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computedProperties.html
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<!DOCTYPE html>
<html lang="zh-TW">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Computed Properties</title>
<link rel="stylesheet" href="./style.css">
</head>
<body>
<h1>Computed Properties & Watch</h1>
<h2>Computed Properties</h2>
<p>模版可以放運算邏輯，但是不建議放太複雜的邏輯，會降低可讀性</p>
<p>有<code>get</code>和<code>set</code>可以使用</p>
<p class="tip"><code>data</code>，就是監聽自己的變化進行更新，<code>computed的get</code>就是監聽很多變數的變化進行更新 - by curt</p>
<b>example</b>
<pre>//javascript
var vm = new Vue({
el: '#example',
data: {
message: 'Hello'
},
computed: {
// a computed getter
reversedMessage: function () {
// `this` points to the vm instance
return this.message.split('').reverse().join('')
}
}
})</pre>
<div id="example">
<pre>< p >Original message: "{ { data value } }"< /p ></pre>
<p>Original message: "{{ message }}"</p>
<pre>< p >Computed reversed message: "{ { function name } }"< /p ></pre>
<p>Computed reversed message: "{{ reversedMessage }}"</p>
</div>
<h2>Computed Caching vs Methods</h2>
<p>computed比method快！computed有「快取住結果」，method沒有。</p>
<div id="now">
<!-- <p>this is :"{{ test }}"</p> -->
<pre>now: "{ { now } }"</pre>
<p>now: "{{ now }}"<code>//只是值</code></p>
<pre>nowC: "{ { nowC } }"</pre>
<p>nowC: "{{ nowC }}"<code>用computed</code></p>
<pre>nowM: "{ { nowM() } }"</pre>
<p>nowM: "{{ nowM() }}"<code>用methods, 要加括號</code></p>
</div>
<h2>Computed vs Watched Property</h2>
<p>watching <code>dataKey1</code>, when <code>dataKey1</code> change, update <code>dataKey2</code></p>
<pre>{ { key } }</pre>
<pre> watch: {
dataKey1: function (dataValue1) {
this.dataKey2 = dataValue1 + ' ' + this.dataKey3
}
}</pre>
<b>example</b>
<pre>watch: {
firstName: function (val) {
this.fullName = val + ' ' + this.lastName
},
lastName: function (val) {
this.fullName = this.firstName + ' ' + val
}
}</pre>
<div id="demo">
<pre>fullName: { { fullName } }</pre>
<p>fullName: {{ fullName }}</p>
<pre>fullNameC: { { fullNameC } }</pre>
<p>fullNameC: {{ fullNameC }}</p>
<pre>firstName: { { firstName } }</pre>
<p>firstName: {{ firstName }}</p>
<pre>lastName: { { lastName } }</pre>
<p>lastName: {{ lastName }}</p>
</div>
<h2>Setter of Computed</h2>
<p>像C#這樣的用法</p>
<pre>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]
}
}
}</pre>
<div id="demo1">
<pre>fullName: { { fullName } }</pre>
<p>fullName: {{ fullName }}</p>
<pre>firstName: { { firstName } }</pre>
<p>firstName: {{ firstName }}</p>
<pre>lastName: { { lastName } }</pre>
<p>lastName: {{ lastName }}</p>
</div>
<h2>Watchers</h2>
<p>非<code>watch</code>不可的時候。</p>
<p>This is most useful when you want to perform asynchronous or expensive operations in response to changing data</p>
<b>watch implement</b>
<div id="watch-example">
<p> Ask a yes/no question:
<input v-model="question">
</p>
<p>{{ answer }}</p>
</div>
<b>computed implement</b>
<div id="watch-example1">
<p> Ask a yes/no question:
<input v-model="question">
</p>
<p>{{ answer }}</p>
</div>
<h2>所以...</h2>
<table>
<thead>
<tr>
<th></th>
<th>觸發方式</th>
<th>呼叫方式<br>必須和變數同名?</th>
<th>呼叫</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>data</code></td>
<td>「變數」被賦值，渲染view</td>
<td>o</td>
<td><code>vue.value</code></td>
</tr>
<tr>
<td><code>watch</code></td>
<td>「同名的變數」被賦值就執行</td>
<td>o</td>
<td><code>vue.value</code></td>
</tr>
<tr>
<td><code>computed</code></td>
<td>「取用的變數」被賦值就執行</td>
<td>x</td>
<td><code>vue.xValue</code></td>
</tr>
<tr>
<td><code>method</code></td>
<td>被呼叫就執行</td>
<td>x</td>
<td><code>vue.value()</code></td>
</tr>
</tbody>
</table>
<script src="https://unpkg.com/vue"></script>
<script type="text/javascript">
var vm = new Vue({
el: '#example',
data: {
message: 'Hello'
},
computed: {
// a computed getter
reversedMessage: function () {
// `this` points to the vm instance
return this.message.split('').reverse().join('')
}
}
})
var now = new Vue({
el: '#now',
data: {
test: 'Hello',
now: Date()
},
computed: {
nowC: function () {
return this.now
}
},
methods: {
nowM: function () {
return Date()
}
}
})
var name = new Vue({
el: '#demo',
data: {
firstName: 'Foo',
lastName: 'Bar',
fullName: 'Foo Bar'
},
computed: {
fullNameC: function () {
return this.firstName + ' ' + this.lastName
}
},
watch: {
firstName: function (val) {
this.fullName = val + ' ' + this.lastName
},
lastName: function (val) {
this.fullName = this.firstName + ' ' + val
}
}
})
var name = new Vue({
el: '#demo1',
data: {
firstName: 'Foo',
lastName: 'Bar',
},
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]
}
}
}
})
</script>
<!-- Since there is already a rich ecosystem of ajax libraries -->
<!-- and collections of general-purpose utility methods, Vue core -->
<!-- is able to remain small by not reinventing them. This also -->
<!-- gives you the freedom to just use what you're 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: {
// 如果 question 发生改变，这个函数就会运行
question: function (newQuestion, oldQuestion) {
console.log(this)
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
// 学习更多关于 _.debounce function (and its cousin
// _.throttle), 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
})
},
// 这是我们为用户停止输入等待的毫秒数
500
)
}
})
var watchExampleVM1 = new Vue({
el: '#watch-example1',
data: {
questionText: '',
answer: 'I cannot give you an answer until you ask a question!'
},
computed: {
// 如果 question 发生改变，这个函数就会运行
question: {
set: function (newQuestion) {
// != newQuestion
console.log(this)
this.answer = 'Waiting for you to stop typing...'
this.questionText = newQuestion
this.getAnswer(newQuestion)
},
get: function () {
console.log(this)
return this.questionText;
}
}
},
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
// 学习更多关于 _.debounce function (and its cousin
// _.throttle), 参考: https://lodash.com/docs#debounce
getAnswer: _.debounce(
function (newQuestion) {
if (newQuestion.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
})
},
// 这是我们为用户停止输入等待的毫秒数
500
)
}
})
</script>
</body>
</html>