Vue advanced technology: in-depth understanding of the implementation principles of v-if, v-show, v-else, v-else-if

Vue advanced technology: in-depth understanding of the implementation principles of v-if, v-show, v-else, v-else-if, specific code examples are required

In Vue, v-if, v-show, v-else and v-else-if are commonly used conditional rendering instructions. They can control the display and hiding of elements based on conditions. Although these instructions are common in development, their implementation principles are not very clear. This article will deeply analyze the implementation principles of v-if, v-show, v-else and v-else-if, and give specific code examples.

1. The implementation principle of v-if
v-if is a conditional rendering instruction, which determines whether to render an element or component based on conditions. When the condition is true, the corresponding element or component is rendered; when the condition is false, the corresponding element or component is not rendered. The implementation principle of v-if is as follows:

1. Vue will first evaluate the expression of v-if to determine whether the condition is true.
2. If the condition is true, Vue will create and insert the corresponding element or component.
3. If the condition is false, Vue will destroy the corresponding element or component and remove it from the DOM.

Code example:

<template>
  <div>
    <h1 v-if="show">Hello World!</h1>
  </div>
</template>

<script>
export default {
  data() {
    return {
      show: true
    }
  }
}
</script>

In the above code example, when show is true, render "

Hello World!

"; when show is When false, the element is not rendered.

2. The implementation principle of v-show
v-show is also a conditional rendering instruction. It has a similar function to v-if and can control the display and hiding of elements based on conditions. The difference is that v-show does not destroy the element, but controls the display and hiding of the element by modifying the element's display attribute.

The implementation principle of v-show is as follows:

1. Vue will first evaluate the expression of v-show to determine whether the condition is true.
2. If the condition is true, Vue will set the display attribute of the element to its original value.
3. If the condition is false, Vue will set the element's display attribute to none.

Code example:

<template>
  <div>
    <h1 v-show="show">Hello World!</h1>
  </div>
</template>

<script>
export default {
  data() {
    return {
      show: true
    }
  }
}
</script>

In the above code example, when show is true, the element is displayed by setting "display: block"; when show is false, by setting "display: none" to hide the element.

3. Implementation principles of v-else and v-else-if
v-else and v-else-if are supplementary instructions of v-if. They can be used after v-if. Use For handling multiple conditions.

The implementation principles of v-else and v-else-if are as follows:

1. v-else will occur only when the previous v-if or v-else-if condition is false will take effect.
2. v-else-if will take effect when the previous v-if condition is false and its own condition is true.

Code example:

<template>
  <div>
    <h1 v-if="score >= 90">优秀</h1>
    <h1 v-else-if="score >= 60">及格</h1>
    <h1 v-else>不及格</h1>
  </div>
</template>

<script>
export default {
  data() {
    return {
      score: 85
    }
  }
}
</script>

In the above code example, according to the value of score, the corresponding content is displayed through v-if, v-else-if and v-else.

To sum up, v-if, v-show, v-else and v-else-if are commonly used conditional rendering instructions in Vue. They are essentially implemented by controlling the display and hiding of elements. of. An in-depth understanding of their implementation principles will help us better use and optimize Vue's conditional rendering function.

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