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请参阅此代码示例：

dispatch_queue_t downloadQueue=dispatch_queue_create("test", NULL);

        dispatch_async(downloadQueue, ^{

            //do some core data fetching stuff
            //create UIImage            

            //Need to go back to the main thread since this is UI related
            dispatch_async(dispatch_get_main_queue(), ^{

                    if(someView)
                    {
                        [someView addSubview:self];
                    }
                }

            });
        });
        dispatch_release(downloadQueue);

如果在这段代码的主线程部分运行时“someView”不存在怎么办？显然这会导致 EXC_BAD_ACCESS 崩溃，但我想知道处理这种情况的最佳方法是什么。它最常发生在用户导航到需要几秒钟才能加载的页面，但随后决定在当前页面仍在加载时返回上一页。

我可以想象在尝试添加子视图之前要检查某种全局标志，但这似乎很老套。

4

2 回答 2

0 
// Capture a strong reference to someView, to make sure it's still around later on.
__block UIView *v = someView;

//Need to go back to the main thread since this is UI related
dispatch_async(dispatch_get_main_queue(), ^{
   [v addSubview:self]
   // Release v (assuming ARC). If nothing else has retained someView, this means
   // that someView will be deallocated - but we're on the main thread, 
   // so that's fine. What we don't want is a background dealloc.
   v = nil;
});

或者

// Capture a strong reference to someView, to make sure it's still around later on.
__weak UIView *weakRef = someView;


//Need to go back to the main thread since this is UI related
dispatch_async(dispatch_get_main_queue(), ^{
   // This is thread-safe because someView should only be de-alloced on the main thread.
   // consequently our access to weakRef is safe.
   UIView *strongRef = weakRef;
   [strongRef addSubview:self];
});

你不能做的是这个。

   UIView *strongRef = whatever;
    dispatch_async(downloadQueue, ^{
       dispatch_async(dispatch_get_main_queue(), ^{
          [someView addSubview:self]
       });
       // If, by the time we get here, the dispatch to the main queue has already run,
       // then we will send a release to someView on a background thread. If it's the last
       // remaining reference - and it might be, unlikely as it is, then we just dealloced 
       // a UIView in the background. This is bad. 
    });
于 2014-07-10T15:09:10.127 回答
0 
dispatch_async(downloadQueue, ^{
    UIView* v = someView;

现在，如果此时someView已不存在，并且如果您使用的是 ARC，v则为 nil，您可以对其进行测试。

另一方面，如果someView存在，并且如果您正在使用 ARC，那么v它是对它的强引用，并且对于块嵌套的其余部分，它不会消失。然后，您可以（此时或稍后在块的主线程部分）询问v.window是否发现视图仍在界面中；如果没有，则不值得继续。

于 2013-04-30T16:20:09.937 回答