Composable functions in Android Kotlin - Mini App: Build & Ship

Build: Greeting Screen

This screen shows a greeting message using a composable function. It displays a simple text greeting centered on the screen.

Target UI

---------------------
|                   |
|                   |
|    Hello, User!    |
|                   |
|                   |
---------------------

Create a composable function named Greeting that takes a name as a parameter and displays 'Hello, <name>!'

Use the Greeting composable inside the main screen composable to show the greeting message.

Center the text both vertically and horizontally on the screen.

Starter Code

Android Kotlin

import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview

@Composable
fun Greeting(name: String) {
    // TODO: Display greeting text here
}

@Composable
fun GreetingScreen() {
    Surface(
        modifier = Modifier.fillMaxSize(),
        color = MaterialTheme.colorScheme.background
    ) {
        // TODO: Call Greeting composable here
    }
}

@Preview(showBackground = true)
@Composable
fun GreetingScreenPreview() {
    GreetingScreen()
}

Task 1

Task 2

Task 3

Solution

Android Kotlin

import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview

@Composable
fun Greeting(name: String) {
    Text(text = "Hello, $name!")
}

@Composable
fun GreetingScreen() {
    Surface(
        modifier = Modifier.fillMaxSize(),
        color = MaterialTheme.colorScheme.background
    ) {
        Box(
            modifier = Modifier.fillMaxSize(),
            contentAlignment = Alignment.Center
        ) {
            Greeting(name = "User")
        }
    }
}

@Preview(showBackground = true)
@Composable
fun GreetingScreenPreview() {
    GreetingScreen()
}

We created a composable function Greeting that takes a name parameter and displays a greeting message using Text. In GreetingScreen, we used a Box with contentAlignment = Alignment.Center to center the greeting text both vertically and horizontally. We passed the string "User" to the Greeting composable to show the message "Hello, User!" on the screen.

Final Result

Completed Screen

---------------------
|                   |
|                   |
|    Hello, User!    |
|                   |
|                   |
---------------------

The screen shows the greeting text 'Hello, User!' centered.

No buttons or inputs; static display.

Stretch Goal

Modify the Greeting composable to accept a second parameter for a greeting prefix (e.g., 'Hi', 'Welcome') and display it before the name.

💡 Hint

Add a new parameter with a default value to the Greeting function and update the text to use it.

Practice

1. What is the main purpose of a @Composable function in Android Kotlin?

easy

A. To manage database operations

B. To handle network requests

C. To define app navigation routes

D. To build and display UI components

Solution

1. Step 1: Understand the role of @Composable

   The @Composable annotation marks functions that create UI elements in Jetpack Compose.

2. Step 2: Identify the correct purpose

   These functions are used to build and display UI components, not for networking or database tasks.

3. Final Answer:

   To build and display UI components -> Option D

4. Quick Check:

   @Composable = UI builder [OK]

Hint: Remember: @Composable means UI code [OK]

Common Mistakes:

• Thinking @Composable handles logic like networking
• Confusing @Composable with lifecycle methods
• Assuming it manages navigation or data storage

2. Which of the following is the correct way to declare a composable function in Kotlin?

easy

A. @Composable fun MyComponent() { /* UI code */ }

B. fun MyComponent() { /* UI code */ }

C. @Composable MyComponent() { /* UI code */ }

D. fun @Composable MyComponent() { /* UI code */ }

Solution

1. Step 1: Recall the syntax for composable functions

   Composable functions must be annotated with @Composable before the function keyword.

2. Step 2: Check each option

   @Composable fun MyComponent() { /* UI code */ } correctly places @Composable before fun. The other options either lack the annotation entirely or place it incorrectly (after fun or without fun).

3. Final Answer:

   @Composable fun MyComponent() { /* UI code */ } -> Option A

4. Quick Check:

   Annotation before fun = correct syntax [OK]

Hint: Always put @Composable before fun [OK]

Common Mistakes:

• Omitting the @Composable annotation
• Placing @Composable after the function name
• Using incorrect syntax like missing fun

3. What will be displayed when this composable function is called?

  @Composable
  fun Greeting() {
    Text(text = "Hello, Compose!")
  }

medium

A. A text label showing 'Hello, Compose!'

B. A compile-time error due to missing imports

C. An empty screen with no text

D. A button labeled 'Hello, Compose!'

Solution

1. Step 1: Understand the Text composable

   The Text composable displays a text label on the screen with the given string.

2. Step 2: Analyze the function output

   The function shows the text "Hello, Compose!" as a label, not a button or empty screen.

3. Final Answer:

   A text label showing 'Hello, Compose!' -> Option A

4. Quick Check:

   Text shows text label [OK]

Hint: Text composable always displays text on screen [OK]

Common Mistakes:

• Confusing Text with Button composable
• Assuming missing imports cause runtime error
• Expecting no output from a valid composable

4. Identify the error in this composable function:

  @Composable
  fun Welcome() {
    Text(text = Hello World)
  }

medium

A. Missing @Composable annotation

B. Missing quotes around the string in Text

C. Function name should be lowercase

D. Text composable cannot be used inside functions

Solution

1. Step 1: Check the Text parameter

   The string passed to Text must be in quotes. Here, "Hello World" is missing quotes.

2. Step 2: Verify other parts

   The function has the @Composable annotation, function name casing is allowed, and Text is valid inside composables.

3. Final Answer:

   Missing quotes around the string in Text -> Option B

4. Quick Check:

   Strings need quotes in Kotlin [OK]

Hint: Strings must be in quotes inside Text composable [OK]

Common Mistakes:

• Forgetting quotes around string literals
• Thinking function names must be lowercase
• Assuming @Composable is missing when it is present

5. How can you create a reusable composable that shows a greeting with a dynamic name?
Choose the correct function declaration:

  ?

hard

A. @Composable fun Greeting() {
Text(text = "Hello, name!")
}

B. fun Greeting(name: String) {
Text(text = "Hello, ${name}!")
}

C. @Composable fun Greeting(name: String) {
Text(text = "Hello, ${name}!")
}

D. @Composable fun Greeting(name) {
Text(text = "Hello, ${name}!")
}

Solution

1. Step 1: Check for @Composable annotation and parameter

   The function must be annotated with @Composable and accept a String parameter for the name.

2. Step 2: Verify string interpolation and parameter syntax

   @Composable fun Greeting(name: String) { Text(text = "Hello, ${name}!") } correctly uses @Composable, declares name: String, and interpolates it inside the text. The version without @Composable misses the annotation, the one without a parameter hardcodes the name, and the one missing the parameter type is invalid.

3. Final Answer:

   @Composable fun Greeting(name: String) { Text(text = "Hello, ${name}!") } -> Option C

4. Quick Check:

   Composable + typed parameter + string interpolation = correct [OK]

Hint: Composable needs annotation and typed parameters [OK]

Common Mistakes:

• Omitting @Composable annotation
• Not declaring parameter type
• Hardcoding text instead of using parameter