主队列上的performSelectorOnMainThread和dispatch_async有什么区别？

Question

我在修改线程内的视图时遇到问题.我试图添加一个子视图,但显示大约需要6秒或更长时间.我终于搞定了,但我不知道究竟是怎么回事.所以我想知道为什么它有效,以下方法之间有什么区别:

//this worked -added the view instantly
dispatch_async(dispatch_get_main_queue(), ^{
    //some UI methods ej
    [view addSubview: otherView];
}

//this took around 6 or more seconds to display
[viewController performSelectorOnMainThread:@selector(methodThatAddsSubview:) withObject:otherView
 waitUntilDone:NO];

//Also didnt work: NSNotification methods -  took also around 6 seconds to display
//the observer was in the viewController I wanted to modify
//paired to a method to add a subview.
[[NSNotificationCenter defaultCenter] postNotificationName:
 @"notification-identifier" object:object];

作为参考,这是在ACAccountStore类的Completetion Handler中调用的.

accountStore requestAccessToAccountsWithType:accountType withCompletionHandler:^(BOOL granted, NSError *error) {
            if(granted) {
            //my methods were here
            }
}

编辑:当我说它不起作用时,我的意思是大约需要6秒才能显示我添加的视图.

Answer 1

默认情况下,-performSelectorOnMainThread:withObject:waitUntilDone:仅调度选择器以默认运行循环模式运行.如果运行循环处于另一种模式(例如跟踪模式),则在运行循环切换回默认模式之前它不会运行.您可以使用变体-performSelectorOnMainThread:withObject:waitUntilDone:modes:(通过传递您希望它运行的所有模式)来解决这个问题.

另一方面,dispatch_async(dispatch_get_main_queue(), ^{ ... })只要主运行循环将控制流返回到事件循环,就会运行该块.它不关心模式.因此,如果您不想关心模式,dispatch_async()可能是更好的方法.