Keep-alive for caching components in Vue

Introduction

Keep-alive helps save the state of components so they don't reset when you switch views. It makes your app faster and smoother.

You want to keep form inputs filled when switching between pages.

You want to save scroll position in a list when going back and forth.

You want to avoid reloading data every time a component appears.

You want to improve user experience by keeping components alive in memory.

Syntax

Vue

<keep-alive>
  <component-to-cache />
</keep-alive>

Wrap the component you want to cache inside the tags.

Only works with dynamic or routed components.

Examples

This caches MyComponent so it keeps its state when hidden.

Vue

<keep-alive>
  <MyComponent />
</keep-alive>

This caches only components named MyComponent when used dynamically.

Vue

<keep-alive include="MyComponent">
  <component :is="currentView" />
</keep-alive>

This caches all components except OtherComponent.

Vue

<keep-alive exclude="OtherComponent">
  <component :is="currentView" />
</keep-alive>

Sample Program

This example shows two counters. Switching between them keeps their counts because they are wrapped in <keep-alive>. The counters do not reset when you switch views.

Vue

<template>
  <div>
    <button @click="currentView = 'CounterA'">Show Counter A</button>
    <button @click="currentView = 'CounterB'">Show Counter B</button>

    <keep-alive>
      <component :is="currentView" />
    </keep-alive>
  </div>
</template>

<script setup>
import { ref } from 'vue'

const currentView = ref('CounterA')

const CounterA = {
  template: `<div>
    <p>Counter A: {{ count }}</p>
    <button @click="count++">Increment A</button>
  </div>`,
  data() {
    return { count: 0 }
  }
}

const CounterB = {
  template: `<div>
    <p>Counter B: {{ count }}</p>
    <button @click="count++">Increment B</button>
  </div>`,
  data() {
    return { count: 0 }
  }
}

const views = { CounterA, CounterB }

// Register components globally or locally
defineExpose({ views })
</script>

Important Notes

Keep-alive only caches components that are currently rendered dynamically or via routing.

Use include and exclude props to control which components to cache.

Cached components keep their state and DOM, improving performance and user experience.

Summary

Keep-alive caches components to keep their state when switching views.

Wrap dynamic or routed components inside <keep-alive> tags.

Use it to improve speed and keep user input or scroll positions.

Practice

1. What is the main purpose of the <keep-alive> component in Vue?

easy

A. To cache components and preserve their state when switching views

B. To permanently remove components from the DOM

C. To reload components every time they are displayed

D. To style components with CSS automatically

Solution

1. Step 1: Understand what <keep-alive> does

   The <keep-alive> component caches inactive components instead of destroying them.

2. Step 2: Identify the effect on component state

   Because components are cached, their state (like user input or scroll position) is preserved when switching back.

3. Final Answer:

   To cache components and preserve their state when switching views -> Option A

4. Quick Check:

   Keep-alive caches components [OK]

Hint: Remember: keep-alive means keep the component's state alive [OK]

Common Mistakes:

• Thinking keep-alive reloads components every time
• Confusing keep-alive with styling or layout features
• Assuming keep-alive removes components permanently

2. Which of the following is the correct syntax to use keep-alive with a dynamic component named MyComponent?

easy

A. <keep-alive><component :is="MyComponent" /></keep-alive>

B. <keep-alive :is="MyComponent" />

C. <component :is="MyComponent" keep-alive />

D. <keep-alive><MyComponent /></keep-alive>

Solution

1. Step 1: Recall how to wrap dynamic components with keep-alive

   The <keep-alive> component wraps around a dynamic component using <component :is="..." />.

2. Step 2: Check each option's syntax

   <keep-alive><component :is="MyComponent" /></keep-alive> correctly wraps <component :is="MyComponent" /> inside <keep-alive>. Options A and C misuse attributes or tags. <keep-alive><MyComponent /></keep-alive> tries to wrap a component tag directly, which is invalid for dynamic components.

3. Final Answer:

   <keep-alive><component :is="MyComponent" /></keep-alive> -> Option A

4. Quick Check:

   Wrap dynamic component inside keep-alive [OK]

Hint: Wrap inside tags [OK]

Common Mistakes:

• Trying to add keep-alive as a prop to component
• Using component tags directly without :is for dynamic components
• Placing :is on keep-alive instead of component

3. Given this Vue template:

<keep-alive>
  <component :is="currentView" />
</keep-alive>

and currentView switches between 'ViewA' and 'ViewB'. What happens to the state of 'ViewA' when switching to 'ViewB' and back?

medium

A. The state of 'ViewA' is reset to initial values

B. 'ViewA' state is preserved because it is cached

C. An error occurs because keep-alive cannot cache dynamic components

D. 'ViewA' is destroyed and recreated fresh

Solution

1. Step 1: Understand keep-alive behavior with dynamic components

   When wrapped in <keep-alive>, inactive components are cached, not destroyed.

2. Step 2: Apply this to switching views

   Switching from 'ViewA' to 'ViewB' caches 'ViewA'. Returning to 'ViewA' restores its previous state.

3. Final Answer:

   'ViewA' state is preserved because it is cached -> Option B

4. Quick Check:

   Keep-alive caches dynamic components [OK]

Hint: Switching views inside keep-alive preserves previous state [OK]

Common Mistakes:

• Assuming components reset on every switch
• Thinking keep-alive destroys inactive components
• Believing keep-alive causes runtime errors with dynamic components

4. What is wrong with this code snippet?

<keep-alive>
  <MyComponent :is="currentView" />
</keep-alive>

Assuming currentView is a string with component name.

medium

A. keep-alive cannot wrap components

B. Missing closing tag for keep-alive

C. currentView should be a component object, not a string

D. Using :is on a component tag instead of <component>

Solution

1. Step 1: Identify correct usage of dynamic components

   Dynamic components require the <component :is="..." /> syntax, not <MyComponent :is="..." />.

2. Step 2: Check the code snippet

   The snippet incorrectly uses :is on a named component tag, which is invalid and causes errors.

3. Final Answer:

   Using :is on a component tag instead of <component> -> Option D

4. Quick Check:

   Dynamic components need <component :is> syntax [OK]

Hint: Use for dynamic components, not named tags [OK]

Common Mistakes:

• Placing :is on a named component tag
• Forgetting to close keep-alive tags
• Thinking keep-alive cannot wrap components

5. You want to cache only the 'UserProfile' component but not 'UserSettings' when switching views. Which is the correct way to use <keep-alive> with the include prop to achieve this?

hard

A. <keep-alive include={UserProfile}><component :is="currentView" /></keep-alive>

B. <keep-alive exclude="UserProfile"><component :is="currentView" /></keep-alive>

C. <keep-alive include="UserProfile"><component :is="currentView" /></keep-alive>

D. <keep-alive include="UserSettings"><component :is="currentView" /></keep-alive>

Solution

1. Step 1: Understand the include prop on keep-alive

   The include prop accepts a string or array of component names to cache only those components.

2. Step 2: Apply include to cache 'UserProfile' only

   Setting include="UserProfile" caches only the 'UserProfile' component and excludes others like 'UserSettings'.

3. Final Answer:

   <keep-alive include="UserProfile"><component :is="currentView" /></keep-alive> -> Option C

4. Quick Check:

   Use include="UserProfile" to cache only that component [OK]

Hint: Use include prop with component name to cache selectively [OK]

Common Mistakes:

• Using exclude instead of include to cache desired component
• Passing include as an object instead of string
• Including wrong component name in include prop