Redux selectors in React Native - Build, Publish & Deploy

Build & Publish - Redux selectors

Performance Impact of Redux Selectors

Redux selectors help your app efficiently read data from the store. They reduce unnecessary re-renders by returning cached results when data hasn't changed. This keeps your UI smooth, aiming for 60 frames per second, which feels natural and responsive on mobile devices.

Without selectors, your app might re-render too often, wasting CPU and battery. Selectors also help keep memory use low by avoiding duplicate data copies.

💻

How to Optimize Redux Selectors for 60fps Rendering

Use memoized selectors (like those created with reselect) to cache results and avoid recalculations. This means your app only updates components when the relevant data changes.

Keep selectors simple and focused on small slices of state. Avoid heavy computations inside selectors; if needed, move complex logic outside or use asynchronous data fetching.

Test your selectors with React DevTools and Redux DevTools to ensure components only re-render when necessary.

Impact on App Bundle Size and Startup Time

Adding selectors, especially memoized ones, adds minimal size to your app bundle—usually just a few kilobytes from libraries like reselect. This has a negligible effect on startup time.

Well-structured selectors can improve perceived startup speed by reducing unnecessary renders during initial load.

iOS vs Android Differences for Redux Selectors

Redux selectors work the same on both iOS and Android in React Native apps. However, performance differences may arise from platform-specific JavaScript engine optimizations.

On iOS, JavaScriptCore is used, while Android uses Hermes or V8. Hermes is optimized for React Native and can improve selector performance and memory use.

Ensure your selectors are efficient to benefit both platforms equally and test on real devices to catch platform-specific issues.

Relevant Store Review Guidelines and Requirements

Using Redux selectors does not directly affect app store guidelines. However, efficient state management helps your app stay responsive and stable, which aligns with Apple's Human Interface Guidelines and Google Play's quality requirements.

Ensure your app does not freeze or crash due to inefficient state updates, as this can cause rejection during review.

Keep your app's memory use reasonable; excessive memory from poor state handling can lead to app termination by the OS.

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

It's likely your selectors are not memoized, causing repeated heavy calculations and unnecessary re-renders. This slows down rendering and delays screen load.

Another cause could be selectors reading large or deeply nested state without optimization, increasing CPU and memory use.

Check if your selectors use reselect or similar memoization and simplify state shape if needed.

Key Result

Redux selectors improve app performance by minimizing unnecessary UI updates through memoization, keeping your React Native app smooth and responsive on both iOS and Android with minimal impact on bundle size and startup time.

Practice

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

easy

A. To create UI components

B. To extract specific data from the Redux store state

C. To define the initial state of the store

D. To dispatch actions to update the store

Solution

1. Step 1: Understand Redux store role

   The Redux store holds the entire app state in one place.

2. Step 2: Role of selectors

   Selectors are functions that pick specific parts of that state for use in components.

3. Final Answer:

   To extract specific data from the Redux store state -> Option B

4. Quick Check:

   Selector = extract data [OK]

Hint: Selectors pick data from state, not change it [OK]

Common Mistakes:

• Confusing selectors with action creators
• Thinking selectors update state
• Using selectors to create UI components

2. Which of the following is the correct syntax to define a simple Redux selector function named selectUser that returns state.user?

easy

A. const selectUser = () => state.user;

B. const selectUser = (state) => { state.user }

C. function selectUser() { return user; }

D. const selectUser = state => state.user;

Solution

1. Step 1: Check function parameter

   A selector receives the whole state as an argument, so it must have state parameter.

2. Step 2: Check return statement

   The selector returns state.user directly. Arrow function with implicit return is correct.

3. Final Answer:

   const selectUser = state => state.user; -> Option D

4. Quick Check:

   Arrow function with state param and return [OK]

Hint: Selector must take state and return part of it [OK]

Common Mistakes:

• Omitting the state parameter
• Not returning the value properly
• Using braces without return statement

3. Given the selector const selectCompletedTodos = state => state.todos.filter(todo => todo.completed); and the state { todos: [{id:1, completed:true}, {id:2, completed:false}, {id:3, completed:true}] }, what will selectCompletedTodos(state) return?

medium

A. [{id:1, completed:true}, {id:3, completed:true}]

B. [{id:2, completed:false}]

C. []

D. undefined

Solution

1. Step 1: Understand selector logic

   The selector filters todos where completed is true.

2. Step 2: Apply filter to given state

   Todos with id 1 and 3 have completed:true, so they are included.

3. Final Answer:

   [{id:1, completed:true}, {id:3, completed:true}] -> Option A

4. Quick Check:

   Filter completed todos = correct list [OK]

Hint: Filter returns only items matching condition [OK]

Common Mistakes:

• Selecting incomplete todos instead
• Returning empty array by mistake
• Confusing filter with map

4. What is wrong with this selector code?

const selectUserName = state => { state.user.name }

medium

A. It does not return any value explicitly

B. It uses wrong parameter name

C. It should dispatch an action instead

D. It should be a class method

Solution

1. Step 1: Check function body

   The function uses braces but no return statement, so it returns undefined.

2. Step 2: Fix by adding return

   Either remove braces or add return to return state.user.name.

3. Final Answer:

   It does not return any value explicitly -> Option A

4. Quick Check:

   Missing return in arrow function with braces [OK]

Hint: Arrow with braces needs explicit return [OK]

Common Mistakes:

• Forgetting return inside braces
• Confusing selectors with actions
• Using wrong parameter names

5. You want to create a memoized selector using reselect to get the count of completed todos from state. Which code correctly implements this?

hard

A. import { createSelector } from 'redux';
const selectCompletedCount = createSelector(state => state.todos, todos => todos.length);

B. const selectCompletedCount = state => state.todos.filter(todo => todo.completed).length;

C. import { createSelector } from 'reselect';
const selectTodos = state => state.todos;
const selectCompletedCount = createSelector(
selectTodos,
todos => todos.filter(todo => todo.completed).length
);

D. const selectCompletedCount = createSelector(state => state.todos, todos => todos.completed.length);

Solution

1. Step 1: Use correct import and createSelector

   The createSelector function is imported from 'reselect' and used properly.

2. Step 2: Define input selector and output function

   selectTodos gets todos array; output function filters completed and counts them.

3. Step 3: Check other options for errors

   The plain selector const selectCompletedCount = state => state.todos.filter(todo => todo.completed).length is not memoized; importing createSelector from 'redux' is wrong; todos => todos.completed.length accesses property incorrectly.

4. Final Answer:

   import { createSelector } from 'reselect'; const selectTodos = state => state.todos; const selectCompletedCount = createSelector(selectTodos, todos => todos.filter(todo => todo.completed).length); -> Option C

5. Quick Check:

   Use reselect createSelector with input and output functions [OK]

Hint: Use createSelector from 'reselect' with input and output functions [OK]

Common Mistakes:

• Importing createSelector from wrong package
• Not memoizing selector
• Incorrectly accessing properties in output function