Article From:https://www.cnblogs.com/jasonlai2016/p/9969563.html

1.Calculated attribute

For any complex logic, you should use computational properties

```<div id="example">
<p>Original message: "{{ message }}"</p>
<p>Computed reversed message: "{{ reversedMessage }}"</p>
</div>
var vm = new Vue({
el: '#example',
data: {
message: 'Hello'
},
computed: {
// Getter for calculating attributesReversed Message: function () {// `this` points to the VM instanceReturn this. message. sPlit ('). reverse (). join (')}}}```

a.Computing Attribute Cache vs Method

Computational properties are cached based on their dependencies.

Why do we need caching? Suppose we have a computational attribute A with high performance overhead, which needs to traverse a huge array and do a lot of computations. Then we may have other computational properties that depend on A. Without caching, we will inevitably execute A get multiple timesTer! If you don’t want a cache, use a method instead.

b.Computing attributes vs listening attributes

It’s usually better to use computational attributes rather than imperative watch callbacks (listening attributes)

```var vm = new Vue({
el: '#demo',
data: {
firstName: 'Foo',
lastName: 'Bar',
fullName: 'Foo Bar'
},
watch: {
firstName: function (val) {
this.fullName = val + ' ' + this.lastName
},
lastName: function (val) {
this.fullName = this.firstName + ' ' + val
}
}
})```
```var vm = new Vue({
el: '#demo',
data: {
firstName: 'Foo',
lastName: 'Bar'
},
computed: {
fullName: function () {
return this.firstName + ' ' + this.lastName
}
}
})```

c.Setter for calculating attributes

By default, getter is the only computational attribute, but you can also provide a setter if you need to:

```// ...
computed: {
fullName: {
// getter
get: function () {
return this.firstName + ' ' + this.lastName
},
// setter
set: function (newValue) {
var names = newValue.split(' ')
this.firstName = names[0]
this.lastName = names[names.length - 1]
}
}
}```

2.Monitor

Used when data needs to change.Executing asynchrony or overheadOperation.

```<div id="watch-example">
<p>
Ask a yes/no question:
<input v-model="question">
</p>
</div>```
```<!-- Because the ecosystem of AJAX libraries and general tools is already quite rich, the Vue core code is not duplicated.-->
<!-- Provide these features to keep it lean. It also gives you the freedom to choose tools that you are more familiar with.-->
<script src="https://cdn.jsdelivr.net/npm/axios@0.12.0/dist/axios.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/lodash@4.13.1/lodash.min.js"></script>
<script>
var watchExampleVM = new Vue({
el: '#watch-example',
data: {
question: '',
answer: 'I cannot give you an answer until you ask a question!'
},
watch: {
// If `question'changes, the function runs
question: function (newQuestion, oldQuestion) {
this.answer = 'Waiting for you to stop typing...'
}
},
created: function () {
// `_.debounce` Lodash is a function that limits the operating frequency.
// In this example, we want to limit the frequency of access to yesno.wtf/api
// AJAX The request will not be sent until the user has finished typing. Want to know more about
// `_.debounce` Knowledge of functions (and their close relatives `throttle'),
// Refer to: https://lodash.com/docs#debounce
},
methods: {
getAnswer: function () {
if (this.question.indexOf('?') === -1) {
this.answer = 'Questions usually contain a question mark. ;-)'
return
}
var vm = this
axios.get('https://yesno.wtf/api')
.then(function (response) {