iOS新闻资讯页面仿网易新闻上拉关闭效果

• 前言： APP当中有资讯的页面，想仿照网易新闻，模仿上拉关闭的效果。<大神忽略，小弟记录知识要点而已嗟^_>
• 行动：网上搜寻看网友有没有好的实现思路，或者好的实现方式，找到了。如下参考：
  http://www.qingpingshan.com/rjbc/ios/202959.html iOS网易新闻详情页上拉关闭当前页面效果
• 接下来就运用在自己的工程当中了，根据以下几步来做：
  1.创建一个类，遵守<UIViewControllerAnimatedTransitioning >协议。

@interface KKAnimaTool : NSObject <UIViewControllerAnimatedTransitioning>
@end

2.实现这个类的协议方法

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext

- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
    return 1.2f;
}
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
    //获取转场动画开始，结束的控制器
    UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
    UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
    //确定结束控制器的视图大小
    CGRect finalFrameForVC = [transitionContext finalFrameForViewController:toViewController];
    UIView *containerView = [transitionContext containerView];
    //设置结束控制器的视图大小
    toViewController.view.frame = finalFrameForVC;
    toViewController.view.alpha = 1.0f;
    [containerView addSubview:toViewController.view];
    [containerView sendSubviewToBack:toViewController.view];
    //开始控制器视图的快照
    UIView *snapshotView = [fromViewController.view snapshotViewAfterScreenUpdates:NO];
    snapshotView.frame = fromViewController.view.frame;
    [containerView addSubview:snapshotView];
    [fromViewController.view removeFromSuperview];
    
    //执行关键帧动画
    NSTimeInterval duration = [self transitionDuration:transitionContext];
    void (^anima)() = ^(){
        for (int i = 0; i < 5; i++) {
            //添加5帧动画，缩放Y轴的大小
            [UIView addKeyframeWithRelativeStartTime:((i/5) * duration) relativeDuration:(0.2)*duration animations:^{
                snapshotView.transform = CGAffineTransformMakeScale(1, 1 - (i+1)/5);
                snapshotView.alpha = 0.0;
            }];
        }
    };
    
    [UIView animateKeyframesWithDuration:duration delay:0 options:UIViewKeyframeAnimationOptionCalculationModeLinear animations:anima completion:^(BOOL finished) {
        
        if (finished) {
            [snapshotView removeFromSuperview];
            [transitionContext completeTransition:YES];
        }
    }];

}

3.在需要实现关闭效果的类当中，遵守协议<UINavigationControllerDelegate>，实现代理方法：

#pragma mark - 上拉关闭（仿网易云资讯）
- (id<UIViewControllerAnimatedTransitioning>)navigationController:(UINavigationController *)navigationController animationControllerForOperation:(UINavigationControllerOperation)operation fromViewController:(UIViewController *)fromVC toViewController:(UIViewController *)toVC
{
    KKAnimaTool *customNavigationAnimationController = [KKAnimaTool new];
    return customNavigationAnimationController;
}

4.我的资讯界面用的WebView,在上拉的时候触发关闭的动作：

    [_detailWebView.scrollView addFooterWithTarget:self action:@selector(popLastVC)];
        _detailWebView.scrollView.footerPullToRefreshText = @"上拉关闭当前页";
        _detailWebView.scrollView.footerReleaseToRefreshText = @"释放关闭当前页";
        _detailWebView.scrollView.footerRefreshingText = @"";

- (void)popLastVC
{
    if (_detailWebView.isLoading) {
        [_detailWebView.scrollView footerEndRefreshing];
    }else{
        self.navigationController.delegate = self;
        [self.navigationController popViewControllerAnimated:YES];
    }
}

5.到此为止，可以实现上拉关闭的动画效果，但是如果按了返回键，不希望出现这种效果，那么还需要处理一下，在pop/push到新的界面的时候，代理置空：

    self.navigationController.delegate = nil;
    [self.navigationController popViewControllerAnimated:YES];

• 补充知识点
  1.参考iOS关键帧动画的实现，关键帧动画的原理 ，iOS UIView Animation - Keyframe ： http://www.devtalking.com/articles/uiview-keyframe-animation/
  2.屏幕快照的原理解释，snapshotViewAfterScreenUpdates ：
  http://www.jianshu.com/p/11fac364403c
  3.动画的放大缩小，CGAffineTransformMakeScale & CGAffineTransformScale: http://www.jianshu.com/p/0ee900339103