好吧 - 找到了答案。它需要被放置在不同的线程，使UI更新里面的：

Okay - found the answer. It needs to be placed inside a different thread that allows the UI to update:

dispatch_async(dispatch_get_main_queue(), {
    // code here
})

的Objective-C版本

dispatch_async(dispatch_get_main_queue(), ^{
    // code here
});