2018-02-01 22:58:37 +08:00
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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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2018-02-01 22:59:52 +08:00
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2018-02-01 22:58:37 +08:00
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