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iPhone Developer : Assembling Views and Animations - Recipe: Fading a View In and Out

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At times, you want to add information to your screen that overlays your view but does not of itself do anything. For example, you might show a top scores list or some instructions or provide a context-sensitive tooltip. Recipe 1 demonstrates how to use a UIView animation block to fade a view into and out of sight. This recipe follows the most basic animation approach. It creates a surrounding view animation block and then adds the single line of code that sets the alpha property.

One thing this recipe does not do is wait for the animation to finish. The change in the bar button item gets called as soon as the animations are committed, nearly a second before they end. If you tap the Fade In/Fade Out button quickly (you may want to slow the animation duration to see this better), you discover that the new animation starts up, replacing the old one, creating a visual discontinuity.

To address this, you might want to add a call to UIView with setAnimationBeginsFromCurrentState:, setting the argument to YES. This tells the iPhone to use the current state of the ongoing animation to start the next animation, avoiding that jump.

Recipe 1. Animating Transparency Changes to a View’s Alpha Property
@implementation TestBedViewController

- (void) fadeOut: (id) sender
{
CGContextRef context = UIGraphicsGetCurrentContext();
[UIView beginAnimations:nil context:context];
[UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];
[UIView setAnimationDuration:1.0];
[[self.view viewWithTag:999] setAlpha:0.0f];
[UIView commitAnimations];
self.navigationItem.rightBarButtonItem =
BARBUTTON(@"Fade In",@selector(fadeIn:));
}

- (void) fadeIn: (id) sender
{
CGContextRef context = UIGraphicsGetCurrentContext();
[UIView beginAnimations:nil context:context];
[UIView setAnimationCurve:UIViewAnimationCurveEaseInOut];
[UIView setAnimationDuration:1.0];
[[self.view viewWithTag:999] setAlpha:1.0f];
[UIView commitAnimations];
self.navigationItem.rightBarButtonItem =
BARBUTTON(@"Fade Out",@selector(fadeOut:));
}

- (void) viewDidLoad
{
self.navigationItem.rightBarButtonItem =
BARBUTTON(@"Fade Out",@selector(fadeOut:));
}
@end

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