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203 lines
4.5 KiB
Markdown
203 lines
4.5 KiB
Markdown
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## 定时器的常见方法
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- setInterval():循环定时器。周而复始的执行(循环执行)
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- setTimeout():定时炸弹。用完以后立刻报废(只执行一次)
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### 定义定时器的方式
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**方式一:**匿名函数
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每间隔一秒打印一次:
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```javascript
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setInterval(function () {
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console.log(1);
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},1000);
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```
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**方式二:**
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每间隔一秒打印一次:
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```javascript
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setInterval(fn,1000);
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function fn(){
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console.log(1);
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}
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```
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### 定时器高级:清除定时器
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定时器的返回值可以用来清除定时器。具体方法是:假设定时器setInterval()的返回值是`参数1`,那么`clearInterval(参数1)`就可以清除定时器。
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setTimeout()的道理是一样的。
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代码举例:
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```html
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<script>
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var num = 1;
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var timer = setInterval(function () {
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console.log(num); //每间隔一秒,打印一次num的值
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num ++;
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if(num ===5) { //打印四次之后,就清除定时器
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clearInterval(timer);
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}
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}, 1000);
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</script>
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```
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## 定时器举例
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### 举例一:5秒后关闭网页两侧的广告栏
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假设网页两侧的广告栏为两个img标签,它们的样式为:
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```html
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<style>
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...
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...
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</style>
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```
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5秒后关闭广告栏的js代码为:
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```html
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<script>
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window.onload = function () {
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//获取相关元素
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var imgArr = document.getElementsByTagName("img");
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//设置定时器:5秒后关闭两侧的广告栏
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setTimeout(fn,5000);
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function fn(){
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imgArr[0].style.display = "none";
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imgArr[1].style.display = "none";
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}
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}
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</script>
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```
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### 举例二:关闭京东顶部广告栏(带动画效果关闭)
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我们在[之前的文章](https://github.com/smyhvae/Web/blob/master/03-JavaScript%E5%9F%BA%E7%A1%80/07-JavaScript%E5%9F%BA%E7%A1%80%EF%BC%9ADOM%E6%93%8D%E4%BD%9C.md)中做过这道题。但是现在,要求广告栏在关闭的时候,带动画效果:**点击关闭按钮后,顶部广告栏慢慢地变透明,直到全部关闭。**
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我们可以用定时器来做。完整版代码如下:
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```html
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<!DOCTYPE html>
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<html>
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<head lang="en">
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<meta charset="UTF-8">
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<title></title>
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<style>
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* {
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padding: 0;
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margin: 0;
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}
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.top-banner {
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background-color: pink;
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height: 80px;
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}
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.w {
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width: 1210px;
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margin: 10px auto;
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position: relative;
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}
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img {
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display: block;
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width: 1210px;
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height: 80px;
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background-color: blue;
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}
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a {
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position: absolute;
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top: 5px;
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right: 5px;
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color: #fff;
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background-color: #000;
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text-decoration: none;
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width: 20px;
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height: 20px;
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font: 700 14px/20px "simsum";
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text-align: center;
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}
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.hide {
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display: none !important;
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}
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.search {
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width: 1210px;
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height: 80px;
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background-color: green;
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margin: 0 auto;
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}
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</style>
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</head>
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<body>
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<div class="top-banner" id="topBanner" style="opacity: 1">
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<div class="w">
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<img src="blue" alt=""/>
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<a href="#" id="closeBanner">×</a>
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</div>
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</div>
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<div class="search">
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</div>
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<script>
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//需求:点击关闭按钮,先让top-banner这个盒子透明度变为0,紧接着display:none;
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//1.获取事件源和相关元素
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var closeBanner = document.getElementById("closeBanner");
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var topBanner = document.getElementById("topBanner");
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//定义定时器
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var timer = null;
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//2.绑定事件
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closeBanner.onclick = function () {
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//3.书写事件驱动程序(定时器,透明度变为0,清除定时器,并隐藏盒子)
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timer = setInterval(function () {
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topBanner.style.opacity -= 0.1;
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if (topBanner.style.opacity < 0) {
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topBanner.style.display = "none";
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clearInterval(timer);
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}
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}, 50);
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}
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</script>
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</body>
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</html>
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```
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代码解释:
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注意,我们要实现给顶部的div加一个行内样式`style="opacity: 1"`,然后才能通过定时器判断`topBanner.style.opacity`的值。
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定时器的返回值其实是number类型的,但我们习惯性地设置初始值为null。
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实现效果:
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![](http://img.smyhvae.com/20180201_2240.gif)
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