Lifting state up in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does "lifting state up" mean in React Native?

It means moving shared state from child components to their closest common parent so they can share and update data together.

beginner

Why do we lift state up in React Native apps?

To keep data consistent between components that need to share or sync information, avoiding duplicated or conflicting states.

beginner

In lifting state up, where should the state live?

The state should live in the closest common parent component of all components that need to access or change that state.

intermediate

How do child components update lifted state?

The parent passes down functions as props to children. Children call these functions to update the state in the parent.

intermediate

What is a simple example of lifting state up?

Two input fields share the same text state lifted to their parent. When one input changes, the parent updates the state and passes it down to both inputs.

What is the main reason to lift state up in React Native?

A To share state between sibling components

B To make components load faster

C To avoid using props

D To reduce the number of components

Lifting state up allows sibling components to share and sync state by moving it to their common parent.

Where should the lifted state be stored?

A In a global variable

B In the child component that uses it

C In the closest common parent component

D In a separate file

The state should live in the closest common parent so all children needing it can access it via props.

How do child components update the lifted state?

A By directly changing the parent's state

B By reloading the app

C By using global variables

D By calling functions passed down as props

Parents pass down update functions as props; children call these to request state changes.

Which of these is NOT a benefit of lifting state up?

A Keeps data consistent

B Makes components independent of each other

C Avoids duplicated state

D Simplifies data flow

Lifting state up actually connects components by sharing state, so they are not independent.

If two sibling components need to display the same data, what should you do?

A Lift the state up to their parent

B Use local state in each component

C Duplicate the state in both components

D Use a third-party library immediately

Lifting state up to the parent allows both siblings to share the same source of truth.

Explain in your own words what "lifting state up" means and why it is useful in React Native.

Think about how siblings can share information through their parent.

Describe a simple example where lifting state up improves your app's behavior.

Imagine two text inputs showing the same text.

Practice

1. What is the main purpose of lifting state up in React Native?

easy

A. To create multiple independent states in child components

B. To move shared state to a common parent component so children can access it

C. To store state only inside child components without sharing

D. To avoid using props for data sharing between components

Solution

1. Step 1: Understand the concept of shared state

   Lifting state up means moving state to a common parent so multiple children can use it.

2. Step 2: Identify the purpose of lifting state up

   This helps keep data consistent and allows children to communicate through the parent.

3. Final Answer:

   To move shared state to a common parent component so children can access it -> Option B

4. Quick Check:

   Lifting state up = shared parent state [OK]

Hint: Shared data lives in parent to sync children easily [OK]

Common Mistakes:

• Thinking each child should have its own separate state
• Believing lifting state up removes the need for props
• Confusing lifting state up with component styling

2. Which of the following is the correct way to pass a state update function from a parent to a child component in React Native?

easy

A.

B.

C.

D.

Solution

1. Step 1: Identify what to pass as prop

   The parent passes a function to update state, not the state value or setState method directly.

2. Step 2: Choose the correct syntax

   Passing updateStateFunction as a prop is correct to allow child to call it.

3. Final Answer:

   <Child updateState={updateStateFunction} /> -> Option C

4. Quick Check:

   Pass update function as prop = <Child updateState={updateStateFunction} /> [OK]

Hint: Pass functions, not state or setState directly [OK]

Common Mistakes:

• Passing the entire state object instead of a function
• Passing setState method directly without binding
• Passing state value instead of update function

3. Given the code below, what will be the output when the button in Child is pressed?

function Parent() {
  const [count, setCount] = React.useState(0);
  return (
    <>
      <Child count={count} increment={() => setCount(count + 1)} />
      <Text>Count: {count}</Text>
    </>
  );
}

function Child({ count, increment }) {
  return <Button title={`Add`} onPress={increment} />;
}

medium

A. The app will crash due to incorrect state update

B. The count will always stay 0 regardless of button presses

C. The button will not respond to presses

D. The count displayed will increase by 1 each time the button is pressed

Solution

1. Step 1: Understand state and props flow

   The Parent holds the count state and passes it plus an increment function to Child.

2. Step 2: Analyze what happens on button press

   Pressing the button calls increment, which updates count by adding 1.

3. Final Answer:

   The count displayed will increase by 1 each time the button is pressed -> Option D

4. Quick Check:

   State update via lifted function increments count [OK]

Hint: Button calls parent's update function to increase count [OK]

Common Mistakes:

• Thinking count won't update because it's passed as prop
• Assuming setCount is not called correctly
• Confusing local and lifted state

4. Identify the error in this lifting state up example:

function Parent() {
  const [text, setText] = React.useState('');
  return <Child text={text} setText={setText()} />;
}

function Child({ text, setText }) {
  return <TextInput value={text} onChangeText={setText} />;
}

medium

A. setText is called immediately instead of passing the function

B. text state is not initialized properly

C. Child component should not receive setText as prop

D. TextInput should not use onChangeText prop

Solution

1. Step 1: Check how setText is passed as prop

   In Parent, setText={setText()} calls setText immediately instead of passing the function.

2. Step 2: Correct way to pass setText

   It should be setText={setText} to pass the function reference.

3. Final Answer:

   setText is called immediately instead of passing the function -> Option A

4. Quick Check:

   Pass function reference, not call it [OK]

Hint: Pass function, don't call it when passing as prop [OK]

Common Mistakes:

• Calling setState function instead of passing it
• Confusing state value with update function
• Ignoring React hooks rules

5. You have two sibling components, TemperatureInputCelsius and TemperatureInputFahrenheit, that need to share temperature data. How should you use lifting state up to keep both inputs in sync?

hard

A. Move the temperature state to their common parent and pass value and update functions as props

B. Keep separate states in each sibling and update independently

C. Store temperature state only in one sibling and let the other read it directly

D. Use global variables outside React components to share temperature

Solution

1. Step 1: Identify the need for shared state

   Both siblings need to reflect the same temperature, so state must be shared.

2. Step 2: Apply lifting state up

   Move temperature state to their common parent and pass it plus update functions down as props.

3. Step 3: Avoid incorrect approaches

   Separate states cause inconsistency; global variables break React's data flow.

4. Final Answer:

   Move the temperature state to their common parent and pass value and update functions as props -> Option A

5. Quick Check:

   Lift shared state to parent for syncing siblings [OK]

Hint: Parent holds shared state; siblings get props and update functions [OK]

Common Mistakes:

• Keeping separate states causing out-of-sync inputs
• Trying to share state via global variables
• Passing state only to one sibling