【Vue 开发实战】基础篇 # 9：合理应用计算属性和侦听器

说明

【Vue 开发实战】学习笔记。

计算属性 computed

• 减少模板中计算逻辑
• 数据缓存.
• 依赖固定的数据类型(响应式数据)

<template>
  <div>
    <p>Reversed message1: "{{ reversedMessage1 }}"</p>
    <p>Reversed message2: "{{ reversedMessage2() }}"</p>
    <p>{{ now }}</p>
    <button @click="() => $forceUpdate()">forceUpdate</button>
    <br />
    <input v-model="message" />
  </div>
</template>
<script>export default {
  data() {
    return {
      message: "hello vue"
    };
  },
  computed: {
    // 计算属性的 getter
    reversedMessage1: function() {
      console.log("执行reversedMessage1");
      return this.message
        .split("")
        .reverse()
        .join("");
    },
    now: function() {
      return Date.now();
    }
  },
  methods: {
    reversedMessage2: function() {
      console.log("执行reversedMessage2");
      return this.message
        .split("")
        .reverse()
        .join("");
    }
  }
};</script>      

侦听器 watch

• 更加灵活、通用
• watch 中可以执行任何逻辑，如函数节流,，Ajax异步获取数据，甚至操作DOM

<template>
  <div>
    {{ $data }}
    <br />
    <button @click="() => (a += 1)">a+1</button>
  </div>
</template>
<script>export default {
  data: function() {
    return {
      a: 1,
      b: { c: 2, d: 3 },
      e: {
        f: {
          g: 4
        }
      },
      h: []
    };
  },
  watch: {
    a: function(val,) {
      this.b.c += 1;
      console.log("new: %s, old: %s", val, oldVal);
    },
    "b.c": function(val,) {
      this.b.d += 1;
      console.log("new: %s, old: %s", val, oldVal);
    },
    "b.d": function(val,) {
      this.e.f.g += 1;
      console.log("new: %s, old: %s", val, oldVal);
    },
    e: {
      handler: function(val,) {
        this.h.push("😄");
        console.log("new: %s, old: %s", val, oldVal);
      },
      deep: true
    },
    h(val,) {
      console.log("new: %s, old: %s", val, oldVal);
    }
  }
};</script>      

computed VS watch

• computed 能做的，watch都能做，反之则不行
• 能用 computed 的尽量用 computed

​

​Computed1.vue​

​

<template>
  <div>
    {{ fullName }}

    <div>firstName: <input v-model="firstName" /></div>
    <div>lastName: <input v-model="lastName" /></div>
  </div>
</template>
<script>export default {
  data: function() {
    return {
      firstName: "Foo",
      lastName: "Bar"
    };
  },
  computed: {
    fullName: function() {
      return this.firstName + " " + this.lastName;
    }
  },
  watch: {
    fullName: function(val,) {
      console.log("new: %s, old: %s", val, oldVal);
    }
  }
};</script>      

<template>
  <div>
    {{ fullName }}

    <div>firstName: <input v-model="firstName" /></div>
    <div>lastName: <input v-model="lastName" /></div>
  </div>
</template>
<script>export default {
  data: function() {
    return {
      firstName: "Foo",
      lastName: "Bar",
      fullName: "Foo Bar"
    };
  },
  watch: {
    firstName: function(val) {
      this.fullName = val + " " + this.lastName;
    },
    lastName: function(val) {
      this.fullName = this.firstName + " " + val;
    }
  }
};</script>