引言

在上一篇博客中，我們已經(jīng)介紹了創(chuàng)建視頻過(guò)渡的實(shí)現(xiàn)方案，步驟非常繁瑣，在生成AVMutableVideoCompositionInstruction和AVMutableVideoCompositionLayerInstruction的計(jì)算也十分復(fù)雜，但其實(shí)還有一個(gè)創(chuàng)建視頻組合的捷徑。不過(guò)我們還是需要理解上一篇博客中我們所討論的步驟，只有理解了那些步驟，才能發(fā)現(xiàn)學(xué)習(xí)這些對(duì)應(yīng)的使用變得容易許多。

創(chuàng)建組合的捷徑

AVVideoComposition定義了一個(gè)十分便捷的初始化方法init(propertiesOf asset: AVAsset)，我們可以將AVCompostion作為參數(shù)來(lái)創(chuàng)建一個(gè)AVVideoComposition。該方法會(huì)為我們創(chuàng)建一個(gè)帶有如下配置的AVVideoComposition對(duì)象：

• instructions ?屬性包含一組完整的基于組合視頻軌道（以及其中包含的片段空間布局）的組合和層指令。
• renderSize ?屬性被設(shè)置為AVComposition對(duì)象的naturalSize，或者如果沒(méi)有設(shè)置，則使用能夠滿足組合視頻軌道中最大視頻維度的尺寸值。
• frameDuration ?設(shè)置為組合視頻軌道中最大nominalFrameRate的值。如果所有軌道的nominalFrameRate值都為0，則frameDuration設(shè)置成默認(rèn)1/30秒（30FPS）。
• renderScale ?始終設(shè)置為1.0。

創(chuàng)建視頻過(guò)渡

下面我們開(kāi)始著手創(chuàng)建視頻過(guò)渡，和之前的實(shí)現(xiàn)方案一樣，先創(chuàng)建一個(gè)遵循PHComposition協(xié)議名為PHTransitionComposition的類，以及遵循PHCompositionBuilder協(xié)議名為PHTransitionCompositionBuilder的類。

PHTransitionComposition負(fù)責(zé)構(gòu)建視頻的可播放和可導(dǎo)出版本。

PHTransitionCompositionBuilder負(fù)責(zé)構(gòu)建PHTransitionComposition，里面會(huì)創(chuàng)建用于視頻編輯的AVMutableComposition，AVMutableAudioMix以及AVMutableVideoComposition。

PHTransitionComposition

代碼實(shí)現(xiàn)如下：

import UIKit
import AVFoundationclass PHTransitionComposition: NSObject,PHComposition {/// 組合軌道var composition:AVMutableComposition!/// 視頻軌道var videoComposition:AVMutableVideoComposition?/// 音頻混合var audioMix:AVMutableAudioMix?init(composition: AVMutableComposition!, videoComposition: AVMutableVideoComposition?, audioMix: AVMutableAudioMix?) {self.composition = compositionself.videoComposition = videoCompositionself.audioMix = audioMix}func makePlayerItem() -> AVPlayerItem? {let playerItem = AVPlayerItem(asset: composition.copy() as! AVAsset)playerItem.videoComposition = videoCompositionplayerItem.audioMix = audioMixreturn playerItem}func makeAssetExportSession() -> AVAssetExportSession? {let exportSession = AVAssetExportSession(asset: composition.copy() as! AVAsset, presetName: AVAssetExportPresetHighestQuality)exportSession?.videoComposition = videoCompositionexportSession?.audioMix = audioMixreturn exportSession!}}

代碼較以前的類相比多了一個(gè)AVMutableVideoComposition屬性，用來(lái)實(shí)現(xiàn)視頻的過(guò)渡效果。

PHTransitionCompositionBuilder

該類中的代碼和以往一樣主要目的是構(gòu)建PHComposition，分成三個(gè)部分，添加視頻到組合軌道，添加音頻到組合軌道，添加背景音樂(lè)到組合軌道。

let defaultTransitionDuration = CMTime(value: 2, timescale: 1)class PHTransitionCompositionBuilder: NSObject,PHCompositionBuilder {/// 資源模型var timeLine:PHTimeLine!/// 組合軌道let composition = AVMutableComposition()init(timeLine: PHTimeLine!) {self.timeLine = timeLine}func buildComposition() -> PHComposition? {// 添加視頻到組合軌道addVideoCompositionTrack()// 創(chuàng)建AVVideoCompositionlet videoComposition = buildVideoComposition()// 添加音頻到組合軌道addAudioCompositionTrack()// 添加背景音樂(lè)到組合軌道let audioMix = addMusicCompositionTrack()return PHTransitionComposition(composition: composition, videoComposition: videoComposition, audioMix: audioMix)}/// 添加視頻到組合軌道func addVideoCompositionTrack() {....}/// 創(chuàng)建AVVideoCompositionfunc buildVideoComposition() -> AVMutableVideoComposition? {....}/// 添加音頻到組合軌道func addAudioCompositionTrack() {let _ = addCompositionTrack(mediaType: .audio, mediaItems: timeLine.audioItems)}/// 添加背景音樂(lè)到組合軌道func addMusicCompositionTrack() -> AVMutableAudioMix?{// 添加背景音樂(lè)var audioMix:AVMutableAudioMix? = nilif timeLine.musicItem != nil {let musicCompositionTrack =  addCompositionTrack(mediaType: .audio, mediaItems: [timeLine.musicItem!])let musicAudioMix = buildAudioMixWithTrack(track: musicCompositionTrack)audioMix = musicAudioMix}return audioMix}/// 私有方法-添加媒體資源軌道/// - Parameters:///  - mediaType: 媒體類型///  - mediaItems: 媒體媒體資源數(shù)組///  - Returns: 返回一個(gè)AVCompositionTrackprivate func addCompositionTrack(mediaType:AVMediaType,mediaItems:[PHMediaItem]?) -> AVMutableCompositionTrack? {if PHIsEmpty(array: mediaItems) {return nil}let trackID = kCMPersistentTrackID_Invalidguard let compositionTrack = composition.addMutableTrack(withMediaType: mediaType, preferredTrackID: trackID) else { return nil }//設(shè)置起始時(shí)間var cursorTime = CMTime.zeroguard let mediaItems = mediaItems else { return nil }for item in mediaItems {//這里默認(rèn)時(shí)間都是從0開(kāi)始guard let asset = item.asset else { continue }guard let assetTrack = asset.tracks(withMediaType: mediaType).first  else { continue }do {try compositionTrack.insertTimeRange(item.timeRange, of: assetTrack, at: cursorTime)} catch {print("addCompositionTrack error")}cursorTime = CMTimeAdd(cursorTime, item.timeRange.duration)}return compositionTrack}/// 創(chuàng)建音頻混合器/// - Parameters:///  - musicTrack: 音樂(lè)軌道func buildAudioMixWithTrack(track:AVMutableCompositionTrack?) -> AVMutableAudioMix? {guard let track = track else { return nil }guard let musicItem = timeLine.musicItem else { return nil }let audioMix = AVMutableAudioMix()let audioMixParam = AVMutableAudioMixInputParameters(track: track)for volumeAutomaition in musicItem.volumeAutomations {audioMixParam.setVolumeRamp(fromStartVolume: volumeAutomaition.startVolume, toEndVolume: volumeAutomaition.endVolume, timeRange: volumeAutomaition.timeRange)}audioMix.inputParameters = [audioMixParam]return audioMix}}

關(guān)于添加音頻和背景音樂(lè)以及創(chuàng)建AVAudioMix的部分，我們?cè)谇懊娴牟┛椭幸呀?jīng)進(jìn)行過(guò)介紹，在這里就不再重復(fù)解釋了，把重點(diǎn)放到添加視頻到組合軌道以及創(chuàng)建AVVideoComposition上。

下面看一下添加視頻到組合軌道的實(shí)現(xiàn)：

    /// 添加視頻到組合軌道func addVideoCompositionTrack() {let compositionTrackA = self.composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)let compositionTrackB = self.composition.addMutableTrack(withMediaType: .video, preferredTrackID: kCMPersistentTrackID_Invalid)let videoTracks = [compositionTrackA,compositionTrackB]let transitionDuration = defaultTransitionDuration// 初始時(shí)間var cursorTime = CMTime.zerolet videoItems = timeLine.videoItmesfor (index,item) in videoItems.enumerated() {let trackIndex = index % 2let currentTrack = videoTracks[trackIndex]guard let asset = item.asset else { continue }guard let assetTrack = asset.tracks(withMediaType: .video).first else { continue }do {try currentTrack?.insertTimeRange(item.timeRange, of: assetTrack, at: cursorTime) } catch {print("insertTimeRange error")}// 更新cursorTimecursorTime = CMTimeAdd(cursorTime, item.timeRange.duration)cursorTime = CMTimeSubtract(cursorTime, transitionDuration)}}

1. 首先從當(dāng)前的AVMutableComposition中創(chuàng)建兩個(gè)新的AVMutableCompositionTrack對(duì)象，兩者都是.video類型，并添加到videoTracks數(shù)組中，提供所需的A-B軌道排列。
2. 遍歷視頻資源，將視頻資源交替插入到AB兩個(gè)軌道中。
3. 計(jì)算cursorTime，需要考慮到減去動(dòng)畫過(guò)程的時(shí)間，因?yàn)檫@段時(shí)間兩個(gè)視頻媒體是重疊的。

創(chuàng)建AVVideoComposition的實(shí)現(xiàn)如下：

    /// 創(chuàng)建AVVideoCompositionfunc buildVideoComposition() -> AVMutableVideoComposition? {let videoComposition = AVMutableVideoComposition(propertiesOf: self.composition)videoComposition.renderSize = CGSize(width: 1920, height: 1080)/// 獲取instructionslet instructions = videoComposition.instructionsvar layerInstructionIndex = 1for instruction in instructions {guard let videoCompositionInstruction = instruction as? AVMutableVideoCompositionInstruction else { continue }let layerInstructions = videoCompositionInstruction.layerInstructions// 判斷是否有兩個(gè)layerInstructionsguard layerInstructions.count == 2 else { continue }// 創(chuàng)建過(guò)渡效果let fromeLayerInstruction = layerInstructions[1 - layerInstructionIndex] as! AVMutableVideoCompositionLayerInstructionlet toLayerInstruction = layerInstructions[layerInstructionIndex] as! AVMutableVideoCompositionLayerInstruction// 設(shè)置動(dòng)畫fromeLayerInstruction.setOpacityRamp(fromStartOpacity: 1.0, toEndOpacity: 0.0, timeRange: videoCompositionInstruction.timeRange)layerInstructionIndex = layerInstructionIndex == 1 ? 0 : 1}return videoComposition}

1. 使用init(propertiesOf asset: AVAsset)創(chuàng)建一個(gè)新的AVVideoComposition實(shí)例，這個(gè)方法會(huì)自動(dòng)創(chuàng)建所需的組合對(duì)象和層指令。并設(shè)置renderSize、renderScale、frameDuration熟悉為相應(yīng)的值。（renderSize需要滿足組合視頻軌道中最大視頻維度的尺寸值）
2. 遍歷videoComposition的instructions屬性，判斷videoCompositionInstruction的layerInstructions個(gè)數(shù)等于2的情況提取重疊的兩個(gè)AVMutableVideoCompositionLayerInstruction。
3. 為兩個(gè)AVMutableVideoCompositionLayerInstruction添加一個(gè)漸變的過(guò)渡動(dòng)畫。
4. 返回AVVideoComposition實(shí)例。

除了上面列出的最簡(jiǎn)單的漸隱過(guò)渡方式，還支持很多其它的過(guò)渡方式。

推入過(guò)渡效果：

            // 推入過(guò)渡效果let identityTransform = CGAffineTransform.identitylet videoWidth = videoComposition.renderSize.widthlet fromeTransform = CGAffineTransform(translationX: -videoWidth, y: 0)let toTransform = CGAffineTransform(translationX: videoWidth, y: 0)fromeLayerInstruction.setTransformRamp(fromStart: identityTransform, toEnd: fromeTransform, timeRange: videoCompositionInstruction.timeRange)toLayerInstruction.setTransformRamp(fromStart: toTransform, toEnd: identityTransform, timeRange: videoCompositionInstruction.timeRange)

擦除過(guò)渡效果：

            // 擦除過(guò)渡效果let width = videoComposition.renderSize.widthlet height = videoComposition.renderSize.heightlet startRect = CGRect(x: 0, y: 0, width: width, height: height)let endRect = CGRect(x: width, y: height, width: width, height: 0.0)fromeLayerInstruction.setCropRectangleRamp(fromStartCropRectangle: startRect, toEndCropRectangle: endRect, timeRange: videoCompositionInstruction.timeRange)

播放

創(chuàng)建PHTransitionCompositionBuilder實(shí)例，構(gòu)建視頻的可播放版本，進(jìn)行播放。

 func player() {guard let delegate = self.delegate else { return }let compositionBuilder = PHTransitionCompositionBuilder(timeLine: timeLine)let composition = compositionBuilder.buildComposition()let playerItem = composition?.makePlayerItem()delegate.replaceCurrentItem(playerItem: playerItem)}

結(jié)語(yǔ)

代碼中我們把工作的重點(diǎn)集中到了A-B軌道的方式交替添加視頻資源，以及構(gòu)建AVMutableVideoComposition并創(chuàng)建過(guò)渡效果上面。但事實(shí)上我們還需要注意到播放進(jìn)度的顯示，創(chuàng)建過(guò)渡后兩個(gè)視頻之間會(huì)有重疊部分，在呈現(xiàn)的時(shí)候需要減去重疊時(shí)間。

需要注意其它軌道的長(zhǎng)度需要小于視頻組合軌道的軌道長(zhǎng)度。

另外最需要注意的是AVMutableVideoComposition的renderSize熟悉，一定要使用能夠滿足組合視頻軌道中最大視頻維度的尺寸值。