Compound components pattern in Vue - Performance & Optimization

Performance: Compound components pattern

MEDIUM IMPACT

This pattern affects rendering speed and interaction responsiveness by managing how nested components communicate and update.

Sharing state between related components in a UI widget

Vue

CompoundComponent.vue
<template>
  <slot name="input" :value="value" :updateValue="updateValue" />
  <slot name="display" :value="value" />
</template>
<script setup>
import { ref } from 'vue'
const value = ref('')
function updateValue(newVal) {
  value.value = newVal
}
</script>

Usage.vue
<template>
  <CompoundComponent>
    <template v-slot:input="{ value, updateValue }">
      <input :value="value" @input="e => updateValue(e.target.value)" />
    </template>
    <template v-slot:display="{ value }">
      <p>{{ value }}</p>
    </template>
  </CompoundComponent>
</template>

Centralizes state in one component and shares it via scoped slots, reducing prop drilling and unnecessary re-renders.

📈 Performance GainSingle reactive source reduces re-renders and improves INP by minimizing DOM updates.

Sharing state between related components in a UI widget

Vue

Parent.vue
<template>
  <div>
    <ChildA :value="value" @update="value = $event" />
    <ChildB :value="value" />
  </div>
</template>
<script setup>
import { ref } from 'vue'
const value = ref('')
</script>

ChildA.vue
<template>
  <input :value="value" @input="$emit('update', $event.target.value)" />
</template>
<script setup>
const props = defineProps({ value: String })
</script>

ChildB.vue
<template>
  <p>{{ value }}</p>
</template>
<script setup>
const props = defineProps({ value: String })
</script>

Passing state and events through multiple props and emits causes many re-renders and prop drilling, increasing DOM updates and slowing interaction.

📉 Performance CostTriggers multiple re-renders on each input, causing INP delays and more DOM updates.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Prop drilling with multiple child components	High - many props and emits	Multiple reflows per input	High paint cost due to frequent updates	[X] Bad
Compound components with shared reactive state	Low - centralized state	Single reflow per input	Lower paint cost due to fewer updates	[OK] Good

Rendering Pipeline

Compound components centralize state management, reducing the number of reactive updates and DOM mutations. This lowers the frequency of layout recalculations and paints during user interactions.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckLayout due to frequent reflows from multiple component updates

Core Web Vital Affected

INP

This pattern affects rendering speed and interaction responsiveness by managing how nested components communicate and update.

Optimization Tips

1Centralize shared state in the parent compound component to avoid prop drilling.

2Use scoped slots to pass state and update functions to child components efficiently.

3Minimize the number of reactive updates to reduce layout recalculations and improve INP.

Performance Quiz - 3 Questions

What is a main performance benefit of using compound components in Vue?

A Reducing unnecessary re-renders by sharing state centrally

B Increasing the number of DOM nodes for better structure

C Using more props to pass data deeply

D Avoiding scoped slots to simplify templates

DevTools: Performance

How to check: Record a performance profile while interacting with the component. Look for frequent scripting and layout events triggered by input.

What to look for: High number of layout recalculations and long scripting times indicate poor compound component usage.

Practice

1. What is the main purpose of the compound components pattern in Vue?

easy

A. To group related UI parts inside one main component for better organization

B. To create multiple unrelated components in one file

C. To avoid using provide and inject for state sharing

D. To write components without any communication between them

Solution

1. Step 1: Understand the pattern goal

   The compound components pattern groups related UI parts to work together inside one main component.

2. Step 2: Compare options

   Only To group related UI parts inside one main component for better organization correctly describes grouping related UI parts for better organization and communication.

3. Final Answer:

   To group related UI parts inside one main component for better organization -> Option A

4. Quick Check:

   Compound components group related parts = B [OK]

Hint: Think: grouping related UI parts inside one component [OK]

Common Mistakes:

• Confusing unrelated components grouping
• Ignoring state sharing between parts
• Thinking it avoids provide/inject

2. Which Vue feature is commonly used to share state between compound components?

easy

A. provide and inject

B. props

C. v-model

D. emit events

Solution

1. Step 1: Identify state sharing methods

   Vue offers several ways to share data, but for compound components, provide and inject are designed for parent-child communication without prop drilling.

