Redux slices and actions in React Native - Mini App: Build & Ship

Build: Counter Screen

A simple counter app screen that uses Redux slices and actions to increase and decrease a number.

Target UI

Counter Screen

+-------------------+
|       Count       |
|        0          |
|                   |
|  [-]       [+]    |
+-------------------+

Display the current count from Redux state

Add two buttons: '+' to increase and '-' to decrease the count

Use Redux slice to define state, actions, and reducer

Connect the screen to Redux store and dispatch actions on button press

Starter Code

React Native

import React from 'react';
import { View, Text, Button, StyleSheet } from 'react-native';
import { useSelector, useDispatch } from 'react-redux';

export default function CounterScreen() {
  const count = useSelector(state => state.counter.value);
  const dispatch = useDispatch();

  // TODO: Add dispatch calls for increment and decrement

  return (
    <View style={styles.container}>
      <Text style={styles.title}>Count</Text>
      <Text style={styles.count}>{count}</Text>
      <View style={styles.buttons}>
        <Button title="-" onPress={() => { /* TODO: dispatch decrement */ }} />
        <Button title="+" onPress={() => { /* TODO: dispatch increment */ }} />
      </View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: { flex: 1, justifyContent: 'center', alignItems: 'center' },
  title: { fontSize: 24, marginBottom: 10 },
  count: { fontSize: 48, marginBottom: 20 },
  buttons: { flexDirection: 'row', width: 150, justifyContent: 'space-between' }
});

// TODO: Create Redux slice with initial state, reducers, and actions

Task 1

Task 2

Task 3

Task 4

Solution

React Native

import React from 'react';
import { View, Text, Button, StyleSheet } from 'react-native';
import { useSelector, useDispatch } from 'react-redux';
import { createSlice } from '@reduxjs/toolkit';

// Redux slice
export const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {
    increment: state => { state.value += 1; },
    decrement: state => { state.value -= 1; }
  }
});

export const { increment, decrement } = counterSlice.actions;
export const counterReducer = counterSlice.reducer;

// React component
export default function CounterScreen() {
  const count = useSelector(state => state.counter.value);
  const dispatch = useDispatch();

  return (
    <View style={styles.container}>
      <Text style={styles.title}>Count</Text>
      <Text style={styles.count}>{count}</Text>
      <View style={styles.buttons}>
        <Button title="-" onPress={() => dispatch(decrement())} />
        <Button title="+" onPress={() => dispatch(increment())} />
      </View>
    </View>
  );
}

const styles = StyleSheet.create({
  container: { flex: 1, justifyContent: 'center', alignItems: 'center' },
  title: { fontSize: 24, marginBottom: 10 },
  count: { fontSize: 48, marginBottom: 20 },
  buttons: { flexDirection: 'row', width: 150, justifyContent: 'space-between' }
});

This example uses @reduxjs/toolkit to create a counter slice. The slice defines the initial state with value: 0 and two reducers: increment and decrement. These reducers update the state by increasing or decreasing the count.

In the React Native component, we use useSelector to read the current count from the Redux store. We use useDispatch to get the dispatch function.

When the user taps the '+' button, the increment action is dispatched, increasing the count. When the '-' button is tapped, the decrement action is dispatched, decreasing the count.

This setup keeps the UI and state logic clean and separated, making it easy to manage and scale.

Final Result

Completed Screen

Counter Screen

+-------------------+
|       Count       |
|        0          |
|                   |
|  [-]       [+]    |
+-------------------+

Tapping '+' increases the number displayed by 1

Tapping '-' decreases the number displayed by 1

Stretch Goal

Add a reset button that sets the count back to zero

💡 Hint

Add a 'reset' reducer and action in the slice, then add a button that dispatches reset

Practice

1. What is the main purpose of a Redux slice in a React Native app?

easy

A. To create UI components automatically

B. To style the app with CSS

C. To handle navigation between screens

D. To group related state and actions together for easier management

Solution

1. Step 1: Understand Redux slice role

   A slice groups state and actions related to a feature in one place.

2. Step 2: Compare with other options

   Options A, C, and D are unrelated to state management.

3. Final Answer:

   To group related state and actions together for easier management -> Option D

4. Quick Check:

   Redux slice = group state and actions [OK]

Hint: Slices bundle state and actions for one feature [OK]

Common Mistakes:

• Confusing slices with UI components
• Thinking slices handle navigation
• Assuming slices style the app

2. Which of the following is the correct way to define an action inside a Redux slice using createSlice?

easy

A. reducers: { increment: (state) => { state.value += 1 } }

B. actions: { increment: () => state.value += 1 }

C. reducers: { increment: () => state.value += 1 }

D. actions: { increment: (state) => { return state.value + 1 } }

Solution

1. Step 1: Recall createSlice syntax

   Reducers are defined as functions with (state) parameter that mutate state directly.

