iphone - 使用 setVolumeRampFromStartVolume 淡入淡出保存音频在 iOS 中不起作用

Question

我正在尝试为 iPhone 项目剪切音频文件。我可以剪切并保存它，但我尝试应用的任何淡入/淡出都不起作用，音频文件只是保存剪切但没有褪色。

我正在使用以下代码:

//
// NO PROBLEMS TO SEE HERE, MOVE ON
//
    NSArray *documentsFolders = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    int currentFileNum = 10;
    NSURL *url = [NSURL fileURLWithPath: [[documentsFolders objectAtIndex:0] stringByAppendingPathComponent:[NSString stringWithFormat:@"%@%d.%@", AUDIO_SOURCE_FILE_NAME ,currentFileNum, AUDIO_SOURCE_FILE_EXTENSION ]]];
    NSDictionary *options = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES]
                                                        forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:url options:options];
    AVAssetExportSession* exporter = [AVAssetExportSession exportSessionWithAsset:asset presetName:AVAssetExportPresetAppleM4A];

    for (NSString* filetype in exporter.supportedFileTypes) {
        if ([filetype isEqualToString:AVFileTypeAppleM4A]) {
            exporter.outputFileType = AVFileTypeAppleM4A;
            break;
        }
    }
    if (exporter.outputFileType == nil) {
        NSLog(@"Needed output file type not found? (%@)", AVFileTypeAppleM4A);
        //return;
    }

    NSString* outPath = [[documentsFolders objectAtIndex:0] stringByAppendingPathComponent:[NSString stringWithFormat:@"%@%d.%@", AUDIO_CUTTED_FILE_NAME ,currentFileNum, AUDIO_SOURCE_FILE_EXTENSION ]];

    NSURL* const outUrl = [NSURL fileURLWithPath:outPath];
    exporter.outputURL = outUrl;

    float endTrimTime = CMTimeGetSeconds(asset.duration);
    float startTrimTime = fminf(AUDIO_DURATION, endTrimTime);
    CMTime startTrimCMTime=CMTimeSubtract(asset.duration, CMTimeMake(startTrimTime, 1));
    exporter.timeRange = CMTimeRangeMake(startTrimCMTime, asset.duration);

//
// TRYING TO APPLY FADEIN FADEOUT, NOT WORKING, NO RESULTS, "CODE IGNORED"
//
    AVMutableAudioMix *exportAudioMix = [AVMutableAudioMix audioMix];

    NSMutableArray* inputParameters = [NSMutableArray arrayWithCapacity:1];

    CMTime startFadeInTime = startTrimCMTime;
    CMTime endFadeInTime = CMTimeMake(startTrimTime+1, 1);
    CMTime startFadeOutTime = CMTimeMake(endTrimTime-1, 1);
    CMTime endFadeOutTime = CMTimeMake(endTrimTime, 1);

    CMTimeRange fadeInTimeRange = CMTimeRangeFromTimeToTime(startFadeInTime, endFadeInTime);

    CMTimeRange fadeOutTimeRange = CMTimeRangeFromTimeToTime(startFadeOutTime, endFadeOutTime);

    AVMutableAudioMixInputParameters *exportAudioMixInputParameters = [AVMutableAudioMixInputParameters audioMixInputParameters];
    [exportAudioMixInputParameters setVolume:0.0 atTime:CMTimeMakeWithSeconds(startTrimTime-0.01, 1)];
    [exportAudioMixInputParameters setVolumeRampFromStartVolume:0.0 toEndVolume:1.0 timeRange:fadeInTimeRange];
    [exportAudioMixInputParameters setVolumeRampFromStartVolume:1.0 toEndVolume:0.0 timeRange:fadeOutTimeRange];

    [inputParameters insertObject:exportAudioMixInputParameters atIndex:0];

    exportAudioMix.inputParameters = inputParameters;
    exporter.audioMix = exportAudioMix;

    [exporter exportAsynchronouslyWithCompletionHandler:^(void) {
        NSString* message;
        switch (exporter.status) {
            case AVAssetExportSessionStatusFailed:
                message = [NSString stringWithFormat:@"Export failed. Error: %@", exporter.error.description];
                [asset release];
                break;
            case AVAssetExportSessionStatusCompleted: {
                [asset release];
                [self reallyConvert:currentFileNum];
                message = [NSString stringWithFormat:@"Export completed: %@", outPath];
                break;
            }
            case AVAssetExportSessionStatusCancelled:
                message = [NSString stringWithFormat:@"Export cancelled!"];
                [asset release];
                break;
            default:
                NSLog(@"Export 4 unhandled status: %d", exporter.status);
                [asset release];
                break;
        }       
    }];

Answer 1

您需要选择轨道。而不是调用:

AVMutableAudioMixInputParameters *exportAudioMixInputParameters = [AVMutableAudioMixInputParameters audioMixInputParameters];

调用:

AVAssetTrack *assetTrack = [[asset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0];

AVMutableAudioMixInputParameters *exportAudioMixInputParameters = [AVMutableAudioMixInputParameters audioMixInputParametersWithTrack:assetTrack];

在您现有的代码中，您还可以像这样指定轨道:

exportAudioMixInputParameters.trackID = [[[asset tracksWithMediaType:AVMediaTypeAudio]objectAtIndex:0] trackID];

祝你好运!