Continue the topic "Interactive Animations" which objc.io talked two years ago. When the article was released(just before WWDC2014 and iOS 8), a key problem can't be resolved by UIView Animation in iOS 7. Two authors of the article implement the above interacitive animation in two ways: UIKit Dynamics, and spring animation effect implemented by CADisplayLink, latter is admirable.

WWDC 2014 Session 236: Building Interruptible and Responsive Interactions explains how to bulid fluid interactive animations. There are three transition problems to resolve:

1. Animation to Gesture: pause or stop animation in current position.
2. Gesture to Animation: add a spring animation with special initial velocity to make smooth transition.
3. Animation to Animation: reverse animation in BeginFromCurrentState or Additive mode.

Almost two weeks ago, Session 216: Advances in UIKit Animations and Transitions introduces UIViewPropertyAnimator in iOS 10 to make completely interactive, interruptible animations.

I implement above animation in two ways:

1. UIView Animation/Core Animation: after iOS 8, it's very easy to achieve the goal; on iOS 7, there is a little limitation that UIView Animation is not additive until iOS 8 and Spring Core Animation is not public until iOS 9.
2. UIViewPropertyAnimator: it's awesome and very flexible, and relatively complex, not a little. You can use it like UIView Animation, or in its style, or combine two styles. I show you how to implement above interactive animation by UIViewPropertyAnimator in three ways in 'iOS10-Control-Panel-Interactive-Animation' project.

两篇详细的博客：

1. 交互式动画(上)：iOS 10 以下的实现，使用 UIView Animation/Core Animation 来实现上述交互动画。
2. 交互式动画(下)：UIViewPropertyAnimator in iOS 10，使用 UIViewPropertyAnimator实现上述交互动画，这个类相当灵活，可以说是面向对象版本的 UIView Animation。但在实现交互控制时，相比 UIView Animation/Core Animation，使用上复杂了许多。

1. Control-Panel-Interactive-Animation: iOS 8+, Swift 2.2+
2. iOS10-Control-Panel-Interactive-Animation: Xcode 8 beta, iOS 10+, Swift 3

Before iOS 10, view controller transition is not completely interactive, interruptible, for example, push and pop in UINavigationController: if start transition in non-interactive, you’ll have to wait until the transition animation is finished before you can do anything; if start transition in interactive, after interaction end, the transition animation is no more interactive.

iOS 10 introduce the protocol UIViewImplicitlyAnimating, which UIViewPropertyAnimator conform to, into view controller transition protocol which is very complex now. Actually, with the help of UIViewPropertyAnimator, we can simplify view controller transition protocol. A demo for this: iOS10PushPop.

详细内容已在 iOS 视图控制器转场详解中更新。