AVFoundation系列一： AVAsset的使用方法

AVAsset 是AVFoundation的操作模型，其中包含音频，视频，字幕，元数据。

GPUImage系列专栏 参考：AVFoundation Programming Guide

AVFoundation系列五：关于音视频的导出 AVFoundation系列四：如何配置一个合格的Camera AVFoundation系列三：音视频编辑 AVFoundation系列二：用AVPlayer播放视频

本文将从以下几个方面介绍AVAsset

Demo 地址

1. AVAsset的加载方式 2. 播放一个AVAsset 3. 获取一个asset的相关属性 4. 从相册加载AVAsset 5. loadValuesAsynchronously的使用 6. 从视频中获取视频帧 图像 7. 通过一个AVAsset导出音频，设置时间裁剪 8. 通过一个AVAsset导出视频，设置时间裁剪

一、AVAsset的加载方式 第一种加载方式

let path = Bundle.main.path(forResource: "3", ofType: "mp4") let asset = AVAsset.init(url: URL.init(fileURLWithPath: path!, isDirectory: true))

第二种加载方式

let options = [AVURLAssetPreferPreciseDurationAndTimingKey:true] let asset = AVURLAsset.init(url: URL.init(fileURLWithPath: path!, isDirectory: true), options: options)

其中：AVURLAssetPreferPreciseDurationAndTimingKey 获取精确时间 通常不会再播放时使用 URL.init(fileURLWithPath: path!, isDirectory: true)如果URL是一个文件路径，苹果建议加上isDirectory

二、播放一个AVAsset 由于生命周期的原因，我们添加两个成员变量：

var player:AVPlayer? var playerItem:AVPlayerItem?

播放：

func plackTrack(track:AVAssetTrack?){ guard let asset = track?.asset else{ return } playerItem = AVPlayerItem.init(asset: asset) player = AVPlayer.init(playerItem: playerItem!) let playerlayer = AVPlayerLayer.init(player: player!) playerlayer.frame = view.bounds self.view.layer.addSublayer(playerlayer) player?.play() }

AVAssetTrack 如： let audioTrack = asset.tracks(withMediaType: .audio) let videoTrack = asset.tracks(withMediaType: .video)

三、获取一个asset的相关属性

func getAssetAttribute(){ let path = Bundle.main.path(forResource: "3", ofType: "mp4") let asset = AVAsset.init(url: URL.init(fileURLWithPath: path!, isDirectory: true)) //时长 let duration = asset.duration.seconds //歌词 let lyrics = asset.lyrics //创建时间 通常从相册加载时会有数据 let creatDate = asset.creationDate?.dataValue //相关信息 如 iso let metadata = asset.metadata(forFormat: .isoUserData) print(duration,lyrics ?? "",creatDate ?? "",metadata) }

四、从相册加载AVAsset 这里没有写相关代码，请参考PhotoKit

五、loadValuesAsynchronously的使用

func asyncLoadInfo(){ let path = Bundle.main.path(forResource: "3", ofType: "mp4") let asset = AVAsset.init(url: URL.init(fileURLWithPath: path!, isDirectory: true)) ///当一个asset加载时，它的部分属性是未知的，因此需要 asset来完成指定的加载 asset.loadValuesAsynchronously(forKeys: ["playable"]) { var error: NSError? let keyStatus = asset.statusOfValue(forKey: "playable", error: &error) } }

六、从视频中获取视频帧 图像

func getImageByAsset(){ let path = Bundle.main.path(forResource: "3", ofType: "mp4") let asset = AVAsset.init(url: URL.init(fileURLWithPath: path!, isDirectory: true)) if asset.tracks(withMediaType: .video).count > 0 { let imgGen = AVAssetImageGenerator.init(asset: asset) //最大尺寸 imgGen.maximumSize = CGSize.init(width: 100, height: 100) //光圈 imgGen.apertureMode = AVAssetImageGenerator.ApertureMode.cleanAperture //异步加载多个 imgGen.generateCGImagesAsynchronously(forTimes: [CMTime.zero as NSValue]) { (time1, cgimg, time2, result, error) in if let _cgimg = cgimg { let img = UIImage.init(cgImage: _cgimg) print("async get a img by asset") } } var actrueTime:CMTime = CMTime.zero //同步加载一个 if let cgimg = try? imgGen.copyCGImage(at: CMTime.zero, actualTime: &actrueTime){ let img = UIImage.init(cgImage: cgimg) print("sync get a img by asset") } } }

generateCGImagesAsynchronously可以一次性异步加载多个 videoComposition 与 appliesPreferredTrackTransform冲突，会导致彼此失效

