iphone - __block 变量未保留

Question

我正在尝试将 block 内的结果分配给 block 变量。这是我的代码:

__block UIImage *latestImage;
ALAssetsLibrary *assetLibrary = [[ALAssetsLibrary alloc] init];

// Enumerate just the photos and videos group by using ALAssetsGroupSavedPhotos.
[assetLibrary   enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos 
                              usingBlock:^(ALAssetsGroup *group, BOOL *stop){
                                  // Within the group enumeration bl  ock, filter to enumerate just photos.
                                  [group setAssetsFilter:[ALAssetsFilter allPhotos]];

                                  // Chooses the photo at the last index
                                  [group  enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:([group numberOfAssets]-1)] 
                                                           options:0
                                                        usingBlock:^(ALAsset *alAsset, NSUInteger index, BOOL *innerStop){   
                                                            // the end of the enumeration is signaled by asset == nil.
                                                            if (alAsset){
                                                                ALAssetRepresentation *representation = [alAsset defaultRepresentation];
                                                                latestImage = [UIImage imageWithCGImage:[representation fullResolutionImage]];
                                                            }
                                                        }];
                              }
                            failureBlock: ^(NSError *error){
                                // handle error
                                NSLog(@"No groups");
                            }
 ];
return latestImage;

我通过在 block 内的 UIImageView 上显示它来确认变量 latestImage 已设置在 block 内。但是，当我尝试如上面的代码所示返回该对象时，它什么也没有返回。

我错过了什么？

谢谢。

Answer 1

enumerateGroupsWithTypes:usingBlock:failureBlock: 是异步的。您在 block 有机会设置变量之前返回 latestImage。来自 ALAssestsLibrary 的文档:

Many of the methods declared by ALAssetsLibrary take a failure block as an argument. These methods are all asynchronous, because the user may need to be asked to grant access to the data.

您可以将整个函数包装在 dispatch_sync() 中，或者使用信号量来等待答案。无论哪种方式，请确保您没有在主线程上执行此操作。