ios - 如何使用 AVFoundation 延迟几秒同时录制和播放捕获的视频？

Question

我正在考虑让我的 Swift iOS 应用程序录制视频并在同一屏幕上延迟 30 秒播放。

我一直在使用 official example录制视频。然后我添加了一个按钮，该按钮将在屏幕上的单独 View 中使用 AVPlayer 触发播放 self.movi​​eFileOutput?.outputFileURL。它接近我想要的，但显然一旦到达写入磁盘的文件末尾它就会停止播放，并且在写入下一个缓冲 block 时不会继续播放。

我可以每 30 秒停止一次视频录制并保存每个文件的 URL，以便我可以播放它，但这意味着视频捕获和播放会中断。

我怎样才能让视频录制永不停止，播放始终在屏幕上出现我想要的任何延迟？

我见过一个类似的问题，所有答案都指向 AVFoundation 文档。我找不到如何让 AVFoundation 在录制时将可预测的视频 block 从内存写入磁盘。

Answer 1

您可以通过录制 30 秒的视频 block 来实现您想要的效果，然后将它们排队到 AVQueuePlayer 以进行无缝播放。在 macOS 上使用 AVCaptureFileOutput 录制视频 block 会非常容易，但遗憾的是，在 iOS 上你不能在不丢帧的情况下创建新 block ，所以你必须使用更冗长、更低级别的 AVAssetWriter API:

import UIKit
import AVFoundation

// TODO: delete old videos
// TODO: audio

class ViewController: UIViewController {
    // capture
    let captureSession = AVCaptureSession()

    // playback
    let player = AVQueuePlayer()
    var playerLayer: AVPlayerLayer! = nil

    // output. sadly not AVCaptureMovieFileOutput
    var assetWriter: AVAssetWriter! = nil
    var assetWriterInput: AVAssetWriterInput! = nil

    var chunkNumber = 0
    var chunkStartTime: CMTime! = nil
    var chunkOutputURL: URL! = nil

    override func viewDidLoad() {
        super.viewDidLoad()

        playerLayer = AVPlayerLayer(player: player)
        view.layer.addSublayer(playerLayer)

        // inputs
        let videoCaptureDevice = AVCaptureDevice.defaultDevice(withMediaType: AVMediaTypeVideo)
        let videoInput = try! AVCaptureDeviceInput(device: videoCaptureDevice)
        captureSession.addInput(videoInput)

        // outputs
        // iOS AVCaptureFileOutput/AVCaptureMovieFileOutput still don't support dynamically
        // switching files (?) so we have to re-implement with AVAssetWriter
        let videoOutput = AVCaptureVideoDataOutput()
        // TODO: probably something else
        videoOutput.setSampleBufferDelegate(self, queue: DispatchQueue.main)
        captureSession.addOutput(videoOutput)

        captureSession.startRunning()
    }

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        playerLayer.frame = view.layer.bounds
    }

    func createWriterInput(for presentationTimeStamp: CMTime) {
        let fileManager = FileManager.default
        chunkOutputURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0].appendingPathComponent("chunk\(chunkNumber).mov")
        try? fileManager.removeItem(at: chunkOutputURL)

        assetWriter = try! AVAssetWriter(outputURL: chunkOutputURL, fileType: AVFileTypeQuickTimeMovie)
        // TODO: get dimensions from image CMSampleBufferGetImageBuffer(sampleBuffer)
        let outputSettings: [String: Any] = [AVVideoCodecKey:AVVideoCodecH264, AVVideoWidthKey: 1920, AVVideoHeightKey: 1080]
        assetWriterInput = AVAssetWriterInput(mediaType: AVMediaTypeVideo, outputSettings: outputSettings)
        assetWriterInput.expectsMediaDataInRealTime = true
        assetWriter.add(assetWriterInput)

        chunkNumber += 1
        chunkStartTime = presentationTimeStamp

        assetWriter.startWriting()
        assetWriter.startSession(atSourceTime: chunkStartTime)
    }
}

extension ViewController: AVCaptureVideoDataOutputSampleBufferDelegate {
    func captureOutput(_ captureOutput: AVCaptureOutput!, didOutputSampleBuffer sampleBuffer: CMSampleBuffer!, from connection: AVCaptureConnection!) {
        let presentationTimeStamp = CMSampleBufferGetPresentationTimeStamp(sampleBuffer)

        if assetWriter == nil {
            createWriterInput(for: presentationTimeStamp)
        } else {
            let chunkDuration = CMTimeGetSeconds(CMTimeSubtract(presentationTimeStamp, chunkStartTime))

            if chunkDuration > 30 {
                assetWriter.endSession(atSourceTime: presentationTimeStamp)

                // make a copy, as finishWriting is asynchronous
                let newChunkURL = chunkOutputURL!
                let chunkAssetWriter = assetWriter!

                chunkAssetWriter.finishWriting {
                    print("finishWriting says: \(chunkAssetWriter.status.rawValue, chunkAssetWriter.error)")
                    print("queuing \(newChunkURL)")
                    self.player.insert(AVPlayerItem(url: newChunkURL), after: nil)
                    self.player.play()
                }
                createWriterInput(for: presentationTimeStamp)
            }
        }

        if !assetWriterInput.append(sampleBuffer) {
            print("append says NO: \(assetWriter.status.rawValue, assetWriter.error)")
        }
    }
}

附注很想知道你 30 秒前在做什么。你到底在做什么？