Vue.js:如何在组件初始化时触发观察器函数

Sometimes I need to call some function in response to a change in a data property. But, I also need that function to fire for the initial value of the data property.

The watcher does not fire when the component initializes since the property being watched technically hasn't changed yet. So, I end up putting the function in the methods object, and then calling that method in the watcher and the mounted hook.

Here's an example:

new Vue({ 
  el: '#app',
  data() {
    return {
      selectedIndex: 0,
    }
  },
  methods: {
    focusSelected() {
      this.$refs.input[this.selectedIndex].focus();
    }
  },
  watch: {
    selectedIndex() {
      this.focusSelected();
    }
  },
  mounted() {
    this.focusSelected();
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.4.2/vue.min.js"></script>
<div id="app">
  <div v-for="i in 4">
    <input ref="input"/>
    <button @click="selectedIndex = (i - 1)">Select</button>
  </div>
</div>

Is there a way for me to be able to have the watcher fire when the component initializes?

解决方案

Watchers in Vue have an option to provide an immediate value:

Passing in immediate: true in the option will trigger the callback immediately with the current value of the expression

In this case, you could set a watcher in the mounted hook:

new Vue({ 
  el: '#app',
  data() {
    return {
      selectedIndex: 0,
    }
  },
  mounted() {
    this.$watch('selectedIndex', (i) => {
      this.$refs.input[i].focus();
    }, { immediate: true });
  }
})

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/2.4.2/vue.min.js"></script>
<div id="app">
  <div v-for="i in 4">
    <input ref="input"/>
    <button @click="selectedIndex = (i - 1)">Select</button>
  </div>
</div>

---

You could also specify the immediate option for a watcher in the watch object like so:

watch: {
  foo: {
    immediate: true,
    handler(value) {
      this.bar = value;
    }
  }
}