如果在mainQueue / mainThread之外运行，NavigationController.pushViewController崩溃

Question

I have code like this

SomeController *controller = [[SomeController alloc] init];
[self.navController setViewControllers:@[controller] animated:YES];

If run this code in non-main thread , then app crashes during resignFirstResponder of viewController which is executing the above code

If run this code in dispatch_async(dispatch_get_main_queue, ^{}) ;

Then it works fine, no crash

Why does it crash in the first case ? I thought if run in non-main thread, then UI wont see the changes but I did not expect it to crash?

Answer 1

Calls to UIKit classes should take place on the main thread. Apple's UIKit documentation states:

NOTE

For the most part, use UIKit classes only from your app's main thread. This is particularly true for classes derived from UIResponder or that involve manipulating your app's user interface in any way.

Because UIKit assumes that it'll be running on the main thread, there are no real guarantees about what will happen if you try and access it on a different thread; your UI may not update, you may get a crash, etc. Some classes may even through an exception if they discover you're calling them on a non-main thread.