Ios9 什么'；s指南识别器状态'；s已更改，应在何时使用？

UIgestureRecognitizerState发生了什么变化，应该在什么时候使用

public enum UIGestureRecognizerState : Int {
    case possible

    case began

    case changed // the recognizer has received touches recognized as a change to the gesture. the action method will be called at the next turn of the run loop

    case ended

    case cancelled

    case failed   
}