浅谈vue.js的computed
computed意为计算属性，官方称任何复杂逻辑，你都可以使用计算属性。 下面就购物车的小例子对computed进行讲解。 下面看2张运行截图：
首先我们看js代码：
var vm = new Vue({
el:"#materialTest",
data:{
//allMaterialList定义了所有的货物列表，属性有id，名称，数量，单价和是否再购物车中5个属性
allMaterialList:[
{id:0,name:"香蕉",num:1,unitPirce:5,isSelect:false},
{id:1,name:"苹果",num:1,unitPirce:1,isSelect:false},
{id:2,name:"菠萝",num:1,unitPirce:2,isSelect:false},
{id:3,name:"桃子",num:1,unitPirce:6,isSelect:false},
{id:4,name:"番茄",num:1,unitPirce:4,isSelect:false}
]
},
methods:{
//购物车增加删除的方法
changeMaterial:function(item,type){
if(!item){
item = this.allMaterialList[0];
}
var temp = this.allMaterialList[item.id];
temp.isSelect = type;
//注意，这里改变数组，或对象，要用下面的方式，简单的push是不能触发双向绑定的。
Vue.set(this.allMaterialList, item.id, temp);
}
},
computed:{
//computed计算选中的的物品
notSelectMaterialList:function(){
return this.allMaterialList.filter(function (item) {
return item.isSelect === false
})
},
//computed计算未选中的的物品
isSelectMaterialList:function(){
return this.allMaterialList.filter(function (item) {
return item.isSelect === true
})
},
//computed计算商品总价格
allCount:function(){
var count = 0;
for(var i = 0 ; i < this.isSelectMaterialList.length ; i++){
count += this.isSelectMaterialList[i].num * this.isSelectMaterialList[i].unitPirce;
}
return count;
}
},
created:function(){
this.changeMaterial('',true);
}
})
上面的js代码基本上显示了整个逻辑，购物车里面的货物可以增减，最终allCount来显示总价格。再看html代码：
<div id="materialTest">
<div class="material-list">
<p v-show="isSelectMaterialList.length > 0">已经选择的水果</p>
<div class="item-list" v-for="item in isSelectMaterialList">
<span v-text="'品名:'+item.name"></span>
<span v-text="'单价:'+item.unitPirce"></span>
<span>数量：<input type="number" v-model="item.num" min="0"></span>
<span @click="changeMaterial(item,false)">-</span>
</div>
</div>
<div class="all-material-list">
<p v-show="notSelectMaterialList.length > 0">所有水果</p>
<div class="item-list" v-for="item in notSelectMaterialList">
<span v-text="'品名:'+item.name"></span>
<span v-text="'单价:'+item.unitPirce"></span>
<span @click="changeMaterial(item,true)">+</span>
</div>
</div>
<p>总价格：${{allCount}}</p>
</div>
这里贴上完整的代码供大家参考：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>computed</title>
<script src="https://cdn.jsdelivr.net/npm/vue@2.4.4/dist/vue.min.js"></script>
<style>
.item-list span:last-child{
border:1px solid;
padding:0px 5px;
cursor: pointer;
}
.item-list{
margin-bottom: 10px;
}
</style>
</head>
<body>
<div id="materialTest">
<div class="material-list">
<p v-show="isSelectMaterialList.length > 0">已经选择的水果</p>
<div class="item-list" v-for="item in isSelectMaterialList">
<span v-text="'品名:'+item.name"></span>
<span v-text="'单价:'+item.unitPirce"></span>
<span>数量：<input type="number" v-model="item.num" min="0"></span>
<span @click="changeMaterial(item,false)">-</span>
</div>
</div>
<div class="all-material-list">
<p v-show="notSelectMaterialList.length > 0">所有水果</p>
<div class="item-list" v-for="item in notSelectMaterialList">
<span v-text="'品名:'+item.name"></span>
<span v-text="'单价:'+item.unitPirce"></span>
<span @click="changeMaterial(item,true)">+</span>
</div>
</div>
<p>总价格：${{allCount}}</p>
</div>
<p>Tips:总结,使用computed计算出了3个变量，notSelectMaterialList、isSelectMaterialList、allCount。大大简化了程序，computed还会缓存计算结果，比watch强。再者，当结果没发生变化时，并不会触发函数，但不意味computed没有观察到变化。</p>
</body>
<script>
var vm = new Vue({
el:"#materialTest",
data:{
allMaterialList:[
{id:0,name:"香蕉",num:1,unitPirce:5,isSelect:false},
{id:1,name:"苹果",num:1,unitPirce:1,isSelect:false},
{id:2,name:"菠萝",num:1,unitPirce:2,isSelect:false},
{id:3,name:"桃子",num:1,unitPirce:6,isSelect:false},
{id:4,name:"番茄",num:1,unitPirce:4,isSelect:false}
]
},
methods:{
changeMaterial:function(item,type){
if(!item){
item = this.allMaterialList[0];
}
var temp = this.allMaterialList[item.id];
temp.isSelect = type;
//注意，这里改变数组，或对象，要用下面的方式，简单的push是不能触发双向绑定的。
Vue.set(this.allMaterialList, item.id, temp);
}
},
computed:{
notSelectMaterialList:function(){ //使用计算属性过滤要显示的物品
return this.allMaterialList.filter(function (item) {
return item.isSelect === false
})
},
isSelectMaterialList:function(){
return this.allMaterialList.filter(function (item) {
return item.isSelect === true
})
},
allCount:function(){
var count = 0;
for(var i = 0 ; i < this.isSelectMaterialList.length ; i++){
count += this.isSelectMaterialList[i].num * this.isSelectMaterialList[i].unitPirce;
}
return count;
}
},
created:function(){
th-
s.-
hangeMaterial('',true);
}
})
</script>
</html>
后记：适当运用computed能大大简化我们的代码，计算属性默认只有 getter ，不过在需要时你也可以提供一个 setter，不过运用的少。最后欢迎评论，一起探讨，共同进步.
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