2. Step 2: Check each option

   reducers: { increment: (state) => { state.value += 1 } } correctly uses reducers with state mutation. Options B and D wrongly use 'actions' key. reducers: { increment: () => state.value += 1 } uses arrow function without braces, which won't mutate state properly.

3. Final Answer:

   reducers: { increment: (state) => { state.value += 1 } } -> Option A

4. Quick Check:

   Reducers mutate state inside createSlice [OK]

Hint: Reducers mutate state; actions key is not used in createSlice [OK]

Common Mistakes:

• Using 'actions' instead of 'reducers' key
• Not mutating state directly inside reducer
• Returning new state instead of mutating

3. Given this slice code:

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {
    increment: (state) => { state.value += 1 },
    decrement: (state) => { state.value -= 1 }
  }
});

const state = { value: 5 };
counterSlice.reducer(state, { type: 'counter/increment' });
console.log(state.value);

What will be logged to the console?

medium

A. 6

B. 5

C. NaN

D. Error

Solution

1. Step 1: Understand reducer call

   The reducer receives state { value: 5 } and action type 'counter/increment'. It runs increment reducer which adds 1 to state.value.

2. Step 2: Calculate new state value

   5 + 1 = 6, so state.value becomes 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Increment action adds 1 to value [OK]

Hint: Reducer mutates state.value by +1 on increment action [OK]

Common Mistakes:

• Thinking state is immutable and unchanged
• Confusing action type format
• Expecting error due to direct mutation

4. You wrote this slice but dispatching the action does not update the state:

const todoSlice = createSlice({
  name: 'todo',
  initialState: [],
  reducers: {
    addTodo: (state, action) => {
      state.push(action.payload);
    }
  }
});

// Later in component
dispatch(todoSlice.actions.addTodo('Buy milk'));

What is the most likely reason the UI does not update?

medium

A. The reducer mutates state directly which is not allowed

B. The initialState should be an object, not an array

C. The slice reducer is not added to the store

D. The action payload is missing

Solution

1. Step 1: Check reducer code

   The reducer correctly mutates state array by pushing new todo, which is allowed with Immer in createSlice.

2. Step 2: Identify missing integration

   If UI does not update, likely the slice reducer is not connected to the Redux store, so dispatch has no effect.

3. Final Answer:

   The slice reducer is not added to the store -> Option C

4. Quick Check:

   Reducer must be in store for dispatch to update state [OK]

Hint: Always add slice reducer to store for updates [OK]

Common Mistakes:

• Thinking direct mutation is forbidden in createSlice
• Assuming initialState must be object
• Forgetting to add reducer to store

5. You want to create a Redux slice that manages a list of users and includes an action to update a user's name by id. Which code snippet correctly implements this?

hard

A. reducers: { updateName: (state, action) => { state.filter(u => u.id !== action.payload.id).push(action.payload); } }

B. reducers: { updateName: (state, action) => { const user = state.find(u => u.id === action.payload.id); if(user) { user.name = action.payload.name; } } }

C. reducers: { updateName: (state, action) => { state = state.map(u => u.id === action.payload.id ? { ...u, name: action.payload.name } : u); } }

D. reducers: { updateName: (state, action) => { state.map(u => u.id === action.payload.id ? { ...u, name: action.payload.name } : u); } }

Solution

1. Step 1: Understand mutation in createSlice

   Reducers can mutate state directly because Immer handles immutability internally.

2. Step 2: Analyze each option

   reducers: { updateName: (state, action) => { const user = state.find(u => u.id === action.payload.id); if(user) { user.name = action.payload.name; } } } finds user by id and mutates name directly, which is correct. reducers: { updateName: (state, action) => { state.map(u => u.id === action.payload.id ? { ...u, name: action.payload.name } : u); } } executes map but discards the new array without returning it or mutating state, incorrect. reducers: { updateName: (state, action) => { state = state.map(u => u.id === action.payload.id ? { ...u, name: action.payload.name } : u); } } tries to assign new array to state variable, which does not update state correctly. reducers: { updateName: (state, action) => { state.filter(u => u.id !== action.payload.id).push(action.payload); } } filters out user and pushes new one, which mutates array incorrectly and may cause bugs.

3. Final Answer:

   reducers: { updateName: (state, action) => { const user = state.find(u => u.id === action.payload.id); if(user) { user.name = action.payload.name; } } } -> Option B

4. Quick Check:

   Direct mutation with find and update is correct in createSlice [OK]

Hint: Use find and mutate user directly inside reducer [OK]

Common Mistakes:

• Trying to reassign state variable inside reducer
• Using filter and push incorrectly
• Not mutating state directly in createSlice