UIViewController 生命周期方法调用注意事项

APP 中希望主动调用生命周期方法时，官方建议不要直接调用四个生命周期方法，建议调用 -beginAppearanceTransition:animated: 和 -endAppearanceTransition 方法来实现目的。 -beginAppearanceTransition:animated: 和 -endAppearanceTransition 需要匹配使用。

[ViewController] Calling -viewWillDisappear: directly on a view controller is not supported, and may result in out-of-order callbacks and other inconsistent behavior. Use the -beginAppearanceTransition:animated: and -endAppearanceTransition APIs on UIViewController to manually drive appearance callbacks instead. Make a symbolic breakpoint at UIViewControllerAlertForAppearanceCallbackMisuse to catch this in the debugger.

-beginAppearanceTransition:YES animated:NO 等效于 -willAppear:NO;
-endAppearanceTransition 等效于 -didAppear:NO;
-beginAppearanceTransition:NO animated:NO 等效于 -willDisappear:NO;
-endAppearanceTransition 等效于 -didDisappear:NO。
调用了 -beginAppearanceTransition:NO animated:NO 这个方法后，在没有调用匹配的 [vc endAppearanceTransition] 方法之前，vc的 willDisappear和 didDisappear 不会被自动调用。
[parentVC.view addSubview:childVC.view] 会自动触发调用childVC的viewDidLoad、viewWillAppear、viewDidAppear方法。
[childVC.view removeFromSuperview] 会自动触发调用childVC的viewWillDisappear、viewDidDisappear方法。
[parentVC addChildViewController:childVC] 的作用：parentVC的生命周期方法调用后，会遍历所有的childVC，然后调用childVC的对应的生命周期方法。