手写系列-防抖和节流
防抖和节流都是优化高频率执行代码的一种手段，例如浏览器的resize，scroll，mousemove等事件在触发时，会频繁的调用绑定在事件上的回调函数，极大的浪费资源，降低了前端的性能，因此就需要使用防抖和节流来优化代码
防抖：n秒后在执行该事件，若在n秒内重复触发，则重新计时
节流：n秒内只运行一次，若在n秒内重复触发，只有一次生效
防抖
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| <!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
* {
margin: 0;
padding: 0;
}
.box {
width: 200px;
height: 200px;
display: flex;
justify-content: center;
align-items: center;
background-color: gray;
}
</style>
</head>
<body>
<div class="box"></div>
<script>
window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
box.addEventListener('mousemove', function () {
box.textContent = count++;
})
})
</script>
</body>
</html>
|
如图所示，鼠标移动count+1，设想如果触发的是ajax请求，就太过频繁了
接下来改进代码：
防抖代码
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| <!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
* {
margin: 0;
padding: 0;
}
.box {
width: 200px;
height: 200px;
display: flex;
justify-content: center;
align-items: center;
background-color: gray;
}
</style>
</head>
<body>
<div class="box"></div>
<script>
window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction() {
box.textContent = count++;
}
box.addEventListener('mousemove', debounce(getUserAction, 1000));
})
function debounce(func, time) {
let timeout;
return function () {
clearTimeout(timeout);
timeout = setTimeout(func, time);
}
}
</script>
</body>
</html>
|
修复this
由于func函数的调用位置发生了改变，使得func的this指向了window而不是box，因此我们需要改变func中this的指向
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| window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction() {
console.log(this)
box.textContent = count++;
}
function debounce(func, time) {
let timeout;
let context;
return function () {
context = this;
clearTimeout(timeout);
timeout = setTimeout(function(){
func.apply(context)
}, time);
}
}
box.addEventListener('mousemove',debounce(getUserAction,1000))})
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修复event
此时在getUserAction中打印event位undefined，而我们希望event可以为mouseEvent
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| window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction(e) {
console.log(this)
console.log(e)
box.textContent = count++;
}
function debounce(func, time) {
let timeout;
let context,args;
return function () {
context = this;
args = arguments;
clearTimeout(timeout);
timeout = setTimeout(function(){
func.apply(context,arguments)
}, time);
}
}
box.addEventListener('mousemove',debounce(getUserAction,1000))})
|
立即执行
我们不希望非要等到事件停止触发后才执行，我希望立刻执行函数，然后等到停止触发 n 秒后，才可以重新触发执行
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| window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction(e) {
console.log(this, e)
box.textContent = count++;
}
function debounce(func, time, immediate) {
let timeout;
let context,args;
return function () {
context = this;
args = arguments;
console.log(arguments)
if(timeout) clearTimeout(timeout);
if (immediate) {
let callNow = !timeout;
timeout = setTimeout(function () {
timeout = null;
}, time);
if(callNow) func.apply(context,args)
} else {
timeout = setTimeout(function () {
func.apply(context, args)
}, time);
}
}
}
box.addEventListener('mousemove', debounce(getUserAction, 1000,true))
})
|
节流
如果持续触发事件，每隔一段时间只执行一次事件
使用时间戳
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| window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction() {
box.textContent = count++;
}
box.addEventListener('mousemove',throttle(getUserAction,1000))
function throttle(func,time){
let previous = 0;
let context,args;
return function(){
let now = +new Date();
context = this;
args = arguments;
if (now - previous >= time){
func.apply(func,args)
previous = now
}
}
}
})
|
使用定时器
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| window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction() {
box.textContent = count++;
}
box.addEventListener('mousemove',throttle(getUserAction,1000))
function throttle(func,time){
let context,args;
let timeout;
return function(){
context = this;
args = arguments;
if(!timeout){
timeout = setTimeout(function(){
timeout = null;
func.apply(context,args)
},time)
}
}
}
})
|
合体
对比上面两种方式，我们可以发现：
- 第一种事件会立刻执行，第二种事件会在 n 秒后第一次执行
- 第一种事件停止触发后没有办法再执行事件，第二种事件停止触发后依然会再执行一次事件
如果我们既需要事件立刻执行，又需要停止触发后依然会再次执行一次事件，那么就可以二者结合
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| window.addEventListener('load', function () {
let box = document.querySelector('.box');
let count = 0;
function getUserAction() {
box.textContent = count++;
}
box.addEventListener('mousemove',throttle(getUserAction,1000))
function throttle(func,time){
let context,args,timeout;
let previous = 0;
function throttled(){
let now = +new Date();
let remain = time - (now - previous);
context = this;
args = arguments;
if(remain <= 0 || remain > time){
if(timeout){
clearTimeout(timeout);
timeout = null;
}
func.apply(context,args)
previous = now;
}else if(!timeout){
timeout = setTimeout(function(){
previous = +new Date();
timeout = null;
func.apply(context,args)
},time)
}
}
return throttled;
}
})
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