Question

我有一个值，我正在从一个块中检索，我想在下面的方法中返回。由于看起来块是异步的，我该如何实现呢？

-(UIImage*) imageAtIndex:(NSUInteger)index
{
    UIImage *image;
    [self.album enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:index] options:0 usingBlock: ^(ALAsset *result, NSUInteger index, BOOL *stop)
     {
            //set image in here
     }];

    return image;
}

Answer 1

过去我这样做了，检查一下。

-(void) imageAtIndex:(NSUInteger)index //since block is async you may not be able to return the image from this method
{

    UIImage *image;
    [self.album enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:index] options:0 usingBlock: ^(ALAsset *result, NSUInteger index, BOOL *stop)
     {
         //set image in here
          dispatch_async(dispatch_get_main_queue(), ^{ //if UI operations are done using this image, it is better to be in the main queue, or else you may not need the main queue.
             [self passImage:image]; //do the rest of the things in `passImage:` method
          });
     }];
}

Answer 2

如果您的块是异步的，则在返回之前无法设置返回的值，因为程序在方法结束之前可能尚未完成异步任务。一些更好的解决方案是：

如果您愿意，可以找一个同样的方法来完成同样的工作现在这可能不是最好的选择，因为你的UI会在块运行时锁定。
找到值时调用选择器，并使方法本身无效。

偶然，尝试这样的事情：

-(void) findImageAtIndex:(NSUInteger)index target:(id)target foundSelector:(SEL)foundSEL
{
    __block UIImage *image;
    [self.album enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:index] options:0 usingBlock: ^(ALAsset *result, NSUInteger index, BOOL *stop)
    {
        //set image in here
        if ([target respondsToSelector:foundSEL]) { //If the object we were given can call the given selector, call it here
            [target performSelector:foundSEL withObject:image];
            return;
        }
    }];
    //Don't return a value
}

然后你可以这样称呼它：

    ...
    //Move all your post-finding code to bar:
    [self findImageAtIndex:foo target:self foundSelector:@selector(bar:)];
}

- (void)bar:(UIImage *)foundImage {
     //Continue where you left off earlier
     ...
}

返回GCD块中设置的值

2 个答案: