React.memo and useMemo in React Native - Build, Publish & Deploy

Build & Publish - React.memo and useMemo

Performance Impact

Using React.memo and useMemo helps your React Native app avoid unnecessary re-rendering. This keeps the frame rate smooth, aiming for 60fps or higher, which means animations and interactions feel fast and natural. Without these, your app might waste CPU and memory on components that don't need to update, causing janky scrolling or slow UI responses. Efficient memoization also reduces battery drain by lowering CPU usage.

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How to Optimize for 60fps Rendering

Use React.memo to wrap functional components that receive props but don't always need to update. This tells React to skip re-rendering if props haven't changed. Use useMemo to memoize expensive calculations or objects so they don't get recreated on every render. Avoid overusing these hooks; only memoize when you see performance issues or when components are heavy. Profiling with React DevTools helps identify where memoization is beneficial.

Impact on App Bundle Size and Startup Time

Both React.memo and useMemo are built-in React APIs and add no extra bundle size. They do not increase startup time. Their impact is purely on runtime performance by reducing unnecessary work. Using them wisely can improve perceived app speed without affecting app size.

iOS vs Android Differences

React Native uses the same React APIs on both iOS and Android, so React.memo and useMemo behave identically across platforms. However, performance gains might feel different due to device hardware and OS optimizations. Android devices vary more in CPU power, so memoization can have a bigger impact on lower-end devices. iOS devices tend to have more consistent performance, but memoization still helps maintain smooth UI.

Store Review Guidelines and Requirements

Neither Apple App Store nor Google Play Store have specific rules about using React.memo or useMemo. However, both stores require apps to be responsive and not freeze or crash. Efficient rendering helps meet these requirements by preventing UI hangs. Also, avoid excessive memory use or battery drain, which can cause app rejection. Using memoization correctly supports these guidelines by improving app stability and user experience.

Self-Check: Your app takes 5 seconds to load this screen. What's likely wrong?

If your screen is slow to load, it might be because components are re-rendering too often or expensive calculations run on every render. Check if you are missing React.memo on components that receive unchanged props or if you need useMemo to cache heavy computations. Also, verify that you are not creating new objects or functions inline that cause re-renders. Profiling with React DevTools can help find these issues.

Key Result

Using React.memo and useMemo in React Native improves app performance by preventing unnecessary re-renders, leading to smoother UI at 60fps, no increase in bundle size, and consistent behavior across iOS and Android.

Practice

1. What is the main purpose of React.memo in React Native?

easy

A. To manage component state

B. To prevent a component from re-rendering if its props have not changed

C. To fetch data from an API

D. To memorize a value between renders

Solution

1. Step 1: Understand React.memo's role

   React.memo is a higher-order component that caches the rendered output of a component.

2. Step 2: Identify its effect on re-rendering

   It skips re-rendering if the component's props have not changed, improving performance.

3. Final Answer:

   To prevent a component from re-rendering if its props have not changed -> Option B

4. Quick Check:

   React.memo = skip re-render if props unchanged [OK]

Hint: React.memo skips re-render when props stay the same [OK]

Common Mistakes:

• Confusing React.memo with useMemo
• Thinking React.memo manages state
• Believing React.memo fetches data

2. Which of the following is the correct syntax to use useMemo in a React Native functional component?

easy

A. const memoizedValue = useMemo(computeValue(), dependencies);

B. const memoizedValue = useMemo(computeValue, dependencies);

C. const memoizedValue = useMemo(() => computeValue, dependencies);

D. const memoizedValue = useMemo(() => computeValue(), [dependencies]);

Solution

1. Step 1: Recall useMemo syntax

   useMemo takes a function returning a value and an array of dependencies.

2. Step 2: Match correct syntax

   const memoizedValue = useMemo(() => computeValue(), [dependencies]); correctly uses an arrow function and dependency array.

3. Final Answer:

   const memoizedValue = useMemo(() => computeValue(), [dependencies]); -> Option D

4. Quick Check:

   useMemo syntax = function + dependencies array [OK]

Hint: useMemo needs a function and dependency array [OK]

Common Mistakes:

• Passing dependencies without array brackets
• Calling computeValue immediately instead of in a function
• Omitting the arrow function wrapper

3. Given the code below, what will be logged when MyComponent re-renders with the same count prop?

const MyComponent = React.memo(({ count }) => {
  console.log('Rendered');
  return {count};
});

medium

A. Rendered logged once

B. Rendered logged twice

C. No logs

D. Error due to duplicate components

Solution

1. Step 1: Understand React.memo behavior on same props

   React.memo skips re-render if props are unchanged, so the component renders only once.

2. Step 2: Analyze console.log calls

   React.memo prevents re-rendering when props are the same on subsequent renders. Thus, 'Rendered' is logged only once, on the initial render.

3. Final Answer:

   Rendered logged once -> Option A

4. Quick Check:

   React.memo skips re-render = one log [OK]

Hint: React.memo skips re-render if props unchanged, so one log [OK]

Common Mistakes:

• Assuming component renders twice
• Thinking React.memo disables all rendering
• Confusing prop changes with state changes

4. Identify the error in this React Native code using useMemo:

const memoizedValue = useMemo(computeValue(), [dependency]);

medium

A. computeValue is called immediately instead of being passed as a function

B. Dependency array should be empty

C. useMemo cannot be used in functional components

D. Missing import of useMemo

Solution

1. Step 1: Check how useMemo expects its first argument

   useMemo expects a function, not the result of a function call.

2. Step 2: Identify the mistake in the code

   computeValue() is called immediately; it should be wrapped in a function like () => computeValue().

3. Final Answer:

   computeValue is called immediately instead of being passed as a function -> Option A

4. Quick Check:

   useMemo needs a function, not a direct call [OK]

Hint: Pass function to useMemo, not function call [OK]

Common Mistakes:

• Calling function instead of passing it
• Ignoring dependency array importance
• Assuming useMemo works without import

5. You have a component that receives a large list as a prop and a function to filter it. You want to optimize rendering and avoid recalculating the filtered list unnecessarily. Which combination is best?

const FilteredList = React.memo(({ list, filterFunc }) => {
  const filtered = useMemo(() => list.filter(filterFunc), [list, filterFunc]);
  return {filtered.length};
});

hard

A. Only use useMemo for filtered list, no React.memo needed

B. Only use React.memo on the component, no useMemo needed

C. Use React.memo on the component and useMemo for filtered list calculation

D. Neither React.memo nor useMemo are useful here

Solution

1. Step 1: Understand React.memo role

   React.memo prevents re-render if props (list, filterFunc) do not change.

2. Step 2: Understand useMemo role

   useMemo caches the filtered list calculation, avoiding expensive recalculation unless dependencies change.

3. Step 3: Combine both for best optimization

   Using both together avoids unnecessary renders and expensive recalculations.

4. Final Answer:

   Use React.memo on the component and useMemo for filtered list calculation -> Option C

5. Quick Check:

   React.memo + useMemo = best performance [OK]

Hint: Combine React.memo and useMemo for props and expensive calculations [OK]

Common Mistakes:

• Using only one optimization and missing the other
• Not including dependencies in useMemo
• Assuming useMemo alone prevents re-render