计算属性（computed），主要用于处理一些复杂逻辑。
基础例子
<div id="app">
<p>原始字符串: {{ message }}</p>
<p>计算后反转字符串: {{ reversedMessage }}</p>
</div>
<script>
var vm = new Vue({
el: '#app',
data: {
message: 'Runoob!'
},
computed: {
// 计算属性的 getter
reversedMessage: function () {
// `this` 指向 vm 实例
return this.message.split('').reverse().join('')
}
}
})
</script>
computed vs methods
我们可以使用 methods 来替代 computed，效果上两个都是一样的，但是 computed 是基于它的依赖缓存，只有相关依赖发生改变时才会重新取值。而使用 methods ，在重新渲染的时候，函数总会重新调用执行。使用 computed 性能会更好，但是如果你不希望缓存，可以使用 methods 属性。
例1：购物车价格计算
<!DOCTYPE html>
<html class="no-js">
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
<title>vue.js computed练习-计算购物车总价</title>
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<style type="text/css">
.container {
display: felx;
width: 370px;
margin: 0 auto;
flex-direction: column;
}
.item {
display: flex;
border: 1px solid #000;
border-radius: 10px;
width: 350px;
height: 50px;
margin-bottom: 10px;
/* 垂直方向居中 */
align-items: center;
/* 水平方向居中 */
/* justify-content: center; */
padding-left: 10px;
padding-right: 10px;
}
.item-id {
flex: 1 1 15%;
}
.item-cover {
flex: 1 1 10%;
}
.item-name {
flex: 1 1 30%;
}
.item-price {
flex: 1 1 20%;
}
.item-count {
flex: 1 1 25%;
}
.goods-count {
width: 20px;
}
.totalPrice {
display: flex;
width: 370px;
justify-content: space-between;
align-items: center;
}
.btn-settle {
width: 100px;
height: 30px;
background-color: #87CEEB;
border-radius: 5px;
border: none;
outline: none;
color: #FFF;
font-size: 16px;
}
</style>
</head>
<body>
<div id="app">
<div class="container">
<div class="item" v-for="goods in goodsList">
<div class="item-id">
{{goods.id}}
</div>
<div class="item-cover">
<a v-bind:href="goods.url" target="_blank"><img :src="goods.cover" width = "35"/></a>
</div>
<div class="item-name">
{{goods.name}}
</div>
<div class="item-price">
{{goods.price}}
</div>
<div class="item-count">
<button type="button" @click="goods.count -= 1" :disabled="goods.count === 0 || settled">-</button>
<input type="text" class="goods-count" v-model="goods.count" />
<button type="button" @click="goods.count += 1" :disabled="settled">+</button>
</div>
</div>
<div class="totalPrice">
<h3>Total Price</h3>
<p>￥{{totalPrice}}</p>
<button type="button" class="btn-settle" @click="settle" :disabled="settled">结算</button>
</div>
<div class="price" v-if="settled">
<p>您购买了{{totalCount}}件商品，需要支付总价为：{{totalPrice}}</p>
</div>
</div>
</div>
<script type="text/javascript">
var app = new Vue({
el: '#app',
data: {
goodsList: [
{
id : 1,
name : 'iPhone 8',
price : 3999,
url: "https://item.jd.com/5089267.html",
cover: "https://ss2.baidu.com/6ONYsjip0QIZ8tyhnq/it/u=3841252853,949538163&fm=58",
count : 1,
},
{
id : 2,
name : 'iPhone X',
price : 6349,
url: "https://item.jd.com/5089253.html",
cover: "https://ss2.baidu.com/6ONYsjip0QIZ8tyhnq/it/u=3159575759,329221210&fm=58",
count : 1,
},
{
id : 3,
name : 'iPhone Xs',
url: "https://item.jd.com/100000177748.html",
cover: "http://2c.zol-img.com.cn/product_small/13_120x90/816/cenfNF9Ndm2Y.jpg",
price : 7899,
count : 1,
},
],
settled: false,
},
methods: {
settle:function() {
this.settled = true;
}
},
computed: {
totalPrice:function() {
var totalPrice = 0;
for(var i = 0; i < this.goodsList.length; i ++) {
totalPrice += this.goodsList[i].price * this.goodsList[i].count;
}
return totalPrice;
},
totalCount:function() {
var totalCount = 0;
for(var i = 0; i < this.goodsList.length; i ++) {
totalCount += this.goodsList[i].count;
}
return totalCount;
}
},
})
</script>
</body>
</html>
例2：搜索页面
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>Vue.js computed练习-搜索页面的实现</title>
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
<script src="https://cdn.jsdelivr.net/npm/vue/dist/vue.js"></script>
<style type="text/css">
* {
margin: 0;
padding: 0;
box-sizing: border-box;
}
a {
text-decoration: none;
color: #333;
}
.container {
width: 95%;
margin: 0 auto;
}
