Button composable in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Button composable in Jetpack Compose?

A Button composable is a clickable UI element in Jetpack Compose that triggers an action when pressed. It is used to create buttons in Android apps with declarative UI.

beginner

How do you create a simple Button with text "Click me" in Jetpack Compose?

Use the Button composable with a lambda for onClick and a Text composable inside it: Button(onClick = { /* action */ }) { Text("Click me") }

beginner

What parameter must you always provide to a Button composable?

You must always provide the onClick parameter, which is a lambda function that defines what happens when the button is pressed.

intermediate

How can you customize the colors of a Button in Jetpack Compose?

You can customize colors by using the colors parameter with ButtonDefaults.buttonColors(), for example: Button( onClick = { }, colors = ButtonDefaults.buttonColors(containerColor = Color.Red) ) { Text("Red Button") }

intermediate

What is the difference between Button and TextButton in Jetpack Compose?

Button has a background and elevation by default, making it look like a raised button. TextButton is a button without background or elevation, appearing as just text that is clickable.

Which parameter is required when creating a Button composable?

A shape

B text

C color

D onClick

The onClick parameter defines the action when the button is pressed and is required.

How do you add text inside a Button composable?

A Use the Text composable inside the Button's lambda

B Set the text parameter of Button

C Use a string directly as Button's argument

D Use a TextView inside Button

Button composable takes a lambda where you can place child composables like Text.

What does ButtonDefaults.buttonColors() help you do?

A Set button size

B Add icons to button

C Customize button colors

D Change button font

ButtonDefaults.buttonColors() provides color customization options for buttons.

Which composable looks like a flat text without background by default?

A TextButton

B Button

C OutlinedButton

D IconButton

TextButton is a button styled as flat text without background or elevation.

What happens if you omit the onClick parameter in a Button composable?

A Button shows default action

B Compilation error

C Button works but does nothing

D Button is disabled automatically

onClick is a required parameter; omitting it causes a compilation error.

Explain how to create a basic Button composable with a click action and text label.

Think about how you tell the button what to do and what it shows.

Describe ways to customize the appearance of a Button in Jetpack Compose.

Colors and styles can be changed using built-in helpers.

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