Picker and date picker in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Picker selection updates state correctly

What will be the value of selectedFruit after selecting "Banana" from the Picker?

React Native

import React, { useState } from 'react';
import { View, Text } from 'react-native';
import { Picker } from '@react-native-picker/picker';

export default function FruitPicker() {
  const [selectedFruit, setSelectedFruit] = useState('Apple');

  return (
    <View>
      <Picker
        selectedValue={selectedFruit}
        onValueChange={(itemValue) => setSelectedFruit(itemValue)}
      >
        <Picker.Item label="Apple" value="Apple" />
        <Picker.Item label="Banana" value="Banana" />
        <Picker.Item label="Cherry" value="Cherry" />
      </Picker>
      <Text>Selected: {selectedFruit}</Text>
    </View>
  );
}

A "Banana"

B "Apple"

C "Cherry"

D undefined

💡 Hint

The onValueChange updates the state with the selected value.

When the user selects "Banana", the onValueChange callback sets selectedFruit to "Banana". The Text component then shows "Selected: Banana".

❓ ui_behavior

intermediate

DatePicker initial date and change behavior

Given the code below, what will be displayed in the Text component after the user picks March 15, 2024?

React Native

import React, { useState } from 'react';
import { View, Text, Button, Platform } from 'react-native';
import DateTimePicker from '@react-native-community/datetimepicker';

export default function DatePickerExample() {
  const [date, setDate] = useState(new Date(2023, 0, 1));
  const [show, setShow] = useState(false);

  const onChange = (event, selectedDate) => {
    setShow(Platform.OS === 'ios');
    if (selectedDate) {
      setDate(selectedDate);
    }
  };

  return (
    <View>
      <Button title="Pick a date" onPress={() => setShow(true)} />
      {show && (
        <DateTimePicker
          value={date}
          mode="date"
          display="default"
          onChange={onChange}
        />
      )}
      <Text>Selected date: {date.toDateString()}</Text>
    </View>
  );
}

A Selected date: Thu Mar 14 2024

B Selected date: Fri Mar 15 2024

C Selected date: Sun Jan 01 2023

D Selected date: undefined

💡 Hint

The onChange updates the date state with the picked date.

When the user picks March 15, 2024, the onChange sets date to that date. The Text component shows the date in a readable string format.

❓ lifecycle

advanced

DatePicker visibility on Android vs iOS

Why does the DateTimePicker disappear immediately after selecting a date on Android but not on iOS in the code below?

React Native

const onChange = (event, selectedDate) => {
  setShow(Platform.OS === 'ios');
  if (selectedDate) {
    setDate(selectedDate);
  }
};

A Because on iOS, <code>setShow(false)</code> is called immediately after selection.

B Because on iOS, the picker does not call <code>onChange</code> after selection.

C Because on Android, the picker is not rendered at all.

D Because on Android, <code>setShow(false)</code> hides the picker after selection; on iOS, it stays visible.

💡 Hint

Check how setShow is set depending on platform.

The code sets show to false on Android after selection, hiding the picker. On iOS, it remains true, so the picker stays visible.

📝 Syntax

advanced

Correct syntax for Picker.Item inside Picker

Which option shows the correct syntax to add a Picker.Item with label "Orange" and value "orange" inside a Picker?

A <Picker.Item label='Orange' value=orange />

B <Picker.Item label=Orange value="orange" />

C <Picker.Item label="Orange" value="orange" />

D <Picker.Item label="Orange" value="orange"> </Picker.Item>

💡 Hint

Remember JSX syntax requires quotes around string props.

Option C uses correct JSX syntax with double quotes around string props and self-closing tag.

🔧 Debug

expert

Why does the DatePicker not update the displayed date?

Given the code below, why does the displayed date never change after picking a new date?

React Native

import React, { useState } from 'react';
import { View, Text, Button } from 'react-native';
import DateTimePicker from '@react-native-community/datetimepicker';

export default function BrokenDatePicker() {
  const [date, setDate] = useState(new Date());
  const [show, setShow] = useState(false);

  const onChange = (event, selectedDate) => {
    setShow(false);
    if (selectedDate) {
      date = selectedDate;
    }
  };

  return (
    <View>
      <Button title="Pick date" onPress={() => setShow(true)} />
      {show && <DateTimePicker value={date} mode="date" onChange={onChange} />}
      <Text>Selected date: {date.toDateString()}</Text>
    </View>
  );
}

A Because the code assigns to the variable 'date' directly instead of using setDate to update state.

B Because the DateTimePicker component is missing the 'display' prop.

C Because the initial state date is not set to a valid Date object.

D Because the onChange callback does not call setShow(true).

💡 Hint

In React, state must be updated with the setter function, not by direct assignment.

The line date = selectedDate; only changes the local variable, not the React state. The component does not re-render with the new date.

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