如何实现动画过渡效果？

简介

动画这个概念非常宽泛，涉及各个领域，这里我们把范围缩小到前端网页应用层面上，不用讲游戏领域的Animate，一切从最简单的开始。

目前大部分网页应用都是基于框架开发的，比如Vue，React等，它们都是基于数据驱动视图的，那么让我们来对比一下，还没有这些框架的时候我们如何实现动画或者过渡效果，然后使用数据驱动又是如何实现的。

传统过渡动画

动画效果对体验有着非常重要的效果，但是对于很多开发者来讲，可能是个非常薄弱的环节。在css3出现之后，很多初学者最常用的动画过渡可能就是css3的能力了。

css过渡动画

css启动过渡动画非常简单，书写transition属性就可以了，下面写一个demo

<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;
}

效果还是很赞的，css3的transition基本满足了大部分动画需求，如果不满足还有真正的css3 animation。
animate-css
大名鼎鼎的css动画库，谁用谁知道。

不管是css3 transition 还是 css3 animation，我们简单使用都是通过切换class类名，如果要做回调处理，浏览器也提供了 ontransitionend ， onanimationend等动画帧事件，通过js接口进行监听即可。

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

ok，这就是css动画的基础了，通过js封装也可以实现大部分的动画过渡需求，但是局限性在与只能控制css支持的属性动画，相对来说控制力还是稍微弱一点。

js动画

js毕竟是自定义编码程序，对于动画的控制力就很强大了，而且能实现各种css不支持的效果。 那么 js 实现动画的基础是什么？
简单来讲，所谓动画就是在 时间轴上不断更新某个元素的属性，然后交给浏览器重新绘制，在视觉上就成了动画。废话少说，还是先来个栗子：

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

#app {
    width: 100px;
    height: 100px;
    background-color: red;
    border-radius: 50%;
}

// Tween仅仅是个缓动函数
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('动画结束重新开始')
        time = 0;
    }
    setTimeout(() => {
        startSport()
    }, fps)
}
startSport()

在时间轴上不断更新属性，可以通过setTimeout或者requestAnimation来实现。至于Tween缓动函数，就是类似于插值的概念，给定一系列变量，然后在区间段上可以获取任意时刻的值，纯数学公式，几乎所有的动画框架都会使用，想了解的可以参考张鑫旭的Tween.js

OK，这个极简demo也是js实现动画的核心基础了，可以看到我们通过程序完美的控制了过渡值的生成过程，所有其他复杂的动画机制都是这个模式。

传统和Vue/React框架对比

通过前面的例子，无论是css过渡还是js过渡，我们都是直接获取到 dom元素的，然后对dom元素进行属性操作。
Vue/React都引入了虚拟dom的概念，数据驱动视图，我们尽量不去操作dom，只控制数据，那么我们如何在数据层面驱动动画呢？

Vue框架下的过渡动画

可以先看一遍文档
Vue过渡动画
我们就不讲如何使用了，我们来分析一下Vue提供的transition组件是如何实现动画过渡支持的。

transition组件

先看transition组件代码，路径 “src/platforms/web/runtime/components/transition.js”
核心代码如下：

// 辅助函数，复制props的数据
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, // 抽象组件，意思是不会真实渲染成dom，辅助开发

  render (h: Function) {
    // 通过slots获取到真实渲染元素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属性，保存通过props传递的数据
    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
  }
}

可以看到，这个组件本身功能比较简单，就是通过slots拿到需要渲染的元素children，然后把 transition的props属性数据copy到data的transtion属性上，供后续注入生命周期使用，mergeVNodeHook就是做生命周期管理的。

modules/transition

接着往下看生命周期相关，路径：
src/platforms/web/runtime/modules/transition.js
先看默认导出：

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)
    } 
  }
} : {}

这里inBrowser就当做true，因为我们分析的是浏览器环境。
接着看enter 和 leave函数，先看enter:

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数据
  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
  }
  // 获取合适的时机应该注入的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)
  // 过渡结束之后的回调处理，删掉进入时的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进入时，添加start class进行过渡
  beforeEnterHook && beforeEnterHook(el)
  if (expectsCSS) {
   // 设置过渡开始之前的默认样式
    addTransitionClass(el, startClass)
    addTransitionClass(el, activeClass)
    // 浏览器渲染下一帧 删除默认样式，添加toClass
    // 添加end事件监听，回调就是上面的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()
  }
}

enter里使用了一个函数whenTransitionEnds，其实就是监听过渡或者动画结束的事件：

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)
}

OK,到了这里，根据上面源代码的注释分析，我们可以发现：

• Vue先是封装了一些列操作dom className的辅助方法addClass/removeClass等。
• 然后在生命周期enterHook之后，马上设置了startClass也就是enterClass的默认初始样式，还有activeClass
• 紧接着在浏览器nextFrame下一帧，移除了startClass，添加了toClass，并且添加了过渡动画的end事件监听处理
• 监听到end事件之后，调动cb，移除了toClass和activeClass

leave的过程和enter的处理过程是一样，只不过是反向添加移除className

结论：Vue的动画过渡处理方式和 传统dom本质上是一样，只不过融入了Vue的各个生命周期里进行处理，本质上还是在dom 添加删除的时机进行处理

React里的过渡动画

噢，我们翻篇了React的文档，也没有发现有过渡动画的处理。嘿，看来官方不原生支持。

但是我们可以自己实现，比如通过useState维护一个状态，在render里根据状态进行className的切换，但是复杂的该怎么办？

所幸在社区找到了一个轮子插件react-transition-group
嗯，直接贴源码，有了前面Vue的分析，这个非常容易理解，反而更简单：

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的时候，更新默认初始状态
  componentDidMount() {
    this.updateStatus(true, this.appearStatus)
  }
 // data更新的时候，更新对应的状态
  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
  }
  // 监听过渡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>
    )
  }
}

可以看到，和Vue是非常相似的，只不过这里变成了在React的各个生命周期函数了进行处理。

到了这里，我们会发现不管是Vue的transiton组件，还是React这个transiton-group组件，着重处理的都是css属性的动画。

数据驱动的动画

而实际场景中总是会遇到css无法处理的动画，这个时候，可以有两种解决方案：

1. 通过ref获取dom，然后采用我们传统的js方案。
2. 通过state状态维护绘制dom的数据，不断通过setState更新state类驱动视图自动刷新