What follows is a category I use to animate UIViews. Adding the animation code where the animation happens feels very messy to me. With these simple categories you can cut down the animation of a UIView in certain situations to a single line of code.

@interface UIView (Extra)

// Animate removing a view from its parent
- (void) removeWithTransition:(UIViewAnimationTransition) transition andDuration:(float) duration;
// Animate adding a subview
- (void) addSubview:(UIView *)view withTransition:(UIViewAnimationTransition) transition withDuration:(float) duration;
// Animate the changing of a views frame
- (void) setFrame:(CGRect) fr withDuration:(float) duration;
// Animate changing the alpha of a view
- (void) setAlpha:(float) a withDuration:(float) duration;

@end

@implementation UIView (Extra)

- (void) removeWithTransition:(UIViewAnimationTransition) transition andDuration:(float) duration
{
	[UIView beginAnimations:nil context:NULL];
	[UIView setAnimationDuration:duration];
	[UIView setAnimationTransition:transition forView:self.superview cache:YES];
	[self removeFromSuperview];
	[UIView commitAnimations];
}

- (void) addSubview:(UIView *)view withTransition:(UIViewAnimationTransition) transition withDuration:(float) duration
{
	[UIView beginAnimations:nil context:NULL];
	[UIView setAnimationDuration:duration];
	[UIView setAnimationTransition:transition forView:self cache:YES];
	[self addSubview:view];
	[UIView commitAnimations];
}

- (void) setFrame:(CGRect) fr withDuration:(float) duration
{
	[UIView beginAnimations:nil context:NULL];
	[UIView setAnimationDuration:duration];
	self.frame = fr;
	[UIView commitAnimations];
}

- (void) setAlpha:(float) a withDuration:(float) duration
{
	[UIView beginAnimations:nil context:NULL];
	[UIView setAnimationDuration:duration];
	self.alpha = a;
	[UIView commitAnimations];
}

@end

Here is an example of using the setFrame:withDuration: to slide a UIDatePicker on and off screen.

- (void) hideDatePicker:(BOOL) hide
{
	CGRect hideFrame = CGRectMake(0, self.view.frame.size.height, datePicker.frame.size.width, datePicker.frame.size.height);
	CGRect showFrame = CGRectMake(0, self.view.frame.size.height - datePicker.frame.size.height, datePicker.frame.size.width, datePicker.frame.size.height);
	
	if(!hide) [datePicker setFrame:showFrame withDuration:.5];
	else [datePicker setFrame:hideFrame withDuration:.5];
}

The following is an example of the flip animation used in a default “utility” application.

// Show the view with animation
[self.view addSubview:myviewcontroller.view withTransition:UIViewAnimationTransitionFlipFromLeft withDuration:.5];
// Remove the view with animation
[myviewcontroller.view removeWithTransition:UIViewAnimationTransitionFlipFromLeft andDuration:.5];

To summarize, categories rock! They make it easy to add functionality to existing classes without the need to subclass. As always questions and comments are welcome.