.input {
display: flex;
width: 100%;
margin: 0 auto;
flex-direction: row;
margin-top: 10px;
margin-bottom: 20px;
}
.input-box {
flex: 1 1 70%;
margin-right: 5%;
border-radius: 5px;
border: 1px solid #eee;
}
.search-btn {
flex: 1 1 30%;
height: 30px;
background-color: #87CEEB;
border-radius: 5px;
border: none;
outline: none;
color: #FFF;
font-size: 16px;
}
.item {
display: flex;
border: 1px solid #eee;
border-radius: 8px;
margin-bottom: 8px;
height: 100%;
}
.item-text {
flex: 1 1 65%;
margin: 8px;
}
.item-title {
font-size: 18px;
font-weight: bold;
}
.item-content {
color: #B4B4B4;
font-size: 15px;
}
.item-thumbnail {
flex: 1 1 35%;
margin: 15px;
display: table-cell;
text-align: center;
vertical-align: middle;
}
.item-thumbnail img {
max-width: 100%;
height: 100px;
}
</style>
</head>
<body>
<div id="app">
<div class="container">
<div class="input">
<input type="text" v-model="searchString" placeholder=" 请输入" class="input-box" />
<button type="button" class="search-btn" @click="search">搜索</button>
</div>
<div v-if="flag">
<div class="item" v-for="article in filteredArticles">
<div class="item-text">
<p class="item-title"><a :href="article.url" target="_blank">{{article.title}}</a></p>
<p class="item-content">{{article.content}}</p>
</div>
<div class="item-thumbnail">
<img :src="article.image">
</div>
</div>
</div>
<div v-else>
<div class="item" v-for="article in articles">
<div class="item-text">
<p class="item-title"><a :href="article.url" target="_blank">{{article.title}}</a></p>
<p class="item-content">{{article.content}}</p>
</div>
<div class="item-thumbnail">
<img :src="article.image">
</div>
</div>
</div>
</div>
</div>
<script type="text/javascript">
var app = new Vue({
el: '#app',
data: {
searchString: "",
// 数据模型
articles: [{
"title": "堪称神器的3款在线工具，你一定用得上！",
"url": "https://www.jianshu.com/p/e83e7999346b",
"image": "https://img.haomeiwen.com/i11438996/56b25f32c9307b4b?imageMogr2/auto-orient/strip%7CimageView2/2/w/640/format/webp",
"content": "一款在线免费GIF编辑神器，提供在线GIF压缩、视频转GIF、GIF合成、GIF裁剪四个功能，用户无需安装任何插件就可以轻松的进行视频格式...",
},
{
"title": "经典面试题：从 URL 输入到页面展现到底发生什么？",
"url": "https://www.jianshu.com/p/45ba3e0d0c7e",
"image": "https://img.haomeiwen.com/i3973862/d90954249a6f6ccd.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1000/format/webp",
"content": "打开浏览器从输入网址到网页呈现在大家面前，背后到底发生了什么？经历怎么样的一个过程？先给大家来张总体流程图，具体步骤请看下文分..."
},
{
"title": "如何免翻墙使用谷歌搜索和Chrome应用商店",
"url": "https://www.jianshu.com/p/484f8e6c88f6",
"image": "https://img.haomeiwen.com/i858154/015a4b083685a3d1.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/800/format/webp",
"content": "可能大家都听过或正在使用谷歌浏览器，但是由于某种原因只能在谷歌浏览器使用百度搜索引擎，至于什么谷歌搜索引擎、谷歌商城、Gmail邮箱..."
},
{
"title": "四款前所未有好用的黑科技APP，绝对的良心实用，赶紧告诉家人",
"url": "https://www.jianshu.com/p/2aec84d269fe",
"image": "https://img.haomeiwen.com/i16042993/168b2cb17fd7ec0c?imageMogr2/auto-orient/strip%7CimageView2/2/w/640/format/webp",
"content": "手机微信、支付宝、淘宝等应用都是我们经常会使用到的APP，除此之外，我们就来就给大家带来几款更加有趣好玩的黑科技APP，绝对的良心实用..."
},
{
"title": "坚持学英语的方法有哪些",
"url": "https://www.jianshu.com/p/0a6a61b0933c",
"image": "https://img.haomeiwen.com/i3525704/c7293758fc59e56b.jpg?imageMogr2/auto-orient/strip%7CimageView2/2/w/960/format/webp",
"content": "学习英语没有什么捷径，至少我认为，我一直以来都是自学英语，从没有听过课堂上老师是怎么讲英语的，都是通过听广播和看视频学会的。我想说..."
}
],
flag : false,
},
methods: {
search: function() {
this.flag =! this.flag;
}
},
computed: {
// 计算函数，匹配搜索
filteredArticles: function() {
var articles_array = this.articles,
searchString = this.searchString;
//搜索关键词为空,则返回原始数据集
if (!searchString) {
return articles_array;
}
//搜索关键词去除无用空格,转换为小写
searchString = searchString.trim().toLowerCase();
//过滤数组中每个元素,如果
articles_array = articles_array.filter(function(item) {
if (item.title.toLowerCase().indexOf(searchString) !== -1 ) {
return item;
}
})
// 返回转化后的数组
return articles_array;
},
}
})
</script>
</body>
</html>
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