Custom components in React Native - Build, Publish & Deploy

Build & Publish - Custom components

Performance Impact of Custom Components

Using custom components helps organize your app UI into smaller pieces. This can improve performance by reducing repeated code and making updates easier. However, creating too many small components can add overhead, causing slower rendering and more memory use. Aim for a balance: components should be reusable but not too tiny.

React Native targets 60 frames per second for smooth animations. Complex custom components with heavy logic or many nested views can drop frames and cause jank. Keep components simple and avoid unnecessary re-renders to maintain smooth UI.

How to Optimize Custom Components for 60fps Rendering

• Use React.memo to prevent re-rendering components when props don't change.
• Keep components focused on one task to reduce complexity.
• Avoid inline functions and objects in props to prevent unnecessary updates.
• Use the useCallback and useMemo hooks to memoize functions and values.
• Minimize the number of nested views inside components to reduce layout calculations.
• Profile your app with React DevTools and React Native Performance Monitor to find slow components.

Impact on App Bundle Size and Startup Time

Custom components themselves add very little to app size since they are just JavaScript code. However, importing many third-party components or large UI libraries inside custom components can increase bundle size significantly.

More components can increase startup time if many are loaded immediately. Use lazy loading or code splitting to load components only when needed, improving startup speed.

iOS vs Android Differences for Custom Components

React Native custom components work similarly on both iOS and Android because they use the same JavaScript code. However, native UI differences can affect appearance and behavior.

For example, some native components like Picker or Switch look different on each platform. Custom components wrapping these should handle platform-specific styles or behaviors using Platform.select.

Performance may vary slightly due to native rendering differences, so test your custom components on both platforms.

Relevant Store Review Guidelines and Requirements

• Apple App Store: Ensure your custom components do not use private APIs or undocumented features. Follow Apple's Human Interface Guidelines for UI consistency and accessibility.
• Google Play Store: Avoid components that cause crashes or excessive battery use. Follow Material Design guidelines for UI and accessibility.
• Both stores require apps to be responsive and accessible. Use proper accessibility props in custom components (like accessibilityLabel and accessible).
• Make sure custom components handle different screen sizes and orientations gracefully.

Self-Check: Your App Takes 5 Seconds to Load This Screen. What's Likely Wrong?

It's likely your custom components are doing too much work on mount or rendering too many nested views. Possible issues:

• Heavy computations or data fetching inside component render functions.
• Not using memoization, causing repeated re-renders.
• Loading large images or assets synchronously in components.
• Too many nested components causing slow layout calculations.

To fix, move heavy work outside render, use React.memo, lazy load assets, and simplify component trees.

Key Result

Custom components improve code reuse and app structure but must be optimized with memoization and simple layouts to maintain 60fps performance and small bundle size. Test on both iOS and Android for UI differences and follow store guidelines for accessibility and responsiveness.

Practice

1. What is a custom component in React Native?

easy

A. A CSS style sheet

B. A built-in React Native library

C. A special type of variable

D. A reusable function that returns UI elements

Solution

1. Step 1: Understand what a component is

   A component is a piece of UI that can be reused in different parts of the app.

2. Step 2: Identify custom components

   Custom components are user-defined functions that return UI elements, unlike built-in libraries or styles.

3. Final Answer:

   A reusable function that returns UI elements -> Option D

4. Quick Check:

   Custom component = reusable function [OK]

Hint: Custom components are functions returning UI [OK]

Common Mistakes:

• Confusing components with styles
• Thinking components are variables
• Mixing built-in libraries with custom components

2. Which of the following is the correct syntax to define a simple custom component named Greeting in React Native?

easy

A. function Greeting() { return Hello; }

B. var Greeting = Hello;

C. const Greeting = () => { Hello }

D. class Greeting extends Component { render() { return 'Hello'; } }

Solution

1. Step 1: Check function component syntax

   Function components return JSX inside the function body with a return statement.

2. Step 2: Evaluate each option

   function Greeting() { return Hello; } correctly defines a function returning JSX. var Greeting = Hello; assigns JSX directly to a variable, which is invalid. const Greeting = () => { Hello } misses the return statement. class Greeting extends Component { render() { return 'Hello'; } } uses class syntax but returns a string, not JSX.

3. Final Answer:

   function Greeting() { return Hello; } -> Option A

4. Quick Check:

   Function component must return JSX with return [OK]

Hint: Function components need a return statement [OK]

Common Mistakes:

• Omitting return in arrow functions
• Assigning JSX directly to variables
• Returning strings instead of JSX

3. What will be displayed on the screen when this component is rendered?

function Welcome(props) {
  return Hi, {props.name}!;
}

// Usage:
<Welcome name="Anna" />

medium

A. Hi, {props.name}!

B. Hi, Anna!

C. Hi, name!

D. Error: props not defined

Solution

1. Step 1: Understand props usage

   The component receives a prop called 'name' and inserts it inside the Text element using curly braces.

2. Step 2: Substitute the prop value

   Since the component is used with name="Anna", the output will be 'Hi, Anna!'.

3. Final Answer:

   Hi, Anna! -> Option B

4. Quick Check:

   Props insert values inside JSX [OK]

Hint: Props values show inside curly braces in JSX [OK]

Common Mistakes:

• Treating props as plain text
• Forgetting curly braces for props
• Assuming props are undefined

4. Identify the error in this custom component code:

function Button() {
  return (
    <TouchableOpacity>
      <Text>Click me</Text>
    </TouchableOpacity>
  )
}

export default Button;

medium

A. Text component cannot be inside TouchableOpacity

B. Function must be named with uppercase first letter

C. Missing import of TouchableOpacity

D. Missing return statement

Solution

1. Step 1: Check component usage

   The component uses TouchableOpacity and Text, which must be imported from 'react-native'.

2. Step 2: Identify missing import

   The code snippet does not show imports, so the likely error is missing import of TouchableOpacity.

3. Final Answer:

   Missing import of TouchableOpacity -> Option C

4. Quick Check:

   All used components must be imported [OK]

Hint: Always import components you use [OK]

Common Mistakes:

• Forgetting to import React Native components
• Thinking Text can't be inside TouchableOpacity
• Confusing function naming rules

5. You want to create a reusable Card component that accepts a title and content as props and displays them inside styled View and Text components. Which of the following is the best way to define this component?

hard

A. function Card({ title, content }) {
return (

{title}
{content}

);
}

B. function Card(props) {
return (

{props.title}
{props.content}

);
}

C. function Card() {
return (

title
content

);
}

D. function Card({ title, content }) {
return (

{title}

);
}

Solution

1. Step 1: Check props usage and destructuring

   function Card({ title, content }) { return ( {title} {content} ); } uses destructuring to get title and content props, which is clear and concise.

2. Step 2: Verify UI structure and styling

   function Card({ title, content }) { return ( {title} {content} ); } wraps texts in a styled View with padding, border, and rounded corners, making it reusable and visually distinct.

3. Step 3: Compare with other options

   function Card(props) { return ( {props.title} {props.content} ); } lacks styling, while the option without props uses literal strings instead of props, and the option missing content display omits the content Text.

4. Final Answer:

   function Card({ title, content }) { return ( {title} {content} ); } -> Option A

5. Quick Check:

   Destructure props and add styles for reusable UI [OK]

Hint: Destructure props and add styles for reusable components [OK]

Common Mistakes:

• Not using props to customize content
• Forgetting to style reusable components
• Using literal strings instead of props