UITabbarItem imageview 实现点击有放大然后还原的动画效果

在UITabBarController代理方法中添加动画，先通过KVC获取UIControl，然后在获取上面的UITabBarSwappableImageView，最后将动画添加到imageview的layer上。

#pragma mark UITabBarControllerDelegate的代理方法
- (BOOL)tabBarController:(UITabBarController *)tabBarController shouldSelectViewController:(UIViewController *)viewController
{
    UIControl*tabBarButton = [viewController.tabBarItem valueForKey:@"view"];
    if (tabBarButton) {
        UIImageView * tabBarSwappableImageView = [tabBarButton valueForKey:@"info"];
        if ([tabBarSwappableImageView isKindOfClass:NSClassFromString(@"UITabBarSwappableImageView")]) {
            CAKeyframeAnimation * animation;
            animation = [CAKeyframeAnimation animationWithKeyPath:@"transform"];
            animation.duration = 0.3;
            animation.removedOnCompletion = YES;
            animation.fillMode = kCAFillModeForwards;
            NSMutableArray *values = [NSMutableArray array];
            [values addObject:[NSValue valueWithCATransform3D:CATransform3DMakeScale(0.9, 0.9, 1.0)]];
            [values addObject:[NSValue valueWithCATransform3D:CATransform3DMakeScale(1.2, 1.2, 1.0)]];
            [values addObject:[NSValue valueWithCATransform3D:CATransform3DMakeScale(1.0, 1.0, 1.0)]];
            animation.values = values;
            [tabBarSwappableImageView.layer addAnimation:animation forKey:nil];
        }
    }
    return YES;
}