上一篇文章介绍了如何进行modal方式的页面切换
自定义的目的控制器在modal切换时完全覆盖源控制器，本篇文章介绍如何实现一个自定义的过渡效果
实现的效果描述：
目的控制器：占据屏幕的二分之一大小，且居中，并在源控制器上覆盖着一个阴影效果，点击阴影时目的控制器返回
UIPresentationController
用于描述目的控制器通过modal方式切换的过渡效果
实现其子类，可以自定义出特殊的效果
实现步骤：
定义过渡效果：实现UIPresentationController子类
目的控制器，遵循过渡协议，设置过渡控制器对象
源控制器进行modal切换
UIPresentationController的属性：
@property(nonatomic, retain, readonly) UIViewController *presentedViewController //目的控制器@property(nonatomic, retain, readonly) UIViewController*presentingViewController //源控制器- (UIView *)presentedView //目的view @property(nonatomic, readonly) UIView *containerView //源view
UIPresentationController的子类应重写的方法
1）init方法，可以创建其他辅助过渡效果的view
- (instancetype)initWithPresentedViewController:(UIViewController*)presentedViewController presentingViewController:(UIViewController*)presentingViewController
如：
- (instancetype)initWithPresentedViewController:(UIViewController*)presentedViewController presentingViewController:(UIViewController*)presentingViewController { if ( self = [super initWithPresentedViewController:presentedViewController presentingViewController:presentingViewController] ) { _shadowBtn = [UIButton buttonWithType:UIButtonTypeCustom]; //阴影按钮的初始状态是隐藏的 _shadowBtn.backgroundColor = [UIColor grayColor]; _shadowBtn.alpha = 0.f; }} 2）重写presentationTransitionWillbegin方法，定义实现过渡效果
- (void)presentationTransitionWillBegin
如：
- (void)presentationTransitionWillBegin{ [self.containerView addSubview:_shadowBtn]; [self.containerView addSubview:self.presentedView]; _shadowBtn.frame = self.containerView.bounds; id coordinate = self.presetingViewController.transitionCoordinator; [coordinate animateAlongsideTransition:^(id _Nonnull context) { _shadowBtn.alpha = 0.5; } completion:nil];} 使用到UIViewController的transitionCoordinator，表示过渡效果的协助器
协助器的animateAlongsideTransition方法定义过渡期间的动画效果
3）重写presentationTransitionDidEnd方法，定义过渡效果后的清理工作。
特别是过渡未完成时的清理动作
- (void)presentationTransitionDidEnd:(BOOL)completed
- (void)presentationTransitionDidEnd:(BOOL)completed{ if ( !completed ) { [_shadowBtn removeFromSuperview]; }} 4）重写frameOfPresentedViewInContainerView，设置被显示视图的frame
- (CGRect)frameOfPresentedViewInContainerView
- (CGRect)frameOfPresentedViewInContainerView{ CGFloat x, y, w, h; w = self.containerView.frame.size.width/2; h = self.containerView.frame.size.height/2; x = self.containerView.frame.size.width/4; y = self.containerView.frame.size.height/4; return CGRectMake(x, y, w, h);} 5）重写dismissTransitionWillBegin方法，设置返回的过渡效果
- (void)dismissalTransitionWillBegin
- (void)dismissalTransitionWillBegin{ id coordinator = self.presetingViewController.transitionCoordinator; [coordinator animateAlongsideTransition:^(id _Nonull context) { _shadowBtn.alpha = 0.01; } completion:nil];} 6）重写dismissalTransitionDidEnd方法，执行清理动作
- (void)dismissalTransitionDidEnd:(BOOL)completed
- (void)dismissalTransitionDidEnd:(BOOL)completed{ if ( completed ) { [_shadowView removeFromSuperview]; }}
目的控制器设置过渡效果
目的控制器遵循代理协议
@interface AMDestViewController ()
设置代理
self.transitioningDelegate = self;
实现代理方法
- (UIPresetationController *) presentationControllerForPresentedViewController:(UIViewController*) presented presentingViewController:(UIViewController*) presenting sourceViewController:(UIViewController*) source{ return [[AMPresentationController alloc] initWithPresentedViewController:presented presentingViewController:presenting];} 源控制器进行modal切换
iOS8.0开始支持这种自定义的过渡效果，主要要设置目的控制器的modalPresentationStyle为自定义。
这种切换方式，在iPhone和iPad上都是可用的。
AMDestViewController * vc = [[AMDestViewController alloct] init];vc.modalPresentationStyle = UIModalPresentationCustom;[self presentViewController:vc animated:YES completion:nil];