Let's talk about computed properties, methods and listeners in Vue
This article will help you learn Vue, talk about the basic theory and practical operation of Vue, and introduce the calculated properties, methods and listeners in Vue. I hope it will be helpful to everyone!
Requirements: In the data, there are two contents: firstName and lastName, and the page displays "Full NamefullName”.
As you can know from the previous article, we can achieve this requirement by directly using interpolation expressions:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>前端一万小时-计算属性、方法与侦听器</title>
<script src="./vue.js"></script>
</head>
<body>
<div id="app">
{{firstName + ' ' + lastName}} <!-- 2️⃣使用插值表达式展示全名。 -->
</div>
<script>
var vm = new Vue({
el: '#app',
data: { // 1️⃣在 data 中定义一组数据，里面有 firstName 和 lastName 两个内容；
firstName: 'Oli',
lastName: 'Zhao'
}
})
</script>
</body>
</html>
But the requirement is achieved , but at the same time there is some logic on the template. The template itself is just for display, there is no need to add logic in it! [Related recommendations: vuejs video tutorial, web front-end development]
The ideal way is to directly add the fullName variable to the interpolation expression. Display full name:
<div id="app">
{{fullName}}
</div>
<script>
var vm = new Vue({
el: '#app',
data: {
firstName: 'Oli',
lastName: 'Zhao',
fullName: 'Oli Zhao' // ❗️data 中添加了 fullName！
}
})
</script>
As you can see, there is no problem when the code is written like this.
But in fact the code is redundant: the original data already has firstName and lastName, which can be used to combine the full name, but it has been redefined again. fullName .
Next, three other methods will be introduced to solve this requirement.
1 Computed propertiescomputed
<div id="app">
{{fullName}}
</div>
<script>
var vm = new Vue({
el: '#app',
data: {
firstName: 'Oli',
lastName: 'Zhao'
},
computed: { // 1️⃣计算属性 computed 属于 Vue 实例的方法，直接定义在实例中；
fullName() { // 2️⃣计算属性的值 fullName 是一个函数；
return this.firstName + ' ' + this.lastName // ❗️一定要写上 return 把计算结果返回！
}
}
})
</script>After using calculated properties, the code will not be redundant at all. fullName is based on firstName and lastName are calculated, and the effect on the page is the same. 
There is another very important knowledge point about computed properties:
Computed properties are cached based on their responsive dependencies.
In other words, When the data on which the calculated attribute depends changes, it will be recalculated; if there is no change, it will not be calculated and the last calculation will be used. Result (This also improves some performance).
In our code, when firstName or lastName changes, fullName will be recalculated, and if it remains unchanged, the cached one will be used. calculation results.
<div id="app">
{{fullName}}
{{age}} <!-- 2️⃣把 age 渲染到模板上； -->
</div>
<script>
var vm = new Vue({
el: '#app',
data: {
firstName: 'Oli',
lastName: 'Zhao',
age: 18 // 1️⃣data 中添加 age 为 18；
},
computed: {
fullName() {
console.log('计算了一次') // 3️⃣每当执行时 console.log 一次。
return this.firstName + ' ' + this.lastName
}
}
})
</script>After saving and refreshing the web page, you can see that the calculated attribute is calculated once. When vm.age = 20 is entered in the console to change age, the page is re-rendered and "18" becomes "20", but the console does not output the calculation again. once, that is, the calculated attribute is not recalculated.
And when we change lastName, it will be recalculated:
2 Methodmethods
##The second is the method that we are already very familiar with methods:
<div id="app">
<!-- ❗️注意：在插值表达式内，计算属性通过属性的名字就可以用，但调用方法时，
一定要加上 () 来进行调用！ -->
{{fullName()}}
{{age}}
</div>
<script>
var vm = new Vue({
el: '#app',
data: {
firstName: 'Oli',
lastName: 'Zhao',
age: 18
},
methods: { // ❗️在 methods 中定义 fullName 方法，逻辑上与计算属性一样；
fullName() {
console.log('计算了一次')
return this.firstName + ' ' + this.lastName
}
}
})
</script>age, it will be calculated again, and when we change lastName, it will also be calculated again: 
method is not cached Mechanically, as long as the page is re-rendered, the fullName method will be re-executed.
3 Listenerwatch
When using a listener, it is inevitable to make the code redundant: <div id="app">
{{fullName}} <!-- 1️⃣-②：插值表达式内不需要 ()； -->
{{age}}
</div>
<script>
var vm = new Vue({
el: '#app',
data: {
firstName: 'Oli',
lastName: 'Zhao',
fullName: 'Oli Zhao', // 1️⃣-①：data 中定义 fullName；
age: 18
},
watch: { // 2️⃣在实例中定义侦听器 watch；
firstName() { /*
2️⃣-①：定义 firstName 方法，对 firstName 进行侦听，
当它改变时，重新对 fullName 赋值；
*/
console.log('计算了一次')
this.fullName = this.firstName + ' ' + this.lastName
},
lastName() { /*
2️⃣-②：定义 lastName 方法，对 lastName 进行侦听，
当它改变时，重新对 fullName 赋值。
*/
console.log('计算了一次')
this.fullName = this.firstName + ' ' + this.lastName
}
}
})
</script>fullName, watch will be displayed without calculation. After modifying the data age that is not related to fullName, since we only used watch to listen to firstName and lastName , so no calculation will be performed. When we modify lastName, a calculation is performed:
Summary:
-
methodswill execute the method once every time the page is rendered; -
watchandcomputedboth have caching mechanisms. But the syntax ofwatchis much more complicated than the syntax ofcomputed.
SoGenerally, when a function can be implemented through the watch, methods and computed methods, The first recommendation is "Computed propertycomputed".
(Learning video sharing: vuejs introductory tutorial, Basic programming video)
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