主队列上的 performSelectorOnMainThread: 和 dispatch_async() 有什么区别? [英] What's the difference between performSelectorOnMainThread: and dispatch_async() on main queue?

问题描述

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.我终于让它工作了，但我不知道究竟是怎么回事.所以我想知道它为什么有效以及以下方法之间有什么区别:

I was having problems modifying a view inside a thread. I tried to add a subview but it took around 6 or more seconds to display. I finally got it working, but I don't know how exactly. So I was wondering why it worked and what's the difference between the following methods:

1. 这奏效了 - 立即添加了视图:

dispatch_async(dispatch_get_main_queue(), ^{
    //some UI methods ej
    [view addSubview: otherView];
}

1. 这需要大约 6 秒或更长时间才能显示:

[viewController performSelectorOnMainThread:@selector(methodThatAddsSubview:) withObject:otherView
    waitUntilDone:NO];

1. NSNotification 方法 - 也花了大约 6 秒来显示观察者在 viewController 中，我想修改成对添加子视图的方法.

1. 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，这些是在 ACAccountStore 类的这个 CompletionHandler 中调用的.

For reference these were called inside this CompletionHandler of the class ACAccountStore.

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

推荐答案

默认情况下，-performSelectorOnMainThread:withObject:waitUntilDone: 只安排选择器以默认运行循环模式运行.如果 run loop 处于另一种模式(例如跟踪模式)，它不会运行，直到 run loop 切换回默认模式.您可以使用变体 -performSelectorOnMainThread:withObject:waitUntilDone:modes: 解决此问题(通过传递您希望它运行的所有模式).

By default, -performSelectorOnMainThread:withObject:waitUntilDone: only schedules the selector to run in the default run loop mode. If the run loop is in another mode (e.g. the tracking mode), it won't run until the run loop switches back to the default mode. You can get around this with the variant -performSelectorOnMainThread:withObject:waitUntilDone:modes: (by passing all the modes you want it to run in).

另一方面，dispatch_async(dispatch_get_main_queue(), ^{ ... }) 将在主运行循环将控制流返回到事件循环时立即运行该块.它不关心模式.因此，如果您也不想关心模式，dispatch_async() 可能是更好的选择.

On the other hand, dispatch_async(dispatch_get_main_queue(), ^{ ... }) will run the block as soon as the main run loop returns control flow back to the event loop. It doesn't care about modes. So if you don't want to care about modes either, dispatch_async() may be the better way to go.

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