155 lines
3.8 KiB
Markdown
155 lines
3.8 KiB
Markdown
## computed
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#### 计算属性
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- 计算属性是基于它们的响应式依赖进行缓存的
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```js
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<div id="example">
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<p>Original message: "{{ message }}"</p>
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<p>Computed reversed message: "{{ reversedMessage }}"</p>
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</div>
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var vm = new Vue({
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el: '#example',
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data: {
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message: 'Hello'
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},
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computed: {
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// 计算属性的 getter
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reversedMessage: function () {
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// `this` 指向 vm 实例
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return this.message.split('').reverse().join('')
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}
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}
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})
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<p>Reversed message: "{{ reversedMessage() }}"</p>
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// 在组件中
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methods: {
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reversedMessage: function () {
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return this.message.split('').reverse().join('')
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}
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}
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```
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#### 监听属性
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- 类似watch
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```js
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<div id="demo">{{ fullName }}</div>
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// watch
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var vm = new Vue({
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el: '#demo',
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data: {
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firstName: 'Foo',
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lastName: 'Bar',
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fullName: 'Foo Bar'
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},
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watch: {
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firstName: function (val) {
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this.fullName = val + ' ' + this.lastName
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},
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lastName: function (val) {
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this.fullName = this.firstName + ' ' + val
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}
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}
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})
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//
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var vm = new Vue({
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el: '#demo',
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data: {
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firstName: 'Foo',
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lastName: 'Bar'
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},
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computed: {
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fullName: function () {
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return this.firstName + ' ' + this.lastName
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}
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}
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})
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```
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#### set
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```js
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computed: {
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fullName: {
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// getter
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get: function () {
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return this.firstName + ' ' + this.lastName
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},
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// setter
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set: function (newValue) {
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var names = newValue.split(' ')
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this.firstName = names[0]
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this.lastName = names[names.length - 1]
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}
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}
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}
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// 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。
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```
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## 监听器
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- 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。
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- 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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```js
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<div id="watch-example">
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<p>
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Ask a yes/no question:
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<input v-model="question">
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</p>
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<p>{{ answer }}</p>
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</div>
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<!-- 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 -->
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<!-- 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。 -->
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<script src="https://cdn.jsdelivr.net/npm/axios@0.12.0/dist/axios.min.js"></script>
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<script src="https://cdn.jsdelivr.net/npm/lodash@4.13.1/lodash.min.js"></script>
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<script>
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var watchExampleVM = new Vue({
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el: '#watch-example',
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data: {
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question: '',
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answer: 'I cannot give you an answer until you ask a question!'
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},
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watch: {
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// 如果 `question` 发生改变，这个函数就会运行
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question: function (newQuestion, oldQuestion) {
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this.answer = 'Waiting for you to stop typing...'
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this.debouncedGetAnswer()
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}
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},
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created: function () {
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// `_.debounce` 是一个通过 Lodash 限制操作频率的函数。
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// 在这个例子中，我们希望限制访问 yesno.wtf/api 的频率
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// AJAX 请求直到用户输入完毕才会发出。想要了解更多关于
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// `_.debounce` 函数 (及其近亲 `_.throttle`) 的知识，
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// 请参考：https://lodash.com/docs#debounce
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this.debouncedGetAnswer = _.debounce(this.getAnswer, 500)
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},
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methods: {
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getAnswer: function () {
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if (this.question.indexOf('?') === -1) {
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this.answer = 'Questions usually contain a question mark. ;-)'
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return
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}
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this.answer = 'Thinking...'
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var vm = this
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axios.get('https://yesno.wtf/api')
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.then(function (response) {
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vm.answer = _.capitalize(response.data.answer)
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})
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.catch(function (error) {
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vm.answer = 'Error! Could not reach the API. ' + error
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})
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}
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}
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})
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</script>
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```
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