Surface and shape in Android Kotlin - Mini App: Build & Ship

Build: ShapeSurfaceScreen

This screen shows a colored rectangle with rounded corners and a shadow to demonstrate surface and shape styling in Android using Kotlin.

Target UI

┌───────────────────────────────┐
│ ShapeSurfaceScreen             │
│ ┌─────────────────────────┐   │
│ │                         │   │
│ │   [ Rounded Rectangle ] │   │
│ │                         │   │
│ └─────────────────────────┘   │
│                               │
└───────────────────────────────┘

Create a screen with a centered rectangle shape

The rectangle must have rounded corners with 16dp radius

Add a subtle shadow below the rectangle to simulate elevation

Fill the rectangle with a light blue color

Use Material Design surface and shape theming where possible

Starter Code

Android Kotlin

package com.example.shapesurface

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.background
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.size
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.unit.dp

class ShapeSurfaceScreen : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            ShapeSurfaceContent()
        }
    }
}

@Composable
fun ShapeSurfaceContent() {
    // TODO: Add your code here
}

Task 1

Task 2

Task 3

Solution

Android Kotlin

package com.example.shapesurface

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.foundation.layout.size
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.unit.dp
import androidx.compose.foundation.shape.RoundedCornerShape

class ShapeSurfaceScreen : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            ShapeSurfaceContent()
        }
    }
}

@Composable
fun ShapeSurfaceContent() {
    Box(
        modifier = Modifier.fillMaxSize(),
        contentAlignment = Alignment.Center
    ) {
        Surface(
            modifier = Modifier.size(width = 200.dp, height = 120.dp),
            shape = RoundedCornerShape(16.dp),
            color = Color(0xFFBBDEFB), // Light blue
            shadowElevation = 8.dp
        ) {
            // Empty content inside surface
        }
    }
}

We use a Surface composable to create a rectangle with a background color and elevation (shadow). The RoundedCornerShape with 16dp radius gives the corners a smooth curve. The shadowElevation property adds a subtle shadow below the surface to simulate depth. The surface is centered inside a Box that fills the screen. The light blue color is set using a hex color code. This demonstrates how to style surfaces and shapes in Android Compose with Kotlin.

Final Result

Completed Screen

┌───────────────────────────────┐
│ ShapeSurfaceScreen             │
│                               │
│           ┌───────────────┐   │
│           │               │   │
│           │               │   │
│           │               │   │
│           └───────────────┘   │
│                               │
└───────────────────────────────┘

The rectangle is static and centered on the screen

It has rounded corners and a subtle shadow below

No user interaction is required

Stretch Goal

Add a button below the rectangle that toggles the rectangle's color between light blue and light green when tapped.

💡 Hint

Use a mutable state variable to hold the current color and update it on button click. Use Compose's Button and remember { mutableStateOf(...) }.

Practice

1. What is the primary purpose of SurfaceView in Android development?

easy

A. To provide a dedicated drawing surface for custom graphics

B. To handle user input events like clicks and touches

C. To manage app navigation between screens

D. To display static text content

Solution

1. Step 1: Understand SurfaceView role

   SurfaceView provides a special area where you can draw graphics directly on a separate thread.

2. Step 2: Compare with other options

   Handling input, navigation, or static text are not the main roles of SurfaceView.

3. Final Answer:

   To provide a dedicated drawing surface for custom graphics -> Option A

4. Quick Check:

   SurfaceView = Dedicated drawing surface [OK]

Hint: SurfaceView is for drawing graphics, not UI controls [OK]

Common Mistakes:

• Confusing SurfaceView with View for UI controls
• Thinking SurfaceView handles navigation
• Assuming SurfaceView manages text display

2. Which of the following is the correct way to start drawing on a SurfaceView canvas in Kotlin?

easy

A. val canvas = surfaceView.getCanvas()
canvas.drawColor(Color.RED)
surfaceView.postCanvas(canvas)

B. val canvas = surfaceView.lockCanvas()
canvas.fill(Color.RED)
surfaceView.unlockCanvas(canvas)

C. val canvas = surfaceHolder.getCanvas()
canvas.clear(Color.RED)
surfaceHolder.postCanvas(canvas)

D. val canvas = surfaceHolder.lockCanvas()
canvas.drawColor(Color.RED)
surfaceHolder.unlockCanvasAndPost(canvas)

Solution

1. Step 1: Identify correct SurfaceHolder methods

   To draw on SurfaceView, use surfaceHolder.lockCanvas() to get Canvas and unlockCanvasAndPost() to post.

