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视频编辑与合成

问题​

如何用 AVFoundation 进行视频剪辑和合成？

答案​

AVAsset 与 AVComposition​

import AVFoundation

func mergeVideos(urls: [URL]) async throws -> URL {
    let composition = AVMutableComposition()
    let videoTrack = composition.addMutableTrack(
        withMediaType: .video,
        preferredTrackID: kCMPersistentTrackID_Invalid
    )!
    let audioTrack = composition.addMutableTrack(
        withMediaType: .audio,
        preferredTrackID: kCMPersistentTrackID_Invalid
    )!

    var currentTime = CMTime.zero
    for url in urls {
        let asset = AVURLAsset(url: url)
        let duration = try await asset.load(.duration)
        let timeRange = CMTimeRange(start: .zero, duration: duration)

        if let video = try await asset.loadTracks(withMediaType: .video).first {
            try videoTrack.insertTimeRange(timeRange, of: video, at: currentTime)
        }
        if let audio = try await asset.loadTracks(withMediaType: .audio).first {
            try audioTrack.insertTimeRange(timeRange, of: audio, at: currentTime)
        }
        currentTime = CMTimeAdd(currentTime, duration)
    }

    // 导出
    let outputURL = FileManager.default.temporaryDirectory.appendingPathComponent("merged.mp4")
    let exporter = AVAssetExportSession(asset: composition, presetName: AVAssetExportPresetHighestQuality)!
    exporter.outputURL = outputURL
    exporter.outputFileType = .mp4
    await exporter.export()
    return outputURL
}

添加水印​

func addWatermark(to asset: AVAsset, text: String) -> AVVideoComposition {
    let videoSize = CGSize(width: 1920, height: 1080)

    let watermarkLayer = CATextLayer()
    watermarkLayer.string = text
    watermarkLayer.fontSize = 40
    watermarkLayer.foregroundColor = UIColor.white.withAlphaComponent(0.7).cgColor
    watermarkLayer.frame = CGRect(x: videoSize.width - 300, y: 20, width: 280, height: 50)

    let parentLayer = CALayer()
    parentLayer.frame = CGRect(origin: .zero, size: videoSize)
    let videoLayer = CALayer()
    videoLayer.frame = parentLayer.frame
    parentLayer.addSublayer(videoLayer)
    parentLayer.addSublayer(watermarkLayer)

    let composition = AVMutableVideoComposition()
    composition.frameDuration = CMTime(value: 1, timescale: 30)
    composition.renderSize = videoSize
    composition.animationTool = AVVideoCompositionCoreAnimationTool(
        postProcessingAsVideoLayer: videoLayer, in: parentLayer
    )
    return composition
}

常见面试问题​

Q1: AVAssetExportSession 导出很慢怎么优化？​

答案：

  • 选择合适的 preset（不必总是 HighestQuality）
  • 只处理需要的时间范围
  • 大量操作时使用 AVAssetWriter 替代（更底层、更灵活）

相关链接​