Why does viewDidAppear in UITabBarController exec before the view appears?

Question

I have a UITabBarController that nests a UIView-Subclass (ImageViewer) as it\'s third tab.

In this ImageViewer Subclass I call the viewDidAppear method:

Answer 1

If you want to allow the screen to be re-drawn when your view loads, but to trigger some other updating code in -viewDidAppear:, use performSelector:withObject:afterDelay: like this:

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];
    [self performSelector:@selector(updateUI) withObject:nil afterDelay:0.0];
}

…

- (void)updateUI
{
    // Do your UI stuff here
}

When you do it this way, the current event loop will finish quickly, and UIKit will be able to re-draw the screen after your view has loaded. updateUI will be called in the next event loop. This is a good way to get snappy view transitions if you have to perform computationally intensive calculations or updates after a view has loaded.