2. Step 2: Match pattern usage

   Compound components use provide/inject to share state internally, unlike props or emit which are for direct parent-child or event communication.

3. Final Answer:

   provide and inject -> Option A

4. Quick Check:

   State sharing in compound components = provide/inject [OK]

Hint: Remember: provide/inject share state inside compound components [OK]

Common Mistakes:

• Choosing props which require manual passing
• Confusing emit with state sharing
• Thinking v-model is for internal state sharing

3. Given this Vue compound component setup, what will be the rendered output?

const TabGroup = {
  setup() {
    provide('activeTab', ref('home'))
  },
  template: `
`
}

const Tab = {
  inject: ['activeTab'],
  props: ['name'],
  template: `{{ name }}`
}

// Usage:
// 
//   
//   
// 

medium

A. homeprofile

B. homeprofile

C. homeprofile

D. Error: activeTab is not reactive

Solution

1. Step 1: Understand provide/inject in Options API

   TabGroup provides 'activeTab' as ref('home'). Tab injects ['activeTab'], receiving the ref object (not auto-unwrapped in Options API).

2. Step 2: Evaluate rendering logic

   In template, activeTab (ref object) === name (string) is always false. No 'active' class applied to either button.

3. Final Answer:

   <button>home</button><button>profile</button> -> Option B

4. Quick Check:

   Options API inject ref === string = false [OK]

Hint: Options API inject returns ref object, not value [OK]

Common Mistakes:

• Assuming inject returns raw value, not ref
• Confusing which tab is active
• Thinking activeTab is not reactive

4. What is the main issue in this compound component code?

const Dropdown = {
  setup() {
    const open = ref(false)
    provide('open', open)
  },
  template: `
`
}

const DropdownToggle = {
  inject: ['open'],
  template: `Toggle`
}

medium

A. The provide key 'open' is missing

B. The template syntax is invalid

C. The 'open' variable is not returned from setup in Dropdown

D. The 'open' ref is not reactive inside DropdownToggle

Solution

1. Step 1: Check inject handling in Options API

   Injected refs via inject: ['open'] are not auto-unwrapped in Options API templates. 'open' is the ref object.

2. Step 2: Understand mutation and reactivity issue

   @click="open = !open" reassigns a boolean to the ref variable (breaks link to original ref) and lacks .value access, making the ref unusable reactively as boolean state.

3. Final Answer:

   The 'open' ref is not reactive inside DropdownToggle -> Option D

4. Quick Check:

   Options API injected ref needs .value [OK]

Hint: Use .value for injected refs in Options API templates [OK]

Common Mistakes:

• Thinking provider setup needs to return state for provide
• Template syntax looks invalid
• Provide key seems missing

5. You want to build a compound Tabs component where clicking a tab updates the active tab and content changes accordingly. Which approach correctly implements this pattern in Vue 3?

hard

A. Use local state in each Tab component without sharing activeTab

B. Pass activeTab as prop to each Tab and emit events to Tabs to update state

C. Use provide to share activeTab ref in Tabs, inject in TabList and TabPanels, update activeTab on Tab click

D. Use global event bus to communicate active tab changes between components

Solution

1. Step 1: Identify compound components pattern usage

   The pattern uses provide/inject to share state like activeTab across nested components without prop drilling.

2. Step 2: Evaluate options for state management

   Use provide to share activeTab ref in Tabs, inject in TabList and TabPanels, update activeTab on Tab click correctly uses provide to share activeTab ref and inject in child components, updating activeTab on tab click to reflect changes.

3. Final Answer:

   Use provide to share activeTab ref in Tabs, inject in TabList and TabPanels, update activeTab on Tab click -> Option C

4. Quick Check:

   Compound components share state via provide/inject = D [OK]

Hint: Use provide/inject to share and update active tab state [OK]

Common Mistakes:

• Using props and events causing complex prop drilling
• Using global event bus which is not recommended
• Keeping state local in each tab causing no sync