帧提供有效的 CVPixelBuffer？