objective-c - AVAssetReader/Writer 只适用于某些设备？

Question

我正在使用 this从音乐库中获取歌曲到我指定目录的教程。它对我来说很好，但其他人报告说这首歌永远不会出现。起初我以为存在设备限制，但似乎与我使用相同设备的人遇到了问题。这有点烦人，因为我无法重现他们的问题。

这是我的代码:

{
    NSURL *assetURL = [song valueForProperty:MPMediaItemPropertyAssetURL];
    AVURLAsset *songAsset = [AVURLAsset URLAssetWithURL:assetURL options:nil];

    NSError *assetError = nil;
    AVAssetReader *assetReader = [[AVAssetReader assetReaderWithAsset:songAsset
                                                                error:&assetError]
                                  retain];
    if (assetError) {
        NSLog (@"error: %@", assetError);
        return;
    }

    AVAssetReaderOutput *assetReaderOutput = [[AVAssetReaderAudioMixOutput 
                                               assetReaderAudioMixOutputWithAudioTracks:songAsset.tracks
                                               audioSettings: nil]
                                              retain];
    if (! [assetReader canAddOutput: assetReaderOutput]) {
        NSLog (@"can't add reader output... die!");
        return;
    }
    [assetReader addOutput: assetReaderOutput];


    NSString *exportPath = @"Library/PreferenceBundles/MyAlarmSettings.bundle/export.m4a";
    if ([[NSFileManager defaultManager] fileExistsAtPath:exportPath]) {
        [[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];
    }

    NSURL *exportURL = [NSURL fileURLWithPath:exportPath];
    AVAssetWriter *assetWriter = [[AVAssetWriter assetWriterWithURL:exportURL
                                                           fileType:AVFileTypeAppleM4A
                                                              error:&assetError]
                                  retain];
    if (assetError) {
        NSLog (@"error: %@", assetError);
        return;
    }
    AudioChannelLayout channelLayout;
    memset(&channelLayout, 0, sizeof(AudioChannelLayout));
    channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

    NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [ NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                    [ NSNumber numberWithInt: 2 ], AVNumberOfChannelsKey,
                                    [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                    [ NSData dataWithBytes:&channelLayout length: sizeof( AudioChannelLayout ) ], AVChannelLayoutKey,
                                    [ NSNumber numberWithInt: 320000 ], AVEncoderBitRateKey,
                                    nil];                


    AVAssetWriterInput *assetWriterInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio
                                                                               outputSettings:outputSettings]
                                            retain];
    if ([assetWriter canAddInput:assetWriterInput]) {
        [assetWriter addInput:assetWriterInput];
    } else {
        NSLog (@"can't add asset writer input... die!");
        return;
    }

    assetWriterInput.expectsMediaDataInRealTime = NO;

    [assetWriter startWriting];
    [assetReader startReading];

    AVAssetTrack *soundTrack = [songAsset.tracks objectAtIndex:0];
    CMTime startTime = CMTimeMake (0, soundTrack.naturalTimeScale);
    [assetWriter startSessionAtSourceTime: startTime];

    __block UInt64 convertedByteCount = 0;

    dispatch_queue_t mediaInputQueue = dispatch_queue_create("mediaInputQueue", NULL);
    [assetWriterInput requestMediaDataWhenReadyOnQueue:mediaInputQueue 
                                            usingBlock: ^ 
     {
         // NSLog (@"top of block");
         while (assetWriterInput.readyForMoreMediaData) {
             CMSampleBufferRef nextBuffer = [assetReaderOutput copyNextSampleBuffer];
             if (nextBuffer) {
                 // append buffer
                 [assetWriterInput appendSampleBuffer: nextBuffer];
                 //             NSLog (@"appended a buffer (%d bytes)", 
                 //                    CMSampleBufferGetTotalSampleSize (nextBuffer));
                 convertedByteCount += CMSampleBufferGetTotalSampleSize (nextBuffer);
                 // oops, no
                 // sizeLabel.text = [NSString stringWithFormat: @"%ld bytes converted", convertedByteCount];

                 NSNumber *convertedByteCountNumber = [NSNumber numberWithLong:convertedByteCount];
                 [self performSelectorOnMainThread:@selector(updateSizeLabel:)
                                        withObject:convertedByteCountNumber
                                     waitUntilDone:NO];
             } else {
                 // done!

                 NSString *songtitle =  [[NSString alloc]initWithFormat:@"Alarm sound set to '%@' \n \n Press OK to respring. ",[song valueForProperty:MPMediaItemPropertyTitle]];

                 UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Done!" message:songtitle delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
                 [alert show];
                 [alert release];



                 [assetWriterInput markAsFinished];
                 [assetWriter finishWriting];
                 [assetReader cancelReading];
                 NSDictionary *outputFileAttributes = [[NSFileManager defaultManager]
                                                       attributesOfItemAtPath:exportPath
                                                       error:nil];
                 NSLog (@"done. file size is %llu",
                        [outputFileAttributes fileSize]);
                 NSNumber *doneFileSize = [NSNumber numberWithLong:[outputFileAttributes fileSize]];
                 [self performSelectorOnMainThread:@selector(updateCompletedSizeLabel:)
                                        withObject:doneFileSize
                                     waitUntilDone:NO];
                 // release a lot of stuff


                 [assetReader release];
                 [assetReaderOutput release];
                 [assetWriter release];
                 [assetWriterInput release];
                 [exportPath release];
                 break;
             }
         }

     }];

    NSLog (@"bottom of convertTapped:");
}

Answer 1

如果您从用户的 iPod 库访问 mediaItems，他们可能在云端。使用

[self.mediaItem valueForProperty:MPMediaItemPropertyIsCloudItem]

据我所知，您无法访问这些 Assets 或强制它们流式传输