简化Tween的用法

先放场景示例

const manager = new TweenManager({duration, start, end, easing})
while(manager.next()) {
    await TweenManager.frame()
    const value = manager.currentValue
    // do something
}
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引子

写js动画是件挺麻烦的事.须要缓动函数,须要定时器,递归的话须要考虑边界条件.

并且缓动函数用起来也很别扭,参数多,又难记.t,b,c,d每次用都得去看下注释.

最近用到一个浏览器的API,createTreeWalker,这个方法返回一个TreeWalker对象.能够用来遍历dom树.

用法以下

while(tree.nextNode()){
    const dom = tree.currentNode
    // dosomething
}
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一时间思惟发散.若是按照这种设计方式来搞一个缓动函数的管理类,应该能够简化Tween的使用方式,省的每次去记API.

帧管理

实现上一节提到的还不够.写动画麻烦还在于requestAnimationFrame这个API用起来也是挺麻烦的.须要递归调用.

好比一个回到顶部的动画

function backTop(duration) {
    const doc = document.body.scrollTop ? document.body : document.documentElement;
    const start = doc.scrollTop;
    const stamp = Date.now()

    function step() {
        let currentStep = Date.now() - stamp
        if (currentStep >= duration) {
            doc.scrollTop = 0
            return
        }
        doc.scrollTop = Tween.Linear(currentStep, start, -start, duration)
        requestAnimationFrame(step)
    }

    step()
}
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写法是比较繁琐的.仔细考虑,其实递归只是指望定时执行而已.若是是其余语言.一个sleep就搞定了.

JavaScript虽然没有sleep,但有了async/await,咱们也能够实现一个.

const sleep = time => new Promise(resolve => setTimeout(resolve, time));

 (async () => {
     console.log('1');
     await sleep(1000);
     console.log('2');
 })();
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而requestAnimationFrame其实能够与sleep(16)互相代替,那咱们能够实现一个frame方法.

const frame = () => requestAnimationFrame ? new Promise(requestAnimationFrame) : sleep(16);
 (async () => {
     await frame();
     console.log('1 frame');
 })();
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具体实现

咱们指望的用法肯定了.接下来实现.其实代码很少.

const Tween = {
    Linear: function (t, b, c, d) {
      return c * t / d + b;
    },
}
const dftOption = {
    duration: 300,
    start: 0,
    end: 0,
    easing: Tween.Linear,
}
class TweenManager {
    get distance() {
      return this.$options.end - this.$options.start
    }

    get now() {
      return Date.now ? Date.now() : new Date().getTime()
    }

    get currentStep() {
      return this.now - this.stamp
    }

    get currentValue() {
      const {distance, currentStep} = this
      const {duration, easing, start} = this.$options
      return easing(currentStep, start, distance, duration)
    }

    constructor(opt = {}) {
      this.$options = {...dftOption, ...opt}
      this.stamp = this.now
    }

    next() {
      return this.$options.duration > this.currentStep
    }

    static sleep(time = 0) {
      return new Promise(resolve => setTimeout(resolve, time))
    }

    static frame() {
      return requestAnimationFrame ? new Promise(requestAnimationFrame) : TweenManager.sleep(16)
    }
}
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使用示例

一样是回到顶部的动画,能够简化成这样.

function backTop(duration) {
	const doc = document.body.scrollTop ? document.body : document.documentElement;
	const start = doc.scrollTop;
	const manager = new TweenManager({duration, start, end: 0, easing: Tween.Linear});
	while (manager.next()) {
	  await TweenManager.frame();
	  doc.scrollTop = manager.currentValue;
	}
	console.log('done');
}
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好像没少多少???

嗯,心智负担降了一丢丢.多少仍是方便一点了吧...

溜了溜了...