iOS 添加背景音乐

    最近在家无聊至极，天天刷抖音。突然灵感上来，研究了一下抖音加背景音乐的技术。本文使用两段音频叠加成一段（不是前后合成一段，其实都差不多），类似于背景音乐；

    啥都不说了，先上代码！

- (void)audioComposition{
//1.获取本地音频素材
    NSString *audioPath1 = [[NSBundle mainBundle]pathForResource:@"1" ofType:@"m4a"];
    NSString *audioPath2 = [[NSBundle mainBundle]pathForResource:@"2" ofType:@"m4a"];
    AVURLAsset *audioAsset1 = [AVURLAsset assetWithURL:[NSURL fileURLWithPath:audioPath1]];
    AVURLAsset *audioAsset2 = [AVURLAsset assetWithURL:[NSURL fileURLWithPath:audioPath2]];
//2.创建两个音频轨道,并获取两个音频素材的轨道
    AVMutableComposition *composition = [AVMutableComposition composition];
    //音频轨道
    AVMutableCompositionTrack *audioTrack1 = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:0];
    AVMutableCompositionTrack *audioTrack2 = [composition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:0];
    //获取音频素材轨道
    AVAssetTrack *audioAssetTrack1 = [[audioAsset1 tracksWithMediaType:AVMediaTypeAudio] firstObject];
    AVAssetTrack *audioAssetTrack2 = [[audioAsset2 tracksWithMediaType:AVMediaTypeAudio] firstObject];
//3.将两段音频插入音轨文件,进行合并
    //音频合并- 插入音轨文件
    // `startTime`参数设置为‘kCMTimeZero’，表示合成在一块；
    // `startTime`参数设置为第一段音频的时长，即`audioAsset1.duration`, 表示将第二段音频插入到第一段音频的尾部。

    [audioTrack1 insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset1.duration) ofTrack:audioAssetTrack1 atTime:kCMTimeZero error:nil];
    [audioTrack2 insertTimeRange:CMTimeRangeMake(kCMTimeZero, audioAsset2.duration) ofTrack:audioAssetTrack2 atTime:kCMTimeZero error:nil];
//4. 导出合并后的音频文件
    //`presetName`要和之后的`session.outputFileType`相对应
    AVAssetExportSession *session = [[AVAssetExportSession alloc]initWithAsset:composition presetName:AVAssetExportPresetAppleM4A];
    
    NSString *outPutFilePath = [[self.exportUrl stringByDeletingLastPathComponent] stringByAppendingPathComponent:@"exportUrl.m4a"];
    
    if ([[NSFileManager defaultManager] fileExistsAtPath:outPutFilePath]) {
        [[NSFileManager defaultManager] removeItemAtPath:outPutFilePath error:nil];
    }
    // 查看当前session支持的fileType类型
    NSLog(@"---%@",[session supportedFileTypes]);
    NSLog(@"exportUrl:%@", self.exportUrl);
    session.outputURL = [NSURL fileURLWithPath:self.exportUrl];
    session.outputFileType = AVFileTypeAppleM4A; //与上述的`present`相对应
    session.shouldOptimizeForNetworkUse = YES;   //优化网络
    [session exportAsynchronouslyWithCompletionHandler:^{
        
        switch (session.status) {
            case AVAssetExportSessionStatusUnknown: {
                NSLog(@"AVAssetExportSessionStatusUnknown");
            }  break;
            case AVAssetExportSessionStatusWaiting: {
                NSLog(@"AVAssetExportSessionStatusWaiting");
            }  break;
            case AVAssetExportSessionStatusExporting: {
                NSLog(@"AVAssetExportSessionStatusExporting");
            }  break;
            case AVAssetExportSessionStatusCompleted: {
                NSLog(@"AVAssetExportSessionStatusCompleted");
                _audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:[NSURL fileURLWithPath:outPutFilePath] error:nil];
                [_audioPlayer play];
            }  break;
            case AVAssetExportSessionStatusFailed: {
                NSLog(@"AVAssetExportSessionStatusFailed");
            }  break;
            case AVAssetExportSessionStatusCancelled: {
                NSLog(@"AVAssetExportSessionStatusCancelled");

            }  break;
            default: break;
        }
        
    }];
    
}