React.memo and useMemo in React Native - UI Render Trace

Component - React.memo and useMemo

This component demonstrates how React.memo and useMemo help optimize React Native apps by preventing unnecessary re-renders and expensive recalculations. React.memo wraps a component to skip re-rendering if props don't change. useMemo caches a computed value until dependencies change.

Widget Tree

App
├─ View (container)
│  ├─ MemoizedCounter (React.memo)
│  │  └─ Text (displays count)
│  ├─ Button (Increment)
│  └─ Text (Expensive calculation result)

The root App component contains a View as a container. Inside, there is a MemoizedCounter component wrapped with React.memo that shows the current count. Below it is a Button to increment the count. Finally, a Text component displays the result of an expensive calculation memoized with useMemo.

Render Trace - 4 Steps

Step 1: App

Step 2: MemoizedCounter

Step 3: Button

Step 4: Text (Expensive calculation)

State Change - Re-render

Trigger:User taps Increment button

Before

count = 0

→

After

count = 1

Re-renders:App and children re-render; MemoizedCounter re-renders because count prop changed; memoized calculation recomputes for count 1

UI Quiz - 3 Questions

What does React.memo do in this component?

A Handles button press events

B Caches the result of the expensive calculation

C Prevents MemoizedCounter from re-rendering if count prop is unchanged

D Creates the initial state

Key Insight

Using React.memo and useMemo together helps React Native apps run smoothly by avoiding unnecessary work. React.memo skips re-rendering components when props don't change, saving processing time. useMemo caches expensive calculations so they only run when needed. This keeps the UI responsive and efficient.

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