How do I use uni-app's conditional rendering directives (v-if, v-else, v-show)?

How do I use uni-app's conditional rendering directives (v-if, v-else, v-show)?

In uni-app, conditional rendering directives like v-if, v-else, and v-show allow you to control the rendering of elements based on specific conditions. Here's how to use them:

1. v-if: This directive conditionally renders the element if the expression evaluates to true. If the expression is false, the element and its contained directives/expressions are not compiled or rendered.

   <code class="html"><view v-if="condition">This will be rendered if 'condition' is true</view></code>

2. v-else: This directive must immediately follow a v-if or a v-else-if element. It will render the element only if the previous conditional directive's expression evaluates to false.

   <code class="html"><view v-if="condition">This will be rendered if 'condition' is true</view>
   <view v-else>This will be rendered if 'condition' is false</view></code>

3. v-show: Similar to v-if, this directive toggles the visibility of the element based on the expression. Unlike v-if, the element is always compiled and stays in the DOM, but its visibility is controlled through the display CSS property.

   <code class="html"><view v-show="condition">This will be shown or hidden based on 'condition'</view></code>

What are the performance differences between using v-if and v-show in uni-app?

The performance differences between v-if and v-show in uni-app stem from their different approaches to handling DOM manipulation:

• v-if: This directive has a "real" conditional rendering approach. It fully destroys and recreates parts of the DOM and the underlying Vue instance when the condition changes. This approach can be more expensive in terms of performance because it involves adding and removing elements from the DOM, especially if the condition changes frequently.
• v-show: In contrast, v-show simply toggles the display CSS property of the element. This makes it cheaper in terms of performance because it doesn't involve adding or removing elements from the DOM. The element remains in the DOM and is simply hidden or shown, making it more suitable for situations where the condition is expected to change often.

In summary, use v-if when you need to conditionally render a block of content that does not change often, as it's more resource-efficient over time. Use v-show when you need to toggle something very frequently, as it has less overhead in terms of DOM manipulation.

Can v-else be used independently in uni-app, or does it always need to follow a v-if directive?

In uni-app, v-else cannot be used independently; it must always follow a v-if or a v-else-if directive. The v-else directive serves as an "else block" for v-if, and it will only be rendered if the preceding conditional directive's expression evaluates to false.

Here's an example of incorrect and correct usage:

<code class="html"><!-- Incorrect usage: 'v-else' used independently -->
<view v-else>This is incorrect and won't work</view>

<!-- Correct usage: 'v-else' follows a 'v-if' -->
<view v-if="condition">This is correct</view>
<view v-else>This will work correctly</view></code>

How can I nest conditional rendering directives in uni-app for more complex UI logic?

Nesting conditional rendering directives in uni-app allows you to create more complex UI logic by combining multiple conditions. Here’s an example of how to nest v-if, v-else-if, and v-else:

<code class="html"><view v-if="outerCondition">
  <view v-if="innerCondition1">This is rendered if both outerCondition and innerCondition1 are true</view>
  <view v-else-if="innerCondition2">This is rendered if outerCondition is true and innerCondition2 is true</view>
  <view v-else>This is rendered if outerCondition is true and neither innerCondition1 nor innerCondition2 is true</view>
</view>
<view v-else>This is rendered if outerCondition is false</view></code>

In this example, the outer v-if and v-else control the top-level condition, while the inner v-if, v-else-if, and v-else further refine the rendering based on additional conditions. This nested structure allows you to build complex logic for your UI by combining different conditions and rendering different content based on the state of your application.

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