Question

I am creating a bookmarking feature for my app, I’d like to show the user what happens in a way similar to itunes store, when you buy something it jumps to tabBar. I once watched some WWDC video that explained this, but can’t remember how to do it. Any idea where I should start looking for?

Answer 1

You can take a snapshot of the view you want to animate, then create an image layer, then use Core Animation to animate that to the tab bar. Here’s the code I use to do that:

- (void)animateSnapshotOfView:(UIView *)view toTab:(UINavigationController *)navController
{
    NSUInteger targetTabIndex = [self.tabBarController.viewControllers indexOfObject:navController];
    NSUInteger tabCount = [self.tabBarController.tabBar.items count];
    // AFAIK there's no API (as of iOS 4) to get the frame of a tab bar item, so guesstimate using the index and the tab bar frame.
    CGRect tabBarFrame = self.tabBarController.tabBar.frame;
    CGPoint targetPoint = CGPointMake((targetTabIndex + 0.5) * tabBarFrame.size.width / tabCount, CGRectGetMidY(tabBarFrame));
    targetPoint = [self.window convertPoint:targetPoint fromView:self.tabBarController.tabBar.superview];

    UIGraphicsBeginImageContext(view.frame.size);
    [view.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    CGRect frame = [self.window convertRect:view.frame fromView:view.superview];
    CALayer *imageLayer = [CALayer layer];
    imageLayer.contents = (id)image.CGImage;
    imageLayer.opaque = NO;
    imageLayer.opacity = 0;
    imageLayer.frame = frame;
    [self.window.layer insertSublayer:imageLayer above:self.tabBarController.view.layer];

    CGMutablePathRef path = CGPathCreateMutable();
    CGPoint startPoint = imageLayer.position;
    CGPathMoveToPoint(path, NULL, startPoint.x, startPoint.y);
    CGPathAddCurveToPoint(path,NULL,
                          startPoint.x + 100, startPoint.y,
                          targetPoint.x, targetPoint.y - 100,
                          targetPoint.x, targetPoint.y);
    CAKeyframeAnimation *positionAnimation = [CAKeyframeAnimation animationWithKeyPath:@"position"];
    positionAnimation.path = path;
    CGPathRelease(path);

    CABasicAnimation *sizeAnimation = [CABasicAnimation animationWithKeyPath:@"bounds.size"];
    sizeAnimation.fromValue = [NSValue valueWithCGSize:imageLayer.frame.size];
    sizeAnimation.toValue = [NSValue valueWithCGSize:CGSizeMake(50, 50)];

    CABasicAnimation *opacityAnimation = [CABasicAnimation animationWithKeyPath:@"opacity"];
    opacityAnimation.fromValue = [NSNumber numberWithFloat:0.75];
    opacityAnimation.toValue = [NSNumber numberWithFloat:0];

    CAAnimationGroup *animationGroup = [CAAnimationGroup animation];
    animationGroup.animations = [NSArray arrayWithObjects:positionAnimation, sizeAnimation, opacityAnimation, nil];
    animationGroup.duration = 1.0;
    animationGroup.timingFunction=[CAMediaTimingFunction functionWithName:kCAMediaTimingFunctionEaseIn];
    animationGroup.delegate = self;
    [animationGroup setValue:imageLayer forKey:@"animatedImageLayer"];

    [imageLayer addAnimation:animationGroup forKey:@"animateToTab"];
}