參考資料:Vue.js 官網vue
下面兩個代碼示例分別用計算屬性和方法實現同一個功能:ios
<!-- 計算屬示例 --> <p>Computed reversed message: "{{ reversedMessage }}"</p> <script> // ... computed: { reversedMessage: function () { return this.message.split('').reverse().join(''); } </script>
<!-- 方法示例 --> <p>Reversed message: "{{ reversedMessage() }}"</p> <script> // ... methods: { reversedMessage: function () { return this.message.split('').reverse().join(''); } } </script>
就上述兩個案例而言,咱們的分析以下:axios
偵聽屬性是一個對象,鍵是須要觀察的表達式,值是對應回調函數。值也能夠是方法名,或者包含選項的對象。api
當你有一些數據須要隨着其它數據變更而變更時,或者當須要在數據變化時執行異步或開銷較大的操做時,你可使用 watch。數組
在下面這個示例中,使用 watch 選項容許咱們執行異步操做 (訪問一個 API),限制咱們執行該操做的頻率,並在咱們獲得最終結果前,設置中間狀態。這些都是計算屬性沒法作到的。緩存
<div id="watch-example"> <p> Ask a yes/no question: <input v-model="question"> </p> <p>{{ answer }}</p> </div>
var watchExampleVM = new Vue({ el: '#watch-example', data: { question: '', answer: 'I cannot give you an answer until you ask a question!' }, watch: { // 若是 `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.debouncedGetAnswer() } }, created: function () { this.debouncedGetAnswer = _.debounce(this.getAnswer, 500) }, methods: { getAnswer: 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 }) } } })