七、通过一个AVAsset导出音频、视频，设置时间裁剪

func exportAsset(){ guard let path = Bundle.main.path(forResource: "3", ofType: "mp4") else{ return } let url = URL.init(fileURLWithPath: path) let asset = AVAsset.init(url: url) let videoTracks = asset.tracks(withMediaType: .video) let audioTracks = asset.tracks(withMediaType: .audio) let firstVideoTrack = videoTracks.first let firstAudioTrack = audioTracks.first //视频 let videocomposition = AVMutableComposition.init() if let compositionVideoTrack = videocomposition.addMutableTrack(withMediaType: .video, preferredTrackID: 0){ if firstVideoTrack != nil{ try? compositionVideoTrack.insertTimeRange(firstVideoTrack!.timeRange, of: firstVideoTrack!, at: .zero) videoExportSession(asset: videocomposition) } } //音频 let audiocomposition = AVMutableComposition.init() if let compositionAudioTrack = audiocomposition.addMutableTrack(withMediaType: .audio, preferredTrackID: 0){ if firstAudioTrack != nil{ try? compositionAudioTrack.insertTimeRange(firstAudioTrack!.timeRange, of: firstAudioTrack!, at: .zero) audioExportSession(asset: audiocomposition) } } }

可以调用 cancelExport 来取消 导出 audioExport?.cancelExport() AVMutableComposition 用于控制组件成分，本身是AVAsset的子类，意味着它可以像，AVAsset一样被用于播放。

//导出 func audioExportSession(asset:AVAsset){ let presetNames = AVAssetExportSession.exportPresets(compatibleWith: asset) if presetNames.contains(AVAssetExportPresetAppleM4A) { audioExport = AVAssetExportSession.init(asset: asset, presetName: AVAssetExportPresetAppleM4A) }else{ audioExport = AVAssetExportSession.init(asset: asset, presetName: AVAssetExportPresetPassthrough) } audioExport?.outputURL = getAudioExportURL() audioExport?.outputFileType = AVFileType.m4a audioExport?.shouldOptimizeForNetworkUse = true audioExport?.exportAsynchronously(completionHandler: {[weak self]in print(self?.audioExport?.error) }) } //获取url func getAudioExportURL()->URL{ let path = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first! let fileName = "Quinn_export" + ".m4a" let url = path.appendingPathComponent(fileName) if FileManager.default.fileExists(atPath: url.path){ try? FileManager.default.removeItem(at: url) } print("quinn",url) return url }

其中：videoExport、audioExport 为 AVAssetExportSession,在接下来的文章会介绍到。 presetNames 本视频支持导出的格式 AVAssetExportPresetPassthrough 为模拟器支持格式. 如果想要裁剪 设置相关参数 audioExport?.timeRange = getTimeRange(asset:asset) 时间裁剪方法：

//公共函数 //设置裁剪参数 func getTimeRange(asset:AVAsset)->CMTimeRange{ print(asset.duration.timescale) let start = CMTimeMake(value: Int64(asset.duration.timescale * 10), timescale: asset.duration.timescale) let end = CMTimeMake(value: Int64(asset.duration.timescale * 30), timescale: asset.duration.timescale) let range = CMTimeRange.init(start: start, end: end) return range }

同理 ，视频导出代码如下：

/// 视频相关 extension ViewController{ //导出 func videoExportSession(asset:AVAsset){ // presetNames 本视频支持导出的格式 AVAssetExportPresetPassthrough 为模拟器支持格式 let presetNames = AVAssetExportSession.exportPresets(compatibleWith: asset) //设置 AVAssetExportPreset640x480等可选参数，会导致视频压缩，但还需研究VideoTool,进一步做压缩处理 videoExport = AVAssetExportSession.init(asset: asset, presetName: presetNames.first ?? AVAssetExportPresetPassthrough) videoExport?.outputURL = getVideoExportURL() videoExport?.outputFileType = AVFileType.mp4 ///如果想要裁剪 设置相关参数 // videoExport?.timeRange = getTimeRange(asset:asset) videoExport?.exportAsynchronously(completionHandler: {[weak self]in print(self?.audioExport?.error) }) } //获取url func getVideoExportURL()->URL{ let path = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first! let fileName = "Quinn_export" + ".mp4" let url = path.appendingPathComponent(fileName) if FileManager.default.fileExists(atPath: url.path){ try? FileManager.default.removeItem(at: url) } print("quinn",url) return url } }