<div id="app" class="normal"></div>

.normal {
  width: 100px;
  height: 100px;
  background-color: red;
  transition: all 0.3s;
}
.normal:hover {
  background-color: yellow;
  width: 200px;
  height: 200px;
}

var el = document.querySelector('#app')
el.addEventListener('transitionstart', () => {
  console.log('transition start')
})
el.addEventListener('transitionend', () => {
  console.log('transition end')
})

 <div id="app" class="normal"></div>

// Tween僅僅是個(gè)緩動(dòng)函數(shù)
var el = document.querySelector('#app')
var time = 0, begin = 0, change = 500, duration = 1000, fps = 1000 / 60;
function startSport() {
  var val = Tween.Elastic.easeInOut(time, begin, change, duration);
  el.style.transform = 'translateX(' + val + 'px)';
  if (time <= duration) {
    time += fps
  } else {
    console.log('動(dòng)畫(huà)結(jié)束重新開(kāi)始')
    time = 0;
  }
  setTimeout(() => {
    startSport()
  }, fps)
}
startSport()

// 輔助函數(shù)，復(fù)制props的數(shù)據(jù)
export function extractTransitionData (comp: Component): Object {
 const data = {}
 const options: ComponentOptions = comp.$options
 // props
 for (const key in options.propsData) {
  data[key] = comp[key]
 }
 // events.
 const listeners: ?Object = options._parentListeners
 for (const key in listeners) {
  data[camelize(key)] = listeners[key]
 }
 return data
}

export default {
 name: 'transition',
 props: transitionProps,
 abstract: true, // 抽象組件，意思是不會(huì)真實(shí)渲染成dom，輔助開(kāi)發(fā)

 render (h: Function) {
  // 通過(guò)slots獲取到真實(shí)渲染元素children
  let children: any = this.$slots.default
  
  const mode: string = this.mode

  const rawChild: VNode = children[0]

  // 添加唯一key
  // component instance. This key will be used to remove pending leaving nodes
  // during entering.
  const id: string = `__transition-${this._uid}-`
  child.key = getKey(id)
    : child.key
  // data上注入transition屬性，保存通過(guò)props傳遞的數(shù)據(jù)
  const data: Object = (child.data || (child.data = {})).transition = extractTransitionData(this)
  const oldRawChild: VNode = this._vnode
  const oldChild: VNode = getRealChild(oldRawChild)

  
   // important for dynamic transitions!
   const oldData: Object = oldChild.data.transition = extend({}, data)
 // handle transition mode
   if (mode === 'out-in') {
    // return placeholder node and queue update when leave finishes
    this._leaving = true
    mergeVNodeHook(oldData, 'afterLeave', () => {
     this._leaving = false
     this.$forceUpdate()
    })
    return placeholder(h, rawChild)
   } else if (mode === 'in-out') {
    let delayedLeave
    const performLeave = () => { delayedLeave() }
    mergeVNodeHook(data, 'afterEnter', performLeave)
    mergeVNodeHook(data, 'enterCancelled', performLeave)
    mergeVNodeHook(oldData, 'delayLeave', leave => { delayedLeave = leave })
   }
  return rawChild
 }
}

function _enter (_: any, vnode: VNodeWithData) {
 if (vnode.data.show !== true) {
  enter(vnode)
 }
}
export default inBrowser ? {
 create: _enter,
 activate: _enter,
 remove (vnode: VNode, rm: Function) {
  if (vnode.data.show !== true) {
   leave(vnode, rm)
  } 
 }
} : {}

export function addTransitionClass (el: any, cls: string) {
 const transitionClasses = el._transitionClasses || (el._transitionClasses = [])
 if (transitionClasses.indexOf(cls) < 0) {
  transitionClasses.push(cls)
  addClass(el, cls)
 }
}

export function removeTransitionClass (el: any, cls: string) {
 if (el._transitionClasses) {
  remove(el._transitionClasses, cls)
 }
 removeClass(el, cls)
}
export function enter (vnode: VNodeWithData, toggleDisplay: ?() => void) {
 const el: any = vnode.elm

 // call leave callback now
 if (isDef(el._leaveCb)) {
  el._leaveCb.cancelled = true
  el._leaveCb()
 }
 // 上一步注入data的transition數(shù)據(jù)
 const data = resolveTransition(vnode.data.transition)
 if (isUndef(data)) {
  return
 }

