为什么在Swift上显示警报时需要DispatchQueue？-WhywouldyouneedaDispatchQueuewhenshowingAlertsonSwift?

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It’s a conscious design decision from Apple’s side to not have UIKit be thread-safe. Making it thread-safe wouldn’t buy you much in terms of performance; it would in fact make many things slower. And the fact that UIKit is tied to the main thread makes it very easy to write concurrent programs and use UIKit. All you have to do is make sure that calls into UIKit are always made on the main thread. So according to this the fact that UIKit objects must be accessed on the main thread is a design decision by apple to favor performance.

苹果方面的一个有意识的设计决定是没有UIKit是线程安全的。使其具有线程安全性并不会对性能产生太大影响;事实上它会让很多东西变慢。 UIKit与主线程绑定的事实使得编写并发程序和使用UIKit变得非常容易。您所要做的就是确保始终在主线程上调用UIKit。因此，根据这一点，UIKit对象必须在主线程上访问的事实是苹果的设计决定有利于性能。

for more detailed information you can go through this article

有关更多详细信息，请阅读本文

https://www.objc.io/issues/2-concurrency/thread-safe-class-design/

https://www.objc.io/issues/2-concurrency/thread-safe-class-design/

In your case , You are showing alert from another thread so you have to write code under the MainThread so , you can get the main thread using below code

在您的情况下，您正在显示来自另一个线程的警报，因此您必须在MainThread下编写代码，因此，您可以使用下面的代码获取主线程

DispatchQueue.main.async {
       // Your UI Updation here 
   }

Reason

原因

In Cocoa Touch, the UIApplication i.e. the instance of your application is attached to the main thread because this thread is created by UIApplicatioMain(), the entry point function of Cocoa Touch. It sets up main event loop, including the application’s run loop, and begins processing events. Application's main event loop receives all the UI events i.e. touch, gestures etc.

在Cocoa Touch中，UIApplication即应用程序的实例附加到主线程，因为该线程是由Cocoa Touch的入口点函数UIApplicatioMain（）创建的。它设置主事件循环，包括应用程序的运行循环，并开始处理事件。应用程序的主事件循环接收所有UI事件，即触摸，手势等。

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You´ll for sure notice that the alert will lag if you don´t show the alert on the main thread, that´s because your UI code does always have to be done on your main thread.

如果您没有在主线程上显示警报，那么您肯定会注意到警报会滞后，这是因为您的UI代码始终必须在主线程上完成。

So if you're on a background thread and want to execute code on the main thread, you need to call async(). That´s way you call DispatchQueue.main, which is the main thread.

因此，如果您在后台线程上并且想要在主线程上执行代码，则需要调用async（）。这就是你调用DispatchQueue.main的方式，这是主线程。