Ios AVAssetExportSession进程在ipad上卡住了，但在模拟器上没有

这段代码在模拟器上运行良好。然而，当我尝试在iPad上运行导出时，它总是挂起在进度值0.14583-ish。有人能帮我找出原因吗？这件事已经困扰了好一阵子了

这是我的密码：

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];

}

这个while循环阻塞了主线程。NSLog可能无法正常启动。不用while循环就试试吧？

这就是他为什么要睡觉（1）。我更喜欢[NSThread sleepForTimeInterval:1.0]；

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