vue从入门到进阶：计算属性computed与侦听器watch（三）

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

>
  p>Original message: "{{ message }}">Computed reversed message: "{{ reversedMessage }}">
>

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('')
    }
  }
})

Original message: "Hello"
Computed reversed message: "olleH"

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

<p>Reversed message: "{{ reversedMessage() }}"</p>

 在组件中
methods: {
  reversedMessage:  () {
    )
  }
}

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

="demo">{{ fullName }} Vue({
  el: '#demo''Bar' (val) {
      this.fullName = val + ' ' + this.lastName
    },lastName: this.fullName = this.firstName + ' ' + val
    }
  }
})

this.firstName + ' ' + .lastName
    }
  }
})

 ...
computed: {
  fullName: {
     getter
    get:  setter
    set:  (newValue) {
      var names = newValue.split(' ')
      this.firstName = names[0]
      this.lastName = names[names.length - 1]
    }
  }
}
 ...

="watch-example"
    Ask a yes/no question:
    input v-model="question">{{ answer }}>

<!-- 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 =  Vue({
  el: '#watch-example''I cannot give you an answer until you ask a question!' 如果 `question` 发生改变，这个函数就会运行
    question:  (newQuestion) {
      this.answer = 'Waiting for you to stop typing...'
      .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(
       () {
        if (this.question.indexOf('?') === -1) {
          this.answer = 'Questions usually contain a question mark. ;-)'
          
        }
        this.answer = 'Thinking...'
        
        axios.get('https://yesno.wtf/api')
          .then( (response) {
            vm.answer = _.capitalize(response.data.answer)
          })
          .catch( (error) {
            vm.answer = 'Error! Could not reach the API. ' + error
          })
      },1)"> 这是我们为判定用户停止输入等待的毫秒数
      500
    )
  }
})
</script>

 键路径
vm.$watch('a.b.c', (newVal,oldVal) {
   做点什么
})

="app">
    type="text" v-model="a"  />
    ="b"  />
>

函数
 Vue({
  el: '#app''文本2'this.a + .b
  },oldVal) {
    console.log(newVal,oldVal);
  }
)

var unwatch = vm.$watch('a' 之后取消观察
unwatch()

vm.$watch('someObject'true
})
vm.someObject.nestedValue = 123
 callback is fired

vm.$watch('a'
})
 立即以 `a` 的当前值触发回调