AVAssetExportSession 进度未更新

Question

我使用AVAssetExportSession 使用以下代码将电影导出为 MP4 格式：

第 1 步：用户点击按钮开始转换

@IBAction func clickConvert(_ sender:UIButton) {
    self.convertProgress?.progress = 0
    self.convertProgress?.isHidden = false

    var preset = AVAssetExportPresetHighestQuality
        switch self.qualitySelection?.selectedSegmentIndex {
        case 0:
            preset = AVAssetExportPresetLowQuality
            break
        case 1:
            preset = AVAssetExportPresetMediumQuality
            break
        case 2:
            preset = AVAssetExportPresetHighestQuality
            break
        default:
            break
    }


    DispatchQueue.global(qos: .background).async {
        let formatter = DateFormatter()
        formatter.dateFormat = "yyyyMMddHHmmss"
        let fileName = formatter.string(from: Date()) + ".mp4"

        let convertGroup = DispatchGroup()
        convertGroup.enter()

        do {
            let documentDirectory = try self.fileManager.url(for: .documentDirectory, in: .userDomainMask, appropriateFor:nil, create:false)
            let filePath = documentDirectory.appendingPathComponent(fileName)
            if(self.videoURL != nil) {
                self.convertVideo(fromURL: self.videoURL!, toURL: filePath, preset: preset, dispatchGroup: convertGroup)
            } else {
                print("nil Video URL")
            }
            convertGroup.notify(queue: DispatchQueue.main) {
                // reset Convert button state
                self.convertButton?.titleLabel?.text = "Convert"
                self.convertButton?.isEnabled = true

                self.delegate?.updateFileList()
                // Take back to old VC, update file list
                if let navController = self.navigationController {
                    navController.popViewController(animated: true)
                }
            }
        } catch {
            print(error)
        }
    }
}

第二步：触发转换视频功能

func convertVideo(fromURL: URL, toURL: URL, preset:String, dispatchGroup: DispatchGroup) {
    let outFileType = AVFileType.mp4
    let inAsset = AVAsset(url: fromURL)
    let startDate = Date()

    AVAssetExportSession.determineCompatibility(ofExportPreset: preset, with: inAsset, outputFileType: outFileType, completionHandler: { (isCompitable) in
        if !isCompitable {
            return
        }

        guard let export = AVAssetExportSession(asset: inAsset, presetName: preset) else {
            return
        }
        export.outputFileType = outFileType
        export.outputURL = toURL
        export.shouldOptimizeForNetworkUse = true
        let start = CMTimeMakeWithSeconds(0.0, preferredTimescale: 0)
        let range = CMTimeRangeMake(start: start, duration: inAsset.duration)
        export.timeRange = range

        // Timer for progress updates
        self.exportTimer = Timer()
        if #available(iOS 10.0, *) {
            print("start exportTimer")
            self.exportTimer = Timer.scheduledTimer(withTimeInterval: 0.05, repeats: true, block: { _ in
                let progress = Float(export.progress)
                print("Export Progress: \(progress)")

                self.updateProgressDisplay(progress: progress)

                if progress < 0.99 {
                    let dict:[String: Float] = ["progress": progress]
                    NotificationCenter.default.post(name: Notification.Name(Constants.Notifications.ConvertProgress.rawValue), object: nil, userInfo: dict)
                }
            })
        }

        export.exportAsynchronously { () -> Void in
            // Handle export results
            switch export.status {
                case .exporting:
                    print("Exporting...")
                    self.updateProgressDisplay(progress: export.progress)
                    break
                case .failed:
                    print("Error: %@!", export.error!)
                    break
                case .cancelled:
                    print("export cancelled")
                    break
                case .completed:
                    let endDate = Date()
                    let elapsed = endDate.timeIntervalSince(startDate)
                    print("Elapsed: \(elapsed)")
                    print("successful")
                    self.exportTimer?.invalidate() // Stop the timer
                    self.generateThumbnail(path: toURL)
                    break
                default:

                    break
            }
            dispatchGroup.leave()
        }
    })
}

但是，状态更新不起作用，因为计时器 exportTimer 永远不会触发（尝试 1），而 exportSession.exporting 案例永远不会触发（尝试 2）。

附言视频可以毫无问题地转换

附言通知已添加到viewDidLoad()，如下：

NotificationCenter.default.addObserver(self, selector: #selector(onDidReceiveConvertProgress(_:)), name: Notification.Name(Constants.Notifications.ConvertProgress.rawValue), object: nil)

状态更新函数（两种尝试）如下：

@objc func onDidReceiveConvertProgress(_ notification:Notification) {
    print ("onDidReceiveConvertProgress")
    if let data = notification.userInfo as? [String:Float] {
        print("Progress: \(String(describing: data["progress"]))")
        self.convertProgress?.progress = data["progress"]!
    }
}

func updateProgressDisplay(progress: Float) {
    print("updateProgressDisplay")
    self.convertProgress?.progress = progress
}

我错过了什么？

Answer 1

我不确定你是否明白这一点，但以防其他人尝试你的代码，进度计时器没有触发的问题是因为你错过了两件事。

1. 您从未调用该函数来启动计时器。例如self.exportTimer.fire()
2. 您必须确保在主队列上更新此计时器。

我遇到了同样的问题，做这两件事解决了我的问题。