 /* istanbul ignore if */
 if (isDef(el._enterCb) || el.nodeType !== 1) {
  return
 }

 const {
  css,
  type,
  enterClass,
  enterToClass,
  enterActiveClass,
  appearClass,
  appearToClass,
  appearActiveClass,
  beforeEnter,
  enter,
  afterEnter,
  enterCancelled,
  beforeAppear,
  appear,
  afterAppear,
  appearCancelled,
  duration
 } = data

 
 let context = activeInstance
 let transitionNode = activeInstance.$vnode

 const isAppear = !context._isMounted || !vnode.isRootInsert

 if (isAppear && !appear && appear !== '') {
  return
 }
 // 獲取合適的時(shí)機(jī)應(yīng)該注入的className
 const startClass = isAppear && appearClass
  ? appearClass
  : enterClass
 const activeClass = isAppear && appearActiveClass
  ? appearActiveClass
  : enterActiveClass
 const toClass = isAppear && appearToClass
  ? appearToClass
  : enterToClass

 const beforeEnterHook = isAppear
  ? (beforeAppear || beforeEnter)
  : beforeEnter
 const enterHook = isAppear
  ? (typeof appear === 'function' ? appear : enter)
  : enter
 const afterEnterHook = isAppear
  ? (afterAppear || afterEnter)
  : afterEnter
 const enterCancelledHook = isAppear
  ? (appearCancelled || enterCancelled)
  : enterCancelled

 const explicitEnterDuration: any = toNumber(
  isObject(duration)
   ? duration.enter
   : duration
 )

 const expectsCSS = css !== false && !isIE9
 const userWantsControl = getHookArgumentsLength(enterHook)
 // 過(guò)渡結(jié)束之后的回調(diào)處理，刪掉進(jìn)入時(shí)的class
 const cb = el._enterCb = once(() => {
  if (expectsCSS) {
   removeTransitionClass(el, toClass)
   removeTransitionClass(el, activeClass)
  }
  if (cb.cancelled) {
   if (expectsCSS) {
    removeTransitionClass(el, startClass)
   }
   enterCancelledHook && enterCancelledHook(el)
  } else {
   afterEnterHook && afterEnterHook(el)
  }
  el._enterCb = null
 })


 // dom進(jìn)入時(shí)，添加start class進(jìn)行過(guò)渡
 beforeEnterHook && beforeEnterHook(el)
 if (expectsCSS) {
  // 設(shè)置過(guò)渡開(kāi)始之前的默認(rèn)樣式
  addTransitionClass(el, startClass)
  addTransitionClass(el, activeClass)
  // 瀏覽器渲染下一幀 刪除默認(rèn)樣式，添加toClass
  // 添加end事件監(jiān)聽(tīng)，回調(diào)就是上面的cb
  nextFrame(() => {
   removeTransitionClass(el, startClass)
   if (!cb.cancelled) {
    addTransitionClass(el, toClass)
    if (!userWantsControl) {
     if (isValidDuration(explicitEnterDuration)) {
      setTimeout(cb, explicitEnterDuration)
     } else {
      whenTransitionEnds(el, type, cb)
     }
    }
   }
  })
 }

 if (vnode.data.show) {
  toggleDisplay && toggleDisplay()
  enterHook && enterHook(el, cb)
 }

 if (!expectsCSS && !userWantsControl) {
  cb()
 }
}

export let transitionEndEvent = 'transitionend'
export let animationEndEvent = 'animationend'
export function whenTransitionEnds (
 el: Element,
 expectedType: ?string,
 cb: Function
) {
 const { type, timeout, propCount } = getTransitionInfo(el, expectedType)
 if (!type) return cb()
 const event: string = type === TRANSITION ? transitionEndEvent : animationEndEvent
 let ended = 0
 const end = () => {
  el.removeEventListener(event, onEnd)
  cb()
 }
 const onEnd = e => {
  if (e.target === el) {
   if (++ended >= propCount) {
    end()
   }
  }
 }
 setTimeout(() => {
  if (ended < propCount) {
   end()
  }
 }, timeout + 1)
 el.addEventListener(event, onEnd)
}

class Transition extends React.Component {
 static contextType = TransitionGroupContext

 constructor(props, context) {
  super(props, context)
  let parentGroup = context
  let appear =
   parentGroup && !parentGroup.isMounting ? props.enter : props.appear

  let initialStatus

  this.appearStatus = null

  if (props.in) {
   if (appear) {
    initialStatus = EXITED
    this.appearStatus = ENTERING
   } else {
    initialStatus = ENTERED
   }
  } else {
   if (props.unmountOnExit || props.mountOnEnter) {
    initialStatus = UNMOUNTED
   } else {
    initialStatus = EXITED
   }
  }

  this.state = { status: initialStatus }

  this.nextCallback = null
 }

 // 初始dom的時(shí)候，更新默認(rèn)初始狀態(tài)
 componentDidMount() {
  this.updateStatus(true, this.appearStatus)
 }
 // data更新的時(shí)候，更新對(duì)應(yīng)的狀態(tài)
 componentDidUpdate(prevProps) {
  let nextStatus = null
  if (prevProps !== this.props) {
   const { status } = this.state

