试图获取视频缩略图

我试图从保存在Documents /文件夹中的视频中获取视频缩略图，并将其显示在UITableViewCell内的UIImageView中 . 这是我获取缩略图的功能：

- (UIImage*) thumbnailImageForVideo:(NSURL *)sourceURL
{
    AVAsset *asset = [AVAsset assetWithURL:sourceURL];
    AVAssetImageGenerator *imageGenerator = [[AVAssetImageGenerator alloc]initWithAsset:asset];
    NSError *err = NULL;
    CMTime time = CMTimeMake(1, 1);
    CGImageRef imageRef = [imageGenerator copyCGImageAtTime:time actualTime:NULL error:&err];
    NSLog(@"err==%@, imageRef==%@", err, imageRef);
    UIImage *thumbnail = [[UIImage alloc] initWithCGImage:imageRef];
    CGImageRelease(imageRef); // CGImageRef won't be released by ARC
    return thumbnail;
}

这是我使用它的地方：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    LibraryCell *libraryCell = [tableView dequeueReusableCellWithIdentifier:@"LibraryCell" forIndexPath:indexPath];

    NSString *videoPath = [NSString stringWithFormat:@"%@/%@", [NSHomeDirectory() stringByAppendingPathComponent:@"Documents"], [libraryFiles objectAtIndex:indexPath.item]];
    libraryCell.itemImage.image = [self thumbnailImageForVideo:[NSURL URLWithString:videoPath]];

    return libraryCell;
}

该应用程序不会崩溃，但没有图像 . 这是我在控制台中看到的：

2013-04-13 23:44:46.828 GeekOut[14433:907] err==Error Domain=NSURLErrorDomain Code=-1 "unknown error" UserInfo=0x1f0b4610 {NSUnderlyingError=0x1f0b3220 "The operation couldn’t be completed. (OSStatus error -12935.)", NSLocalizedDescription=unknown error}, imageRef==(null)
2013-04-13 23:44:46.859 GeekOut[14433:907] err==Error Domain=NSURLErrorDomain Code=-1 "unknown error" UserInfo=0x1dd8c660 {NSUnderlyingError=0x1dd8c5c0 "The operation couldn’t be completed. (OSStatus error -12935.)", NSLocalizedDescription=unknown error}, imageRef==(null)
2013-04-13 23:44:46.893 GeekOut[14433:907] err==Error Domain=NSURLErrorDomain Code=-1 "unknown error" UserInfo=0x1f0bec80 {NSUnderlyingError=0x1f0be9b0 "The operation couldn’t be completed. (OSStatus error -12935.)", NSLocalizedDescription=unknown error}, imageRef==(null)