我试图压缩/降低本地m4a或mp3文件的质量以使其更小。
最初，我使用AVAssetExportSession将AVAsset导出到临时目录，但我无法控制压缩/降低质量（您只能使用预设，其中只有.wav文件格式支持质量下降）。
然后，在Stack Overflow上遵循了几个示例，我尝试使用AVAssetReader / AVAssetWriter来执行此“导出”操作。
我创建我的读取器/编写器如下：
NSString *exportPath = [NSHomeDirectory() stringByAppendingPathComponent:@"out.m4a"];
NSURL *exportURL = [NSURL fileURLWithPath:outPath];
// reader
NSError *readerError = nil;
AVAssetReader *reader = [[AVAssetReader alloc] initWithAsset:asset
error:&readerError];
AVAssetTrack *track = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];
AVAssetReaderTrackOutput *readerOutput = [[AVAssetReaderTrackOutput alloc] initWithTrack:track
outputSettings:nil];
[reader addOutput:readerOutput];
// writer
NSError *writerError = nil;
AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:exportURL
fileType:AVFileTypeAppleM4A
error:&writerError];
AudioChannelLayout channelLayout;
memset(&channelLayout, 0, sizeof(AudioChannelLayout));
channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
// use different values to affect the downsampling/compression
NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
[NSNumber numberWithFloat:44100.0], AVSampleRateKey,
[NSNumber numberWithInt:2], AVNumberOfChannelsKey,
[NSNumber numberWithInt:128000], AVEncoderBitRateKey,
[NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey,
nil];
AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio
outputSettings:outputSettings];
[writerInput setExpectsMediaDataInRealTime:NO];
[writer addInput:writerInput];
然后我开始写作...
[writer startWriting];
[writer startSessionAtSourceTime:kCMTimeZero];
[reader startReading];
dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
[writerInput requestMediaDataWhenReadyOnQueue:mediaInputQueue usingBlock:^{
NSLog(@"Asset Writer ready : %d", writerInput.readyForMoreMediaData);
while (writerInput.readyForMoreMediaData) {
CMSampleBufferRef nextBuffer;
if ([reader status] == AVAssetReaderStatusReading && (nextBuffer = [readerOutput copyNextSampleBuffer])) {
if (nextBuffer) {
NSLog(@"Adding buffer");
[writerInput appendSampleBuffer:nextBuffer];
}
} else {
[writerInput markAsFinished];
switch ([reader status]) {
case AVAssetReaderStatusReading:
break;
case AVAssetReaderStatusFailed:
[writer cancelWriting];
break;
case AVAssetReaderStatusCompleted:
NSLog(@"Writer completed");
[writer endSessionAtSourceTime:asset.duration];
[writer finishWriting];
NSData *data = [NSData dataWithContentsOfFile:exportPath];
NSLog(@"Data: %@", data);
break;
}
break;
}
}
}];
我写完后，据说已经写入exportURL的数据是null，并且写入器报告成功完成。有什么想法可能出了什么问题吗？
更新 调用appendSampleBuffer:后，写入器状态为AVAssetWriterStatusFailed，尽管读取器状态成功，并且似乎可以读取整个文件。
[writerInput appendSampleBuffer：]时，写入器状态都为3，根据枚举值看起来像是AVAssetWriterStatusFailed。看起来读取器正在复制我的资产中的所有样本缓冲区 -appendSampleBuffer被调用了很多次。 - Dfowj