vue計算屬性和監聽器實例解析

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

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

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

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

// ...
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]
 }
 }
}
// ...

<div id="watch-example">
 <p>
 Ask a yes/no question:
 <input v-model="question">
 </p>
 <p>{{ answer }}</p>
</div>
<!-- 因為 AJAX 庫和通用工具的生態已經相當豐富，Vue 核心代碼沒有重復 -->
<!-- 提供這些功能以保持精簡。這也可以讓你自由選擇自己更熟悉的工具。 -->
<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` 發生改變，這個函數就會運行
 question: function (newQuestion, oldQuestion) {
 this.answer = 'Waiting for you to stop typing...'
 this.getAnswer()
 }
 },
 methods: {
 // `_.debounce` 是一個通過 Lodash 限制操作頻率的函數。
 // 在這個例子中，我們希望限制訪問 yesno.wtf/api 的頻率
 // AJAX 請求直到用戶輸入完畢才會發出。想要了解更多關于
 // `_.debounce` 函數 (及其近親 `_.throttle`) 的知識，
 // 請參考：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
 })
 },
 // 這是我們為判定用戶停止輸入等待的毫秒數
 500
 )
 }
})
</script>