Why Custom components in React Native? - Purpose & Use Cases

The Big Idea

What if you could fix a bug or change a style once and see it everywhere instantly?

The Scenario

Imagine building a mobile app where you need the same button style on many screens. You copy and paste the button code everywhere, changing colors or text each time.

The Problem

This manual way is slow and risky. If you want to change the button style later, you must find and update every copy. It's easy to miss some, causing inconsistent looks and bugs.

The Solution

Custom components let you create a reusable button once. Then you use it everywhere by just calling its name. Change the button style in one place, and all buttons update automatically.

Before vs After

✗ Before

const Screen1 = () => <Button color="blue" title="Click" />;
const Screen2 = () => <Button color="blue" title="Submit" />;

✓ After

const MyButton = ({title}) => <Button color="blue" title={title} />;
const Screen1 = () => <MyButton title="Click" />;
const Screen2 = () => <MyButton title="Submit" />;

What It Enables

Custom components make your app easier to build, update, and keep looking great everywhere.

Real Life Example

Think of a shopping app where every product card looks the same. Using a custom product card component means you design it once and reuse it for all products, saving time and keeping the app consistent.

Key Takeaways

Manual repetition causes slow updates and errors.

Custom components let you reuse UI parts easily.

One change updates all uses, saving time and effort.

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