当前的 UIViewController 做动画

具体的做法是将当前的 VC 截图，图片做动画，图片在做动画的过程中，改变当前 VC 上子视图的颜色、大小等属性。

//preserve the current view snapshot
    UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, YES, 0.0);
    [self.view.layer renderInContext:UIGraphicsGetCurrentContext()];
    UIImage *coverImage = UIGraphicsGetImageFromCurrentImageContext();
    //insert snapshot view in front of this one
    UIView *coverView = [[UIImageView alloc] initWithImage:coverImage];
    coverView.frame = self.view.bounds;
    [self.view addSubview:coverView];
    //update the view (we'll simply randomize the layer background color)
    CGFloat red = arc4random() / (CGFloat)INT_MAX;
    CGFloat green = arc4random() / (CGFloat)INT_MAX;
    CGFloat blue = arc4random() / (CGFloat)INT_MAX;
    self.view.backgroundColor = [UIColor colorWithRed:red green:green blue:blue alpha:1.0];
    //perform animation (anything you like)
    [UIView animateWithDuration:1.0 animations:^{
        //scale, rotate and fade the view
        CGAffineTransform transform = CGAffineTransformMakeScale(0.01, 0.01);
        transform = CGAffineTransformRotate(transform, M_PI_2);
        coverView.transform = transform;
        coverView.alpha = 0.0;
    } completion:^(BOOL finished) {
        //remove the cover view now we're finished with it
        [coverView removeFromSuperview];
    }];