How to use Vue.js computed properties with listeners

This time I will show you how to use Vue.js calculated properties and listeners. What are the precautions for using Vue.js calculated properties and listeners. The following is a practical case. Let’s take a look. take a look.

1. Overview

Calculated properties

The expression in the template is very convenient, but They were originally designed for simple calculations. Putting too much logic into a template can make it overweight and difficult to maintain. For example:

<p id="example">
 {{ message.split('').reverse().join('') }}
</p>

Here, the template is no longer simple declarative logic. You have to watch for a while to realize that here you want to display the flippedString of the variable message. It becomes more difficult to deal with when you want to reference the flipped string here multiple times in the template.

So, for any complex logic, you should use computed properties.

Basic example

<p id="example">
 <p>Original message: "{{ message }}"</p>
 <p>Computed reversed message: "{{ reversedMessage }}"</p>
</p>
var vm = new Vue({
 el: '#example',
 data: {
  message: 'Hello'
 },
 computed: {
  // 计算属性的 getter
  reversedMessage: function () {
   // `this` 指向 vm 实例
   return this.message.split('').reverse().join('')
  }
 }
})

Result:

## Here we declare a calculated property reversedMessage. The function we provide will be used as the getter function of the property vm.reversedMessage

console.log(vm.reversedMessage) // => 'olleH'
vm.message = 'Goodbye'
console.log(vm.reversedMessage) // => 'eybdooG'

You can bind computed properties in the template just like normal properties. Vue knows that vm.reversedMessage depends on vm.message, so when vm.message changes, all bindings that depend on vm.reversedMessage are also updated. And the best part is that we've created this dependency declaratively: the computed property's getter function has no side effects, making it easier to test and understand.

2. Computed property cache vs method

You may have noticed that we can achieve the same effect by calling methods in expressions

<p>Reversed message: "{{ reversedMessage() }}"</p>
// 在组件中
methods: {
 reversedMessage: function () {
  return this.message.split('').reverse().join('')
 }
}

We can define the same function as a method instead of a computed property. The end result is indeed exactly the same both ways. However, the difference is that computed properties are cached based on their dependencies. A computed property is only re-evaluated when its associated dependencies change. This means that as long as the message has not changed, multiple accesses to the reversedMessage calculated property will immediately return the previous calculated result without having to execute the function again.

This also means that the calculated property below will no longer be updated, because Date.now() is not a reactive dependency

computed: {
 now: function () {
  return Date.now()
 }
}

In contrast, whenever a re-render is triggered, calling The method will always execute the function again.

Why do we need caching?

Suppose we have a computational attribute A that has a relatively large performance overhead. It needs to traverse a huge array and do a lot of calculations. Then we might have other computed properties that depend on A . Without caching, we would inevitably execute A's getter multiple times! If you don't want caching, use methods instead.

3. Computed properties vs listening properties

Vue provides a more general way to observe and respond to data on Vue instances Changed: Listening properties. When you have some data that needs to change as

other data changes, it's easy to abuse watches - especially if you've used AngularJS before. However, it is often better to use computed properties instead of imperative watch callbacks.

Think about this example

<p id="demo">{{ fullName }}</p>
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
  }
 }
})

The above code is imperative and repetitive. Compare it to the computed property version:

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

Isn’t it much better.

4. Computed property setters

Computed properties only have getters by default, but you can also provide a setter when needed

// ...
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]
  }
 }
}
// ...

现在再运行 vm.fullName = 'John Doe' 时，setter 会被调用，vm.firstName 和 vm.lastName 也会相应地被更新。

五、侦听器

      虽然计算属性在大多数情况下更合适，但有时也需要一个自定义的侦听器。这就是为什么 Vue 通过 watch 选项提供了一个更通用的方法，来响应数据的变化。当需要在数据变化时执行异步或开销较大的操作时，这个方式是最有用的。

    例如：

<p id="watch-example">
 <p>
  Ask a yes/no question:
  <input v-model="question">
 </p>
 <p>{{ answer }}</p>
</p>
<!-- 因为 AJAX 库和通用工具的生态已经相当丰富，Vue 核心代码没有重复 -->
<!-- 提供这些功能以保持精简。这也可以让你自由选择自己更熟悉的工具。 -->
<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: {
  // 如果 `question` 发生改变，这个函数就会运行
  question: function (newQuestion, oldQuestion) {
   this.answer = 'Waiting for you to stop typing...'
   this.getAnswer()
  }
 },
 methods: {
  // `_.debounce` 是一个通过 Lodash 限制操作频率的函数。
  // 在这个例子中，我们希望限制访问 yesno.wtf/api 的频率
  // AJAX 请求直到用户输入完毕才会发出。想要了解更多关于
  // `_.debounce` 函数 (及其近亲 `_.throttle`) 的知识，
  // 请参考：https://lodash.com/docs#debounce
  getAnswer: _.debounce(
   function () {
    if (this.question.indexOf('?') === -1) {
     this.answer = 'Questions usually contain a question mark. ;-)'
     return
    }
    this.answer = 'Thinking...'
    var vm = this
    axios.get('https://yesno.wtf/api')
     .then(function (response) {
      vm.answer = _.capitalize(response.data.answer)
     })
     .catch(function (error) {
      vm.answer = 'Error! Could not reach the API. ' + error
     })
   },
   // 这是我们为判定用户停止输入等待的毫秒数
   500
  )
 }
})
</script>

    结果：当没有输入？号，那么显示如下：

    当有?时候会输出“yes”或者“no”

 具体案例效果地址：侦听器

 在这个示例中，使用 watch 选项允许我们执行异步操作 (访问一个 API)，限制我们执行该操作的频率，并在我们得到最终结果前，设置中间状态。这些都是计算属性无法做到的。

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