How to add an UIViewController's view as subview

添加一个 UIViewController 作为当前视图的子视图

I feel like all of these answers are slightly incomplete, so here's the proper way to add a viewController's view as a subview of another viewController's view:

    [self addChildViewController:viewControllerToAdd];
    [self.view addSubview:viewControllerToAdd.view];
    [viewControllerToAdd didMoveToParentViewController:self];