在iOS的动画处理中，我们经常会碰到一些页面切换的效果，比如新页面代替旧的页面，为了实现某些特定的效果，我们需要把旧页面先保存下来，然后经过一定的动画处理，过渡到新的页面，这种情况下，renderInContext就派上用场了。

renderInContext可以通过把当前页面内容画到Core Graphics的context，来实现类似于截图的功能。

- (void)viewDidLoad
{
    [super viewDidLoad];

    self.button = [UIButton buttonWithType:UIButtonTypeCustom];
    self.button.frame = CGRectMake(0, 0, 200, 40);
    self.button.center = CGPointMake(CGRectGetMidX(self.view.bounds), CGRectGetMidY(self.view.bounds));
    [self.button setTitle:@"Perform Transition" forState:UIControlStateNormal];
    [self.button setBackgroundColor:[UIColor blueColor]];
    self.button.layer.cornerRadius = 5.0f;
    self.button.layer.masksToBounds = YES;
    [self.button addTarget:self action:@selector(performTransition) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:self.button];
}

- (void)performTransition
{
    // 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 (well 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 animatioin
    [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) {
        [coverView removeFromSuperview];
    }];
}

效果就是下面的样子