What causes a UIViewController to become active?

I am sure this is an easy question, but one that has escaped me for some time now.

Say I have a UIViewController, either defined as a root in an XIB or on a stack. At some point in my code I want to replace it with another view controller. Just flat out replace it. How would I do that?

I have tried defining the controller and assigning, but not sure what actually makes it push on the开发者_运维百科 screen with the absence of a navigation controller.

I think when you say that you want to replace the view controller, what you actually mean is that you want to replace the view. Bear in mind that view controllers aren't visible, but every view controller maps to a view, which can become visible by getting added as a subview of a visible view.

Your solution of replacing self.view with the new view controller's view may work in your particular case, but it's probably not the "correct" answer to your question. There are going to be cases where this solution won't work for you.

Let's say you have a simple view based application with no navigation controller and no tab bar controller. In your app delegate you construct an instance of YourFirstViewController, and you call [window addSubview:yourFirstController];. Your view hierarchy now consists of a UIWindow with a single subview -- the view for YourFirstViewController.

Now let's say the user presses a button on that view, which is handled by an IBAction defined in YourFirstViewController. You want to respond by "replacing" YourFirstViewController's view with a view associated with YourSecondViewController. I put "replacing" in quotes because we more commonly present a view by pushing its view controller onto a navigation stack, or calling presentModalViewController:animated: to present the view modally, but let's assume that you've rejected those options for some reason, and you actually do want to manually replace YourFirstViewController's view with YourSecondViewController's view.

This is a simple matter of manipulating the view hierarchy. You want to remove YourFirstViewController's view from its superview (the UIWindow in this case), and you want to add YourSecondViewController's view as a subview to replace it. Your action would therefore look something like this:

- (IBAction)replaceButtonClicked {
    UIView *mySuperview = self.view.superview;
    YourSecondViewController *secondController = [[YourSecondViewController alloc] init];
    [mySuperview addSubview:secondController.view];
    [self.view removeFromSuperview];
    [secondController release];
}

When we use a methods like -pushViewController:animated: or -presentModalViewController, the receiving controller manipulates the view hierarchy for us. This may make it seem like we're looking at view controllers on the screen, but we're not. We're just looking at a big hierarchy of nested views going all the way up to a UIWindow at the top.

You can present a new view controller modally:

[self presentModalViewController:aViewController animated:YES];

This won't outright replace the current VC, but it will display a new view over the current view.