Custom components in React Native

Introduction

Custom components help you reuse code and build your app faster. They let you create your own building blocks for the app's look and behavior.

You want to show the same button style in many places.

You need a special header that appears on multiple screens.

You want to organize your app into smaller, easy-to-understand parts.

You want to add custom behavior to a group of UI elements.

You want to keep your code clean and avoid repeating yourself.

Syntax

React Native

function MyComponent(props) {
  return (
    <View>
      <Text>{props.title}</Text>
    </View>
  );
}

Custom components are just functions that return UI elements.

You can pass data to them using props.

Examples

A simple component that shows a greeting.

React Native

function Greeting() {
  return <Text>Hello!</Text>;
}

This component uses a prop to show a personalized greeting.

React Native

function Greeting(props) {
  return <Text>Hello, {props.name}!</Text>;
}

Same as above but using arrow function and destructuring props.

React Native

const Greeting = ({ name }) => <Text>Hello, {name}!</Text>;

Sample App

This app shows two cards using the CustomCard component. Each card has a title and description. The cards have simple styling and accessibility labels.

React Native

import React from 'react';
import { View, Text, StyleSheet } from 'react-native';

function CustomCard({ title, description }) {
  return (
    <View style={styles.card} accessible={true} accessibilityLabel={title}>
      <Text style={styles.title}>{title}</Text>
      <Text style={styles.description}>{description}</Text>
    </View>
  );
}

export default function App() {
  return (
    <View style={styles.container}>
      <CustomCard title="Welcome" description="This is a custom card component." />
      <CustomCard title="React Native" description="Build mobile apps with JavaScript." />
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#f0f0f0'
  },
  card: {
    backgroundColor: 'white',
    padding: 16,
    margin: 8,
    borderRadius: 8,
    width: 300,
    shadowColor: '#000',
    shadowOpacity: 0.1,
    shadowRadius: 5,
    shadowOffset: { width: 0, height: 2 },
    elevation: 3
  },
  title: {
    fontSize: 18,
    fontWeight: 'bold',
    marginBottom: 4
  },
  description: {
    fontSize: 14,
    color: '#555'
  }
});

Important Notes

Always give your components clear names that describe what they do.

Use props to make components flexible and reusable.

Remember to add accessibility labels for better app usability.

Summary

Custom components let you build reusable UI pieces.

They are simple functions that return UI elements.

Use props to pass data and customize components.

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