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>
// 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: "htmlcode">
function _enter (_: any, vnode: VNodeWithData) {
 if (vnode.data.show !== true) {
  enter(vnode)
 }
}
export default inBrowser "htmlcode">
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: "htmlcode">
export let transitionEndEvent = 'transitionend'
export let animationEndEvent = 'animationend'
export function whenTransitionEnds (
 el: Element,
 expectedType: "_blank" href="https://github.com/reactjs/react-transition-group" rel="external nofollow" >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:

通过ref获取dom,然后采用我们传统的js方案。
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

以上就是前端如何实现动画过渡效果的详细内容,更多关于前端实现动画过渡效果的资料请关注其它相关文章!

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