將參數發送到主線程從異步塊時遇到問題

Question

我有以下方法，它捕獲照片後從UIImagePicker檢索ALAsset。然後，它試圖通過主線程發送此ALAsset到另外一個我的方法：

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info 
{ 
    NSURL *imageURL = [[info valueForKey:UIImagePickerControllerReferenceURL] retain]; 
    __block ALAsset *result; 

    [self.assetsLibrary assetForURL:imageURL resultBlock:^(ALAsset *asset) 
    { 
     result = [asset retain]; 

     dispatch_async(dispatch_get_main_queue(),^
     { 
      [self loadPhotoImageViewWithAsset:result]; 
      [self dismissModalViewControllerAnimated:YES]; 
      [imageURL release]; 
      [result release]; 
     }); 
    } 
    failureBlock:^(NSError *error) 
    { 

    }]; 
} 

當我進入dispatch_async(dispatch_get_main_queue(),^block，result被顯示爲nil。任何人都知道我在這裏做錯了什麼？

Answer 1

請參閱UIImagePickerController to use for a UIImageView結果顯示爲零。

使用這樣的：

[self.assetsLibrary assetForURL:imageURL resultBlock:^(ALAsset *asset) 
{ 
    result = [asset retain]; 
    [self loadPhotoImageViewWithAsset:result]; 
    [self dismissModalViewControllerAnimated:YES]; 
    [imageURL release]; 
    [result release]; 
} 
failureBlock:^(NSError *error) 
{ 

}]; 

更改loadPhotoImageViewWithAsset方法

-(void)loadPhotoImageViewWithAsset:(ALAsset *)asset 
{ 
    //dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul); 
    //dispatch_async(queue, ^{ 
    dispatch_async(dispatch_get_main_queue(),^
    { 
    //here code for loading image 
    }); 
    // }); 
}