2. Step 2: Check method names and usage

   Only val canvas = surfaceHolder.lockCanvas() canvas.drawColor(Color.RED) surfaceHolder.unlockCanvasAndPost(canvas) uses correct methods: lockCanvas(), drawColor(), unlockCanvasAndPost().

3. Final Answer:

   val canvas = surfaceHolder.lockCanvas()\ncanvas.drawColor(Color.RED)\nsurfaceHolder.unlockCanvasAndPost(canvas) -> Option D

4. Quick Check:

   lockCanvas + unlockCanvasAndPost = correct drawing flow [OK]

Hint: Use lockCanvas() and unlockCanvasAndPost() with SurfaceHolder [OK]

Common Mistakes:

• Using non-existent methods like getCanvas()
• Calling unlockCanvas() instead of unlockCanvasAndPost()
• Trying to draw directly on SurfaceView without Canvas

3. Given the following Kotlin code inside a SurfaceView's drawing thread, what color will fill the canvas?

val paint = Paint()
paint.color = Color.BLUE
val canvas = surfaceHolder.lockCanvas()
canvas.drawRect(0f, 0f, 100f, 100f, paint)
surfaceHolder.unlockCanvasAndPost(canvas)

medium

A. Blue rectangle

B. Red rectangle

C. Transparent rectangle

D. Green rectangle

Solution

1. Step 1: Analyze Paint color setting

   Paint color is set to Color.BLUE, so drawing uses blue color.

2. Step 2: Check drawing command

   drawRect draws a rectangle with the paint color on the canvas.

3. Final Answer:

   Blue rectangle -> Option A

4. Quick Check:

   Paint.color = BLUE means rectangle is blue [OK]

Hint: Paint color sets shape color before drawing [OK]

Common Mistakes:

• Assuming default color is red
• Thinking drawRect clears canvas
• Ignoring paint color setting

4. What is the main issue with this Kotlin SurfaceView drawing code?

val canvas = surfaceHolder.lockCanvas()
canvas.drawCircle(50f, 50f, 30f, paint)

medium

A. Using drawCircle instead of drawOval

B. Missing call to unlockCanvasAndPost(canvas) after drawing

C. Paint object is not initialized

D. Canvas coordinates are out of bounds

Solution

1. Step 1: Check drawing sequence

   After lockCanvas(), you must call unlockCanvasAndPost(canvas) to display the drawing.

2. Step 2: Verify other options

   drawCircle is valid, paint may be initialized, and coordinates are reasonable.

3. Final Answer:

   Missing call to unlockCanvasAndPost(canvas) after drawing -> Option B

4. Quick Check:

   unlockCanvasAndPost needed to show drawing [OK]

Hint: Always unlockCanvasAndPost after lockCanvas [OK]

Common Mistakes:

• Forgetting to post canvas after drawing
• Confusing drawCircle with drawOval
• Assuming drawing appears without unlock

5. You want to draw a red circle with a black border on a SurfaceView. Which Kotlin code snippet correctly achieves this?

hard

A. val canvas = surfaceHolder.lockCanvas()
val paintFill = Paint().apply { color = Color.BLACK; style = Paint.Style.FILL }
val paintStroke = Paint().apply { color = Color.RED; style = Paint.Style.STROKE; strokeWidth = 5f }
canvas.drawCircle(100f, 100f, 50f, paintFill)
canvas.drawCircle(100f, 100f, 50f, paintStroke)
surfaceHolder.unlockCanvasAndPost(canvas)

B. val canvas = surfaceHolder.lockCanvas()
val paint = Paint().apply { color = Color.RED; style = Paint.Style.STROKE; strokeWidth = 5f }
canvas.drawCircle(100f, 100f, 50f, paint)
surfaceHolder.unlockCanvasAndPost(canvas)

C. val canvas = surfaceHolder.lockCanvas()
val paintFill = Paint().apply { color = Color.RED; style = Paint.Style.FILL }
val paintStroke = Paint().apply { color = Color.BLACK; style = Paint.Style.STROKE; strokeWidth = 5f }
canvas.drawCircle(100f, 100f, 50f, paintFill)
canvas.drawCircle(100f, 100f, 50f, paintStroke)
surfaