两个短视频拼接 核心代码

- (void)buildTransitionComposition:(AVMutableComposition *)composition andVideoComposition:(AVMutableVideoComposition *)videoComposition andAudioMix:(AVMutableAudioMix *)audioMix
{
    CMTime nextClipStartTime = kCMTimeZero;
    NSInteger i;
    NSUInteger clipsCount = [self.clips count];
    
    // 使过渡持续时间不大于最短剪辑持续时间的一半.
    CMTime transitionDuration = self.transitionDuration;
    for (i = 0; i < clipsCount; i++ ) {
        NSValue *clipTimeRange = [self.clipTimeRanges objectAtIndex:i];
        if (clipTimeRange) {
            CMTime halfClipDuration = [clipTimeRange CMTimeRangeValue].duration;
            halfClipDuration.timescale *= 2; // You can halve a rational by doubling its denominator.
            transitionDuration = CMTimeMinimum(transitionDuration, halfClipDuration);
            
        }
    }
    
    // 添加两个视频轨道和两个音频轨道.
    AVMutableCompositionTrack *compositionVideoTracks[2];
    AVMutableCompositionTrack *compositionAudioTracks[2];
    compositionVideoTracks[0] = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionVideoTracks[1] = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionAudioTracks[0] = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    compositionAudioTracks[1] = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    
    CMTimeRange *passThroughTimeRanges = alloca(sizeof(CMTimeRange) * clipsCount);
    CMTimeRange *transitionTimeRanges = alloca(sizeof(CMTimeRange) * clipsCount);
    
    // 放置剪辑到交替的视频和音频轨道在合成，重叠过渡持续时间.
    for (i = 0; i < clipsCount; i++ ) {
        NSInteger alternatingIndex = i % 2; // alternating targets: 0, 1, 0, 1, ...
        AVURLAsset *asset = [self.clips objectAtIndex:i];
        NSValue *clipTimeRange = [self.clipTimeRanges objectAtIndex:i];
        CMTimeRange timeRangeInAsset;
        if (clipTimeRange)
            timeRangeInAsset = [clipTimeRange CMTimeRangeValue];
        else
            timeRangeInAsset = CMTimeRangeMake(kCMTimeZero, [asset duration]);
        
        
        NSLog(@"<<<<<<<===:%@", [NSValue valueWithCMTimeRange: timeRangeInAsset]);
        
        AVAssetTrack *clipVideoTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
        [compositionVideoTracks[alternatingIndex] insertTimeRange:timeRangeInAsset ofTrack:clipVideoTrack atTime:nextClipStartTime error:nil];
        
        AVAssetTrack *clipAudioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
        [compositionAudioTracks[alternatingIndex] insertTimeRange:timeRangeInAsset ofTrack:clipAudioTrack atTime:nextClipStartTime error:nil];
        
        //记住这个剪辑应该经过的时间范围。
             // Second剪辑以一个过渡开始。
               //第一个剪辑以一个过渡结束。
             //从时间范围中排除这些转换。
        passThroughTimeRanges[i] = CMTimeRangeMake(nextClipStartTime, timeRangeInAsset.duration);
        if (i > 0) {
            passThroughTimeRanges[i].start = CMTimeAdd(passThroughTimeRanges[i].start, transitionDuration);
            passThroughTimeRanges[i].duration = CMTimeSubtract(passThroughTimeRanges[i].duration, transitionDuration);
        }
        if (i+1 < clipsCount) {
            passThroughTimeRanges[i].duration = CMTimeSubtract(passThroughTimeRanges[i].duration, transitionDuration);
        }
        
        // 这个片段的结尾将通过过渡持续时间与下一个片段的开始重叠。.
        // (注意: this arithmetic falls apart if timeRangeInAsset.duration < 2 * transitionDuration.)
        nextClipStartTime = CMTimeAdd(nextClipStartTime, timeRangeInAsset.duration);
        nextClipStartTime = CMTimeSubtract(nextClipStartTime, transitionDuration);
        
        // 记住过渡到下一项的时间范围.
        if (i+1 < clipsCount) {
            transitionTimeRanges[i] = CMTimeRangeMake(nextClipStartTime, transitionDuration);
        }
    }
    
    // 设置视频合成，如果我们要在剪辑之间进行交叉渐变转换.
    NSMutableArray *instructions = [NSMutableArray array];
    NSMutableArray *trackMixArray = [NSMutableArray array];
    
    // Cycle between "pass through A", "transition from A to B", "pass through B"
    for (i = 0; i < clipsCount; i++ ) {
        NSInteger alternatingIndex = i % 2; // alternating targets
        
        // Pass through clip i.
        AVMutableVideoCompositionInstruction *passThroughInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
        passThroughInstruction.timeRange = passThroughTimeRanges[i];
        AVMutableVideoCompositionLayerInstruction *passThroughLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTracks[alternatingIndex]];
        
        passThroughInstruction.layerInstructions = [NSArray arrayWithObject:passThroughLayer];
        [instructions addObject:passThroughInstruction];
        
        if (i+1 < clipsCount) {
            AVMutableVideoCompositionInstruction *transitionInstruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
            transitionInstruction.timeRange = transitionTimeRanges[i];
            AVMutableVideoCompositionLayerInstruction *fromLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTracks[alternatingIndex]];
            AVMutableVideoCompositionLayerInstruction *toLayer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:compositionVideoTracks[1-alternatingIndex]];
            // 通过设置渐变从0.0到1.0来淡入图层.
            [toLayer setOpacityRampFromStartOpacity:0.0 toEndOpacity:1.0 timeRange:transitionTimeRanges[i]];
            
            transitionInstruction.layerInstructions = [NSArray arrayWithObjects:toLayer, fromLayer, nil];
            [instructions addObject:transitionInstruction];
            
            // 添加AudioMix来减弱音量
            AVMutableAudioMixInputParameters *trackMix1 = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:compositionAudioTracks[0]];
            
            [trackMix1 setVolumeRampFromStartVolume:1.0 toEndVolume:0.0 timeRange:transitionTimeRanges[0]];
            
            [trackMixArray addObject:trackMix1];
            
            AVMutableAudioMixInputParameters *trackMix2 = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:compositionAudioTracks[1]];
            
            [trackMix2 setVolumeRampFromStartVolume:0.0 toEndVolume:1.0 timeRange:transitionTimeRanges[0]];
            [trackMix2 setVolumeRampFromStartVolume:1.0 toEndVolume:1.0 timeRange:passThroughTimeRanges[1]];
            
            [trackMixArray addObject:trackMix2];
        }
        
    }
    
    audioMix.inputParameters = trackMixArray;
    videoComposition.instructions = instructions;
}

具体实现参照 官方dome https://developer.apple.com/library/archive/samplecode/AVCompositionDebugVieweriOS/Introduction/Intro.html
各种专场效果 https://www.jianshu.com/p/d22682830130