ios - 在 AVMutableVideoComposition 中使用 trackId

Question

所以我正在开发这个视频编辑器项目，但在获取 trackID 时遇到了问题我搜索了我所知道的所有地方，但我所能找到的只是一些 Objective-C 代码，但我无法将其转换为 swift

我在这一行收到错误:

layer?.trackID = videoAsset?.tracksWithMediaType(AVMediaTypeVideo)

错误是

Cannot assign to the result of this expression

这是我的代码:

import UIKit
import MobileCoreServices
import AVFoundation
import AVKit

class VideoMakerViewController: UIViewController, UIImagePickerControllerDelegate, UINavigationControllerDelegate, UIVideoEditorControllerDelegate {
    var videoAsset: AVAsset? = nil
    var videoEditor: AVMutableVideoComposition? = nil
    var videoEditorInstroctions: AVVideoCompositionInstruction? = nil
    var layer: AVVideoCompositionLayerInstruction? = nil


    override func viewDidLoad() {
        super.viewDidLoad()
    }


    @IBAction func chooseVideo(sender: AnyObject) {
        var videoPicker = UIImagePickerController()
        videoPicker.delegate = self
        videoPicker.sourceType = .PhotoLibrary
        videoPicker.mediaTypes = [kUTTypeMovie]
        videoPicker.allowsEditing = false
        presentViewController(videoPicker, animated: true, completion: nil)
    }


    func imagePickerController(picker: UIImagePickerController, didFinishPickingMediaWithInfo info: [NSObject : AnyObject]) {
        dismissViewControllerAnimated(true, completion: nil)

        let url = info[UIImagePickerControllerReferenceURL] as! NSURL
        let videoName = url.path!.lastPathComponent
        let documentDirectory = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true).first as! String
        let localPath = documentDirectory.stringByAppendingPathComponent(videoName)
        println(videoName)
        videoAsset = AVURLAsset(URL: url, options: nil)

        videoEditorInstroctions?.backgroundColor = UIColor.blueColor().CGColor

        layer?.trackID = videoAsset?.tracksWithMediaType(AVMediaTypeVideo)
        videoEditor = AVMutableVideoComposition(propertiesOfAsset: videoAsset)
        videoEditor?.frameDuration = CMTimeMake(60, 30)
        videoEditor?.renderSize = CGSize(width: 600, height: 1000)
        videoEditor?.renderScale = Float(1)

        println(videoAsset?.duration)    
    }
}

Answer 1

首先:trackID 是只读的。为了能够为其分配任何内容，您必须使用 AVMutableVideoCompositionLayerInstruction而不是 AVVideoCompositionLayerInstruction。

第二个:tracksWithMediaType返回

An array of AVAssetTrack objects of the asset that present media of mediaType.

而trackID的类型为CMPersistentTrackID 。因此，您不能将其中一个分配给另一个。

但是，您可以/必须从返回数组中的任何对象中检索您正在查找的 ID:

layer?.trackID = (videoAsset?.tracksWithMediaType(AVMediaTypeVideo)[0].trackID)!

或

layer?.trackID = videoAsset!.tracksWithMediaType(AVMediaTypeVideo)[0].trackID