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我正在尝试AVVideoComposition在视频顶部添加一些文本并保存视频。这是我使用的代码：

我 创建一个AVMutableComposition and AVVideoComposition

var mutableComp =          AVMutableComposition()
var mutableVidComp =       AVMutableVideoComposition()
var compositionSize :      CGSize?

func configureAsset(){

    let options =               [AVURLAssetPreferPreciseDurationAndTimingKey : "true"]
    let videoAsset =             AVURLAsset(url: Bundle.main.url(forResource: "Car", withExtension: "mp4")! , options : options)
    let videoAssetSourceTrack =  videoAsset.tracks(withMediaType: AVMediaTypeVideo).first! as AVAssetTrack

    compositionSize = videoAssetSourceTrack.naturalSize

    let mutableVidTrack =       mutableComp.addMutableTrack(withMediaType: AVMediaTypeVideo, preferredTrackID: kCMPersistentTrackID_Invalid)
    let trackRange =            CMTimeRangeMake(kCMTimeZero, videoAsset.duration)

    do {
        try mutableVidTrack.insertTimeRange( trackRange, of: videoAssetSourceTrack, at: kCMTimeZero)

        mutableVidTrack.preferredTransform = videoAssetSourceTrack.preferredTransform

    }catch { print(error) }

    snapshot = …