   if (this.props.in) {
    if (status !== ENTERING && status !== ENTERED) {
     nextStatus = ENTERING
    }
   } else {
    if (status === ENTERING || status === ENTERED) {
     nextStatus = EXITING
    }
   }
  }
  this.updateStatus(false, nextStatus)
 }

 updateStatus(mounting = false, nextStatus) {
  if (nextStatus !== null) {
   // nextStatus will always be ENTERING or EXITING.
   this.cancelNextCallback()

   if (nextStatus === ENTERING) {
    this.performEnter(mounting)
   } else {
    this.performExit()
   }
  } else if (this.props.unmountOnExit && this.state.status === EXITED) {
   this.setState({ status: UNMOUNTED })
  }
 }

 performEnter(mounting) {
  const { enter } = this.props
  const appearing = this.context ? this.context.isMounting : mounting
  const [maybeNode, maybeAppearing] = this.props.nodeRef
   ? [appearing]
   : [ReactDOM.findDOMNode(this), appearing]

  const timeouts = this.getTimeouts()
  const enterTimeout = appearing ? timeouts.appear : timeouts.enter
  // no enter animation skip right to ENTERED
  // if we are mounting and running this it means appear _must_ be set
  if ((!mounting && !enter) || config.disabled) {
   this.safeSetState({ status: ENTERED }, () => {
    this.props.onEntered(maybeNode)
   })
   return
  }

  this.props.onEnter(maybeNode, maybeAppearing)

  this.safeSetState({ status: ENTERING }, () => {
   this.props.onEntering(maybeNode, maybeAppearing)

   this.onTransitionEnd(enterTimeout, () => {
    this.safeSetState({ status: ENTERED }, () => {
     this.props.onEntered(maybeNode, maybeAppearing)
    })
   })
  })
 }

 performExit() {
  const { exit } = this.props
  const timeouts = this.getTimeouts()
  const maybeNode = this.props.nodeRef
   ? undefined
   : ReactDOM.findDOMNode(this)

  // no exit animation skip right to EXITED
  if (!exit || config.disabled) {
   this.safeSetState({ status: EXITED }, () => {
    this.props.onExited(maybeNode)
   })
   return
  }

  this.props.onExit(maybeNode)

  this.safeSetState({ status: EXITING }, () => {
   this.props.onExiting(maybeNode)

   this.onTransitionEnd(timeouts.exit, () => {
    this.safeSetState({ status: EXITED }, () => {
     this.props.onExited(maybeNode)
    })
   })
  })
 }

 cancelNextCallback() {
  if (this.nextCallback !== null) {
   this.nextCallback.cancel()
   this.nextCallback = null
  }
 }

 safeSetState(nextState, callback) {
  // This shouldn't be necessary, but there are weird race conditions with
  // setState callbacks and unmounting in testing, so always make sure that
  // we can cancel any pending setState callbacks after we unmount.
  callback = this.setNextCallback(callback)
  this.setState(nextState, callback)
 }

 setNextCallback(callback) {
  let active = true

  this.nextCallback = event => {
   if (active) {
    active = false
    this.nextCallback = null

    callback(event)
   }
  }

  this.nextCallback.cancel = () => {
   active = false
  }

  return this.nextCallback
 }
 // 監(jiān)聽(tīng)過(guò)渡end
 onTransitionEnd(timeout, handler) {
  this.setNextCallback(handler)
  const node = this.props.nodeRef
   ? this.props.nodeRef.current
   : ReactDOM.findDOMNode(this)

  const doesNotHaveTimeoutOrListener =
   timeout == null && !this.props.addEndListener
  if (!node || doesNotHaveTimeoutOrListener) {
   setTimeout(this.nextCallback, 0)
   return
  }

  if (this.props.addEndListener) {
   const [maybeNode, maybeNextCallback] = this.props.nodeRef
    ? [this.nextCallback]
    : [node, this.nextCallback]
   this.props.addEndListener(maybeNode, maybeNextCallback)
  }

  if (timeout != null) {
   setTimeout(this.nextCallback, timeout)
  }
 }

 render() {
  const status = this.state.status

  if (status === UNMOUNTED) {
   return null
  }

  const {
   children,
   // filter props for `Transition`
   in: _in,
   mountOnEnter: _mountOnEnter,
   unmountOnExit: _unmountOnExit,
   appear: _appear,
   enter: _enter,
   exit: _exit,
   timeout: _timeout,
   addEndListener: _addEndListener,
   onEnter: _onEnter,
   onEntering: _onEntering,
   onEntered: _onEntered,
   onExit: _onExit,
   onExiting: _onExiting,
   onExited: _onExited,
   nodeRef: _nodeRef,
   ...childProps
  } = this.props

  return (
   // allows for nested Transitions
   <TransitionGroupContext.Provider value={null}>
    {typeof children === 'function'
     ? children(status, childProps)
     : React.cloneElement(React.Children.only(children), childProps)}
   </TransitionGroupContext.Provider>
  )
 }
}