计算从ios照片库中选取的图片文件大小

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从 iphone 的 照片库中选取的图片，由于 系统不能返回其文件的具体路径，所以这时要用到 ALAssetsLibrary

代码如下：

     ALAssetsLibrary* alLibrary = [[ALAssetsLibrary alloc] init];
    __block float fileMB  = 0.0;
    
    [alLibrary assetForURL:[info objectForKey:UIImagePickerControllerReferenceURL] resultBlock:^(ALAsset *asset)
     {
         ALAssetRepresentation *representation = [asset defaultRepresentation];
         fileMB = (float)([representation size]/perMBBytes);
         NSLog(@"size of asset in bytes: %0.2f", fileMB);
         iHasAsset = 1;
         [alLibrary release];
     }
              failureBlock:^(NSError *error)
     {
         iHasAsset = 2;
         [alLibrary release];
     }];