Picker and date picker in React Native - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Picker component in React Native?

A Picker is a dropdown menu that lets users select one option from a list. It looks like a spinning wheel or dropdown on mobile devices.

beginner

How do you import the DateTimePicker in React Native?

You import it from '@react-native-community/datetimepicker' package using: <br><code>import DateTimePicker from '@react-native-community/datetimepicker';</code>

beginner

What prop controls the mode of the DateTimePicker?

The mode prop controls whether the picker shows a date, time, or both. Values can be 'date', 'time', or 'datetime'.

beginner

How do you handle the selected value from a Picker?

You use the onValueChange prop to get the selected value and update your component's state to reflect the choice.

intermediate

Why should you use accessibility labels with Picker and DateTimePicker?

Accessibility labels help screen readers describe the picker to users with disabilities, making your app usable by everyone.

Which prop sets the selected value in a React Native Picker?

A currentValue

B value

C defaultValue

D selectedValue

The selectedValue prop sets the current selected option in a Picker.

What package do you install to use DateTimePicker in React Native?

A @react-native-community/datetimepicker

B react-native-picker

C react-native-datepicker

D react-native-date-picker

The official DateTimePicker is from '@react-native-community/datetimepicker'.

Which mode shows only the time in DateTimePicker?

A datetime

B date

C time

D timer

The mode 'time' shows only the time selection.

How do you open the DateTimePicker on Android?

A It opens automatically on render

B Using a button with onPress to set visibility

C By calling a show() method

D By setting a visible state and rendering conditionally

On Android, you usually control visibility with a button press that sets a state to show the picker.

Which accessibility prop helps screen readers identify a Picker?

A accessibilityLabel

B accessibilityHint

C aria-label

D accessibilityRole

The accessibilityLabel describes the Picker for screen readers.

Explain how to implement a Picker in React Native and handle user selection.

Think about how you show options and save the chosen one.

Describe how to use DateTimePicker to select a date and time in a React Native app.

Consider how you open the picker and get the chosen date or time.

Practice

1. What is the main purpose of a Picker component in React Native?

easy

A. To input text data

B. To display images in a gallery

C. To play audio files

D. To let users select one option from a list

Solution

1. Step 1: Understand Picker usage

   A Picker is designed to show a list of options for the user to choose from.

2. Step 2: Compare with other components

   Other options like text input or image gallery serve different purposes, not selection from a list.

3. Final Answer:

   To let users select one option from a list -> Option D

4. Quick Check:

   Picker = select one option [OK]

Hint: Picker is for choosing one item from many [OK]

Common Mistakes:

• Confusing Picker with text input
• Thinking Picker plays media
• Assuming Picker shows images

2. Which of the following is the correct way to import the Picker component from @react-native-picker/picker?

easy

A. import { Picker } from 'react-native';

B. import Picker from '@react-native-picker/picker';

C. import { Picker } from '@react-native-picker/picker';

D. import Picker from 'react-native-picker';

Solution

1. Step 1: Check official import syntax

   The Picker is exported as a named export from '@react-native-picker/picker', so curly braces are needed.

2. Step 2: Eliminate incorrect imports

   Default import from the correct package or imports from wrong packages are incorrect.

3. Final Answer:

   import { Picker } from '@react-native-picker/picker'; -> Option C

4. Quick Check:

   Named import with braces [OK]

Hint: Use curly braces for named imports like Picker [OK]

Common Mistakes:

• Using default import instead of named import
• Importing from wrong package
• Missing curly braces around Picker

3. Given this code snippet, what will be displayed when the user selects "Banana" from the Picker?

const [fruit, setFruit] = useState('Apple');

<Picker selectedValue={fruit} onValueChange={setFruit}>
  <Picker.Item label="Apple" value="Apple" />
  <Picker.Item label="Banana" value="Banana" />
  <Picker.Item label="Cherry" value="Cherry" />
</Picker>

<Text>Selected fruit: {fruit}</Text>

medium

A. Selected fruit: Banana

B. Selected fruit: undefined

C. Selected fruit: Cherry

D. Selected fruit: Apple

Solution

1. Step 1: Understand state update on selection

   The onValueChange calls setFruit with the selected value, updating the fruit state.

2. Step 2: Check selected value after choosing Banana

   When Banana is selected, fruit becomes 'Banana', so the Text shows 'Selected fruit: Banana'.

3. Final Answer:

   Selected fruit: Banana -> Option A

4. Quick Check:

   State updates to selected value = Banana [OK]

Hint: Selected value updates state shown in Text [OK]

Common Mistakes:

• Assuming state does not update on selection
• Confusing label and value
• Expecting default value always

4. Identify the error in this DatePicker usage:

const [date, setDate] = useState(new Date());

<DatePicker
  value={date}
  onChange={(event, selectedDate) => setDate(selectedDate)}
/>

medium

A. The 'value' prop should be 'date' instead of 'value'

B. The DatePicker component requires 'mode' prop

C. The 'onChange' callback parameters are incorrect

D. The 'setDate' function should not be called inside 'onChange'

Solution

1. Step 1: Review DatePicker required props

   React Native DatePicker requires a 'mode' prop like 'date' or 'time' to work properly.

2. Step 2: Check other props correctness

   'value' and 'onChange' usage is correct; 'setDate' is correctly called to update state.

3. Final Answer:

   The DatePicker component requires 'mode' prop -> Option B

4. Quick Check:

   Missing required 'mode' prop [OK]

Hint: DatePicker needs 'mode' prop to work [OK]

Common Mistakes:

• Omitting 'mode' prop causes no date shown
• Confusing 'value' with 'selected' prop
• Ignoring event parameter in onChange

5. You want to create a form with a Picker for selecting a country and a DatePicker for selecting a birthdate. How do you ensure the UI updates correctly when the user changes either input?

hard

A. Use separate state variables for country and birthdate, update each on change

B. Use a single state object and never update it

C. Only update state when the form is submitted

D. Use refs instead of state to track selections

Solution

1. Step 1: Manage state for each input separately

   Use one state variable for country and another for birthdate to track changes independently.

2. Step 2: Update state on user interaction

   Update the corresponding state in onValueChange for Picker and onChange for DatePicker to reflect UI changes.

3. Final Answer:

   Use separate state variables for country and birthdate, update each on change -> Option A

4. Quick Check:

   Separate states updated on change [OK]

Hint: Track each input with its own state variable [OK]

Common Mistakes:

• Using one state object but not updating it
• Delaying state update until submit
• Using refs instead of state for UI updates