Button composable in Android Kotlin - Mini App: Build & Ship

Build: Simple Button Screen

A screen with a button that shows a message when clicked.

Target UI

---------------------
|                   |
|   [Click Me]      |
|                   |
---------------------

Add a Button composable with the text 'Click Me'.

When the button is clicked, show a Toast message 'Button clicked!'.

Center the button on the screen.

Starter Code

Android Kotlin

import android.os.Bundle
import android.widget.Toast
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            Box(
                modifier = Modifier.fillMaxSize(),
                contentAlignment = Alignment.Center
            ) {
                // TODO: Add Button composable here
            }
        }
    }
}

Task 1

Task 2

Task 3

Solution

Android Kotlin

import android.os.Bundle
import android.widget.Toast
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            Box(
                modifier = Modifier.fillMaxSize(),
                contentAlignment = Alignment.Center
            ) {
                Button(onClick = {
                    Toast.makeText(this, "Button clicked!", Toast.LENGTH_SHORT).show()
                }) {
                    Text(text = "Click Me")
                }
            }
        }
    }
}

We use a Box to center the button on the screen by setting contentAlignment = Alignment.Center. Inside the Box, we add a Button composable with the text 'Click Me'. The onClick lambda shows a Toast message 'Button clicked!' using Toast.makeText. This gives immediate feedback when the user taps the button.

Final Result

Completed Screen

---------------------
|                   |
|   [Click Me]      |
|                   |
---------------------

When the user taps the 'Click Me' button, a small popup message 'Button clicked!' appears briefly at the bottom.

Stretch Goal

Change the button color to a custom color and add a second button that resets the Toast message.

💡 Hint

Use the colors parameter in Button and add another Button composable below the first one inside a Column.

Practice

1. What is the main purpose of a Button composable in Android Jetpack Compose?

easy

A. To show lists of items

B. To display images on the screen

C. To create text input fields

D. To let users tap and trigger actions

Solution

1. Step 1: Understand the role of Button composable

   The Button composable is designed to be tapped by users to perform an action.

2. Step 2: Compare with other UI elements

   Other options like displaying images or text input are done by different composables, not Button.

3. Final Answer:

   To let users tap and trigger actions -> Option D

4. Quick Check:

   Button = triggers actions on tap [OK]

Hint: Buttons are for user taps that do something [OK]

Common Mistakes:

• Confusing Button with Image or TextField
• Thinking Button displays content only
• Assuming Button is for layout only

2. Which of the following is the correct syntax to create a Button with a label "Click Me" in Jetpack Compose?

easy

A. Button(onClick = { Text("Click Me") })

B. Button { onClick = { } Text("Click Me") }

C. Button(onClick = { }) { Text("Click Me") }

D. Button(Text("Click Me"), onClick = { })

Solution

1. Step 1: Recall Button syntax in Compose

   The Button composable takes an onClick lambda and a content lambda for the label.

2. Step 2: Check each option

   Button(onClick = { }) { Text("Click Me") } correctly places onClick and Text inside Button's trailing lambda. Others misuse syntax or parameter order.

3. Final Answer:

   Button(onClick = { }) { Text("Click Me") } -> Option C

4. Quick Check:

   Button(onClick) { Text } syntax [OK]

Hint: Button(onClick) { Text("label") } is correct pattern [OK]

Common Mistakes:

• Putting onClick inside Button's content lambda
• Passing Text as a parameter instead of inside lambda
• Wrong order or missing braces

3. What will be displayed when this code runs?

Button(onClick = { println("Tapped") }) {
  Text("Press Here")
}

medium

A. A text label "Press Here" with no button functionality

B. A button labeled "Press Here" that prints "Tapped" when clicked

C. A button labeled "Tapped" that prints "Press Here" when clicked

D. An error because println cannot be used inside onClick

Solution

1. Step 1: Identify the Button label

   The Button shows the Text composable with label "Press Here".

2. Step 2: Understand onClick behavior

   When tapped, the onClick lambda runs println("Tapped"), printing to console.

3. Final Answer:

   A button labeled "Press Here" that prints "Tapped" when clicked -> Option B

4. Quick Check:

   Button label = "Press Here", onClick prints "Tapped" [OK]

Hint: Button label is Text; onClick runs code inside braces [OK]

Common Mistakes:

• Confusing label text with printed output
• Thinking println causes UI error
• Assuming onClick changes label text

4. This code tries to create a Button but causes a compile error:

Button(onClick = println("Clicked")) {
  Text("Tap")
}

What is the error?

medium

A. onClick expects a lambda, but println("Clicked") is called immediately

B. Text composable cannot be inside Button

C. Button requires a label parameter, missing here

D. onClick cannot print text

Solution

1. Step 1: Analyze onClick parameter

   onClick expects a lambda () -> Unit, but println("Clicked") is a function call executed immediately.

2. Step 2: Understand correct lambda usage

   It should be onClick = { println("Clicked") } to pass a lambda that runs println when clicked.

3. Final Answer:

   onClick expects a lambda, but println("Clicked") is called immediately -> Option A

4. Quick Check:

   onClick needs lambda, not immediate call [OK]

Hint: Use curly braces {} for onClick lambda, not direct call [OK]

Common Mistakes:

• Calling function instead of passing lambda
• Misunderstanding onClick parameter type
• Thinking Text inside Button causes error

5. You want a Button that disables itself after being clicked once. Which approach correctly implements this behavior?

hard

A. Use a state variable to track clicks and set enabled = false after first click

B. Call Button(enabled = false) directly without state

C. Put enabled = false inside onClick lambda

D. Use Text composable with onClick instead of Button

Solution

1. Step 1: Understand Button enabled property

   The Button composable has an enabled parameter to control if it can be clicked.

2. Step 2: Use state to track clicks

   To disable after first click, use a state variable that changes on click and updates enabled.

3. Step 3: Evaluate options

   Use a state variable to track clicks and set enabled = false after first click correctly uses state to disable button after click. Others ignore state or misuse enabled.

4. Final Answer:

   Use a state variable to track clicks and set enabled = false after first click -> Option A

5. Quick Check:

   State controls enabled to disable button after click [OK]

Hint: Use state to toggle enabled property after click [OK]

Common Mistakes:

• Setting enabled false without state causes permanent disable
• Trying to disable inside onClick without state
• Using Text instead of Button for clickable UI