From和To
fromView表示当前视图，toView表示要跳转到的视图。
如果是从A视图控制器present到B，则A是from，B是to。从B视图控制器dismiss到A时，B变成了from，A是to。用一张图表示：
From和To.png
Presented和Presenting
不受present或dismiss的影响，如果是从A视图控制器present到B，那么A总是B的presentingViewController,B总是A的presentedViewController。
modalPresentationStyle
枚举类型，表示present时动画的类型。其中可以自定义动画效果的只有两种：FullScreen和Custom，两者的区别在于FullScreen会移除fromView，而Custom不会。
基于block的动画
最简单的转场动画是使用transitionFromViewController方法：
传统的转场动画实现.png
6个参数，前两个表示从哪个VC开始，跳转到哪个VC，中间两个参数表示动画的时间和选项。最后两个参数表示动画的具体实现细节和回调闭包。
自定义present转场动画
转场动画代理，它是一个实现了UIViewControllerTransitioningDelegate协议的对象。
Animator：
它是实现了UIViewControllerAnimatedTransitioning协议的对象，用于控制动画的持续时间和动画展示逻辑，代理可以为present和dismiss过程分别提供Animator，也可以提供同一个Animator。
交互式Animator：和Animator类似，不过它是交互式的，后面会有详细介绍
Presentation控制器：
- 创建动画代理
- 设置B的transitioningDelegate为步骤1中创建的代理对象
- 调用presentViewController:animated:completion:并把参数animated设置为true
- 系统会找到代理中提供的Animator，由Animator负责动画逻辑
//AAPLCrossDissolveTransitionAnimator.h
@import UIKit;
@interface AAPLCrossDissolveTransitionAnimator : NSObject <UIViewControllerAnimatedTransitioning>
@end
//AAPLCrossDissolveTransitionAnimator.m
#import "AAPLCrossDissolveTransitionAnimator.h"
@implementation AAPLCrossDissolveTransitionAnimator
//| ----------------------------------------------------------------------------
- (NSTimeInterval)transitionDuration:(id<UIViewControllerContextTransitioning>)transitionContext
{
return 0.35;
}
//| ----------------------------------------------------------------------------
- (void)animateTransition:(id<UIViewControllerContextTransitioning>)transitionContext
{
UIViewController *fromViewController = [transitionContext viewControllerForKey:UITransitionContextFromViewControllerKey];
UIViewController *toViewController = [transitionContext viewControllerForKey:UITransitionContextToViewControllerKey];
UIView *containerView = transitionContext.containerView;
// For a Presentation:
// fromView = The presenting view.
// toView = The presented view.
// For a Dismissal:
// fromView = The presented view.
// toView = The presenting view.
UIView *fromView;
UIView *toView;
// In iOS 8, the viewForKey: method was introduced to get views that the
// animator manipulates. This method should be preferred over accessing
// the view of the fromViewController/toViewController directly.
// It may return nil whenever the animator should not touch the view
// (based on the presentation style of the incoming view controller).
// It may also return a different view for the animator to animate.
//
// Imagine that you are implementing a presentation similar to form sheet.
// In this case you would want to add some shadow or decoration around the
// presented view controller's view. The animator will animate the
// decoration view instead and the presented view controller's view will
// be a child of the decoration view.
if ([transitionContext respondsToSelector:@selector(viewForKey:)]) {
fromView = [transitionContext viewForKey:UITransitionContextFromViewKey];
toView = [transitionContext viewForKey:UITransitionContextToViewKey];
} else {
fromView = fromViewController.view;
toView = toViewController.view;
}
fromView.frame = [transitionContext initialFrameForViewController:fromViewController];
toView.frame = [transitionContext finalFrameForViewController:toViewController];
fromView.alpha = 1.0f;
toView.alpha = 0.0f;
// We are responsible for adding the incoming view to the containerView
// for the presentation/dismissal.
[containerView addSubview:toView];
NSTimeInterval transitionDuration = [self transitionDuration:transitionContext];
[UIView animateWithDuration:transitionDuration animations:^{
fromView.alpha = 0.0f;
toView.alpha = 1.0;
} completion:^(BOOL finished) {
// When we complete, tell the transition context
// passing along the BOOL that indicates whether the transition
// finished or not.
BOOL wasCancelled = [transitionContext transitionWasCancelled];
[transitionContext completeTransition:!wasCancelled];
}];
}
@end
animateTransition方法的核心则是从转场动画上下文获取必要的信息以完成动画。上下文是一个实现了UIViewControllerContextTransitioning的对象，它的作用在于为animateTransition方法提供必备的信息。您不应该缓存任何关于动画的信息，而是应该总是从转场动画上下文中获取(比如fromView和toView)，这样可以保证总是获取到最新的、正确的信息。
转场动画上下文.png
获取到足够信息后，我们调用UIView.animateWithDuration方法把动画交给Core Animation处理。千万不要忘记在动画调用结束后，执行completeTransition方法。
- present时，要把toView加入到container的视图层级。
- dismiss时，要把fromView从container的视图层级中移除。
交互式(Interactive)转场动画
刚刚我们说到，设置了toViewController的transitioningDelegate属性并且present时，UIKit会从代理处获取animator，其实这里还有一个细节：UIKit还会调用代理的interactionControllerForPresentation:方法来获取交互式控制器，如果得到了nil则执行非交互式动画，这就回到了上一节的内容。
如果获取到了不是nil的对象，那么UIKit不会调用animator的animateTransition方法，而是调用交互式控制器(还记得前面介绍动画代理的示意图么，交互式动画控制器和animator是平级关系)的startInteractiveTransition:方法。
所谓的交互式动画，通常是基于手势驱动，产生一个动画完成的百分比来控制动画效果。整个动画不再是一次性、连贯的完成，而是在任何时候都可以改变百分比甚至取消。这需要一个实现了UIPercentDrivenInteractiveTransition协议的交互式动画控制器和animator协同工作。这看上去是一个非常复杂的任务，但UIKit已经封装了足够多细节，我们只需要在交互式动画控制器和中定义一个时间处理函数(比如处理滑动手势)，然后在接收到新的事件时，计算动画完成的百分比并且调用updateInteractiveTransition来更新动画进度即可。
edit -> http://www.jianshu.com/p/ea0132738057
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