Custom tab bars in React Native - UI Render Trace

Component - Custom tab bars

This UI component shows a custom tab bar at the bottom of the screen. It lets users switch between different screens by tapping on labeled icons. The custom tab bar is styled differently from the default, with unique colors and layout.

Widget Tree

View (root)
├─ View (screen content)
└─ View (custom tab bar)
   ├─ TouchableOpacity (Tab 1)
   │  ├─ Icon
   │  └─ Text (Label 1)
   ├─ TouchableOpacity (Tab 2)
   │  ├─ Icon
   │  └─ Text (Label 2)
   └─ TouchableOpacity (Tab 3)
      ├─ Icon
      └─ Text (Label 3)

The root View holds the whole screen. Inside it, the main screen content is shown above. At the bottom, a View acts as the custom tab bar. It contains three touchable tabs, each with an icon and a label stacked vertically.

Render Trace - 5 Steps

Step 1: View (root)

Step 2: View (screen content)

Step 3: View (custom tab bar)

Step 4: TouchableOpacity (each tab)

Step 5: Icon and Text (inside tab)

State Change - Re-render

Trigger:User taps on a tab button

Before

Selected tab index is 0 (first tab)

→

After

Selected tab index changes to tapped tab index (e.g., 1 or 2)

Re-renders:The entire custom tab bar re-renders to update highlight on selected tab

UI Quiz - 3 Questions

What happens visually when a user taps a different tab?

A The entire screen background color changes

B The tapped tab icon and label change color to show selection

C The tab bar disappears

D Nothing changes visually

Key Insight

Custom tab bars let you create unique navigation styles that match your app's brand. Using touchable areas with icons and labels stacked vertically is a common pattern. Highlighting the selected tab clearly helps users know where they are. Remember to keep the tab bar accessible by ensuring touch targets are large enough and labels are clear.

Practice

1. What is the main purpose of a custom tab bar in React Native navigation?

easy

A. To replace the entire navigation stack with a new one

B. To disable navigation between screens

C. To create a personalized look and behavior for the bottom navigation tabs

D. To automatically fetch data for each tab

Solution

1. Step 1: Understand what a tab bar does

   A tab bar shows navigation options at the bottom of the screen for switching between views.

2. Step 2: Identify the role of a custom tab bar

   A custom tab bar lets you change how these tabs look and behave, instead of using the default style.

3. Final Answer:

   To create a personalized look and behavior for the bottom navigation tabs -> Option C

4. Quick Check:

   Custom tab bar = personalized navigation UI [OK]

Hint: Custom tab bars change appearance and interaction of bottom tabs [OK]

Common Mistakes:

• Thinking it replaces navigation logic entirely
• Assuming it disables navigation
• Confusing it with data fetching

2. Which React Native component is commonly used to make a tab clickable in a custom tab bar?

easy

A. View

B. TouchableOpacity

C. TextInput

D. ScrollView

Solution

1. Step 1: Identify clickable components

   TouchableOpacity is designed to respond to touch events and provide feedback.

2. Step 2: Match with tab interaction needs

   Tabs need to be clickable to switch screens, so TouchableOpacity is the right choice.

3. Final Answer:

   TouchableOpacity -> Option B

4. Quick Check:

   Clickable tab = TouchableOpacity [OK]

Hint: Use TouchableOpacity for clickable tabs in React Native [OK]

Common Mistakes:

• Using View which is not clickable by default
• Choosing TextInput which is for text input
• Using ScrollView which is for scrolling content

3. Given this snippet inside a custom tab bar component:

state.index === 1 ? 'blue' : 'gray'

What does this expression do?

medium

A. Causes a syntax error because of missing parentheses

B. Always sets the tab color to gray

C. Changes the tab color to blue when the app starts

D. Sets the tab color to blue if it is the active tab, otherwise gray

Solution

1. Step 1: Understand the ternary expression

   The expression checks if state.index equals 1; if true, it returns 'blue', else 'gray'.

2. Step 2: Relate to tab color logic

   This means the tab at index 1 is highlighted blue when active, others are gray.

3. Final Answer:

   Sets the tab color to blue if it is the active tab, otherwise gray -> Option D

4. Quick Check:

   state.index === 1 ? 'blue' : 'gray' = active tab color logic [OK]

Hint: Ternary ? : chooses color based on active tab index [OK]

Common Mistakes:

• Thinking it always returns gray
• Assuming it causes syntax error
• Confusing the condition with app start

4. Identify the error in this custom tab bar code snippet:

const CustomTabBar = ({ state, navigation }) => {
  return (
    <View>
      {state.routes.map((route, index) => (
        <TouchableOpacity
          key={index}
          onPress={() => navigation.navigate(route.name)}
        >
          <Text>{route.name}</Text>
        </TouchableOpacity>
      ))}
    </View>
  );
};

medium

A. Missing accessibility props for TouchableOpacity

B. Using index as key causes navigation errors

C. Incorrect use of navigation.navigate inside onPress

D. No error, code works correctly

Solution

1. Step 1: Review accessibility best practices

   TouchableOpacity should have accessibilityRole and accessibilityState for screen readers.

2. Step 2: Check other parts of code

   Using index as key is not ideal but won't cause navigation errors; navigation.navigate usage is correct.

3. Final Answer:

   Missing accessibility props for TouchableOpacity -> Option A

4. Quick Check:

   Accessibility props missing = error [OK]

Hint: Always add accessibilityRole and accessibilityState to touchables [OK]

Common Mistakes:

• Ignoring accessibility props
• Thinking index key breaks navigation
• Assuming navigation.navigate is wrong here

5. You want to create a custom tab bar that highlights the active tab with a bold label and changes the icon color. Which approach correctly uses state and navigation to achieve this?

hard

A. Use state.index to check active tab, and navigation.navigate(route.name) on press; style label bold if active

B. Use navigation.state.index to check active tab, and state.navigate(route.name) on press; style label bold if active

C. Use state.activeTab to check active tab, and navigation.goBack() on press; style label bold if active

D. Use state.index to check active tab, and navigation.push(route.key) on press; style label bold if active

Solution

1. Step 1: Identify correct state usage

   The active tab index is in state.index, not navigation.state or state.activeTab.

2. Step 2: Identify correct navigation method

   navigation.navigate(route.name) is used to switch tabs; goBack or push are not appropriate here.

3. Step 3: Apply styling based on active tab

   Compare current index with tab index to apply bold style and change icon color.

4. Final Answer:

   Use state.index to check active tab, and navigation.navigate(route.name) on press; style label bold if active -> Option A

5. Quick Check:

   state.index + navigation.navigate = correct tab control [OK]

Hint: Use state.index for active tab and navigation.navigate for switching [OK]

Common Mistakes:

• Using wrong property like navigation.state.index
• Using navigation.goBack instead of navigate
• Using state.activeTab which doesn't exist