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.

Answer 2

From the sound of it, if you are actively calling the method, the device might not have time to actually display the view while it is running the "custom code" in your viewDidAppear method. I that case you should let the program call the viewDidAppear method itself.

Your program may also be working on other code which would slow down the appearance of the view, this can be solved using timers. i.e. instead of:

[self otherCode];

you would write:

[NSTimer scheduledTimerWithTimeInterval:.5 
    target:self 
    selector:@selector(otherCode) 
    userInfo:nil 
    repeats:NO];

you might want to try simply delaying your "custom code" with a timer in this way.