Vue v-if 指令

示例

使用 v-if 指令创建一个 <div> 元素，如果条件为 'true'。

<div v-if="createImgDiv">
  <img src="/img_apple.svg" alt="apple">
  <p>This is an apple.</p>
</div>

示例 1

使用 v-if 将数据属性作为条件表达式，并结合 v-else。

示例 2

使用 v-if 将比较检查作为条件表达式，并结合 v-else。

示例 3

结合使用 v-if、v-else-if 和 v-else，根据库存的打字机数量显示状态消息。

示例 4

使用 v-if 将原生 JavaScript 方法作为条件表达式，并结合 v-else。

示例 5

当从 API 接收到数据时，使用 v-if 渲染 <div> 标签。

<template>
  <h1>Example</h1>
  <p>Click the button to fetch data with an HTTP request.</p>
  <p>Each click generates an object with a random user from <a href="https://random-data-api.com/" target="_blank">https://random-data-api.com/</a>.</p>
  <p>The robot avatars are lovingly delivered by <a href="http://Robohash.org" target="_blank">RoboHash</a>.</p>
  <button @click="fetchData">Fetch data</button>
  <div v-if="data" id="dataDiv">
    <img :src="data.avatar" alt="avatar">
    <pre>{{ data.first_name + " " + data.last_name }}</pre>
    <p>"{{ data.employment.title }}"</p>
  </div>
</template>

<script>
  export default {
    data() {
      return {
        data: null,
      };
    },
    methods: {
      async fetchData() {      
        const response = await fetch("https://random-data-api.com/api/v2/users"); 
        this.data = await response.json();
      },    
    }
  };
</script>

<style>
#dataDiv {
  width: 240px;
  background-color: aquamarine;
  border: solid black 1px;
  margin-top: 10px;
  padding: 10px;
}
#dataDiv > img {
  width: 100%;
}
pre {
  font-size: larger;
  font-weight: bold;
}
</style>

示例 6

使用 v-if 创建组件，以便触发 mounted 生命周期钩子。

CompOne.vue:

<template>
    <h2>Component</h2>
    <p>Right after this component is added to the DOM, the mounted() function is called and we can add code to that mounted() function. In this example, an alert popup box appears after this component is mounted.</p>
    <p><strong>Note:</strong> The reason that the alert is visible before the component is visible is because the alert is called before the browser gets to render the component to the screen.</p>
  </template>
  
  <script>
  export default {
    mounted() {
      alert("The component is mounted!");
    }
  }
  </script>

App.vue:

<template>
  <h1>The 'mounted' Lifecycle Hook</h1>
  <button @click="this.activeComp = !this.activeComp">Create component</button>
  <div>
    <comp-one v-if="activeComp"></comp-one>
  </div>
</template>

<script>
export default {
  data() {
    return {
      activeComp: false
    }
  }
}
</script>

<style scoped>
  div {
    border: dashed black 1px;
    border-radius: 10px;
    padding: 20px;
    margin: 10px;
    width: 400px;
    background-color: lightgreen;
  }
</style>

示例 7

使用 v-if 切换 <p> 元素，以便触发动画。

<template>
  <h1>Add/Remove <p> Tag</h1>
  <button @click="this.exists = !this.exists">{{btnText}}</button><br>
  <Transition>
    <p v-if="exists">Hello World!</p>
  </Transition>
</template>

<script>
export default {
  data() {
    return {
      exists: false
    }
  },
  computed: {
    btnText() {
      if(this.exists) {
        return 'Remove';
      }
      else {
        return 'Add';
      }
    }
  }
}
</script>

<style scoped>
  .v-enter-from {
    opacity: 0;
    translate: -100px 0;
  }
  .v-enter-to {
    opacity: 1;
    translate: 0 0;
  }
  .v-leave-from {
    opacity: 1;
    translate: 0 0;
  }
  .v-leave-to {
    opacity: 0;
    translate: 100px 0;
  }
  p {
    background-color: lightgreen;
    display: inline-block;
    padding: 10px;
    transition: all 0.5s;
  }
</style>