AVAssetExportSession進度卡在ipad上，但不在模擬器上

Question

這段代碼在模擬器上正常工作。但是，當我嘗試在iPad上運行導出時，它始終掛起進度值0.14583-ish。有人可以幫我弄清楚爲什麼？已經停留了很長時間。

這裏是我的代碼：

NSArray *compatiblePresets = [AVAssetExportSession exportPresetsCompatibleWithAsset:composition]; 
if ([compatiblePresets containsObject:AVAssetExportPresetLowQuality]) { 
    AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] 
              initWithAsset:composition presetName:AVAssetExportPresetLowQuality]; 


    exportSession.outputURL = [NSURL fileURLWithPath:[[ShowDAO getUserDocumentDir] stringByAppendingString:exportFilename]]; 
    exportSession.outputFileType = AVFileTypeQuickTimeMovie; 

    CMTime start = CMTimeMakeWithSeconds(0, 1); 
    CMTime duration = CMTimeMakeWithSeconds(1000, 1); 
    CMTimeRange range = CMTimeRangeMake(start, duration); 
    exportSession.timeRange = range; 

    [exportSession exportAsynchronouslyWithCompletionHandler:^{ 
     switch ([exportSession status]) { 
      case AVAssetExportSessionStatusCompleted: 
       NSLog(@"Export Completed"); 
       break; 
      case AVAssetExportSessionStatusFailed: 
       NSLog(@"Export failed: %@", [[exportSession error] localizedDescription]); 
       break; 
      case AVAssetExportSessionStatusCancelled: 
       NSLog(@"Export cancelled"); 
       break; 
      default: 
       break; 
     } 


    }]; 

    while(exportSession.progress != 1.0){ 
     NSLog(@"loading... : %f",exportSession.progress); 
     sleep(1); 
    } 
    [exportSession release]; 

} 

Answer 1

while(exportSession.progress != 1.0){ 
    NSLog(@"loading... : %f",exportSession.progress); 
    sleep(1); 
} 

這while循環阻塞主線程。 NSLog可能無法正常啓動。嘗試沒有while循環？