從 iphone 的 照片庫中選取的圖片，由於 系統不能返回其檔案的具體路徑，所以這時要用到 ALAssetsLibrary

程式碼如下：

     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];
     }];