需求: 替换某个视频中的音频
基于 AVFoundation

1.获取素材的地址

   // 创建素材地址
    NSURL *audioUrl = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"audio.mp3" ofType:nil]];
    NSURL *videoUrl = [NSURL fileURLWithPath:[[NSBundle mainBundle] pathForResource:@"video.mp4" ofType:nil]];
 
    AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:audioUrl options:nil];
    AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:videoUrl options:nil];

2.创建一个合成视频的工程文件

    AVMutableComposition* composition = [AVMutableComposition composition];

3.创建轨道 并插入素材

    // 创建音频轨道
    AVMutableCompositionTrack *compositionAudioTrack = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    //在视频轨道插入一个时间段的视频
    [compositionAudioTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset.duration) ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:kCMTimeZero error:nil];
    
    // 创建视频轨道
    AVMutableCompositionTrack *compositionVideoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    //在视频轨道插入一个时间段的音频
    [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration) ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:kCMTimeZero error:nil];

4.导出合成的视频文件

    // 保存地址
    NSString* videoName = @"export.mov";
    NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent:videoName];
    NSURL    *exportUrl = [NSURL fileURLWithPath:exportPath];
    
    if ([[NSFileManager defaultManager] fileExistsAtPath:exportPath])
    {
        [[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];
    }
    
    //导出视频
    AVAssetExportSession* assetExport = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetPassthrough];
    NSLog(@"%@",exportPath);
    assetExport.outputFileType = AVFileTypeQuickTimeMovie;
    NSLog(@"file type %@",assetExport.outputFileType);
    assetExport.outputURL = exportUrl;
    assetExport.shouldOptimizeForNetworkUse = YES;
    
    [assetExport exportAsynchronouslyWithCompletionHandler:
     ^(void ) {
         NSLog(@"status : %ld",assetExport.status);
     }];

这样合成会有个问题,就是如果视频和音频的时长不一 会取更长的时长
后期有时间会补上 视频的一些其他操作方法