objective-c - AVAssetReader/Writer は特定のデバイスでのみ動作しますか?

Question

このチュートリアルを使用して、音楽ライブラリから指定したディレクトリに曲を取得していました。私にとっては問題なく機能しますが、他の人はその曲が決してそこに到達しないと報告しています. 最初は端末の制限かと思ったのですが、どうやら私と同じ端末の人が困っているようです。彼らの問題を再現できないので、ちょっと面倒です。

これが私のコードです：

{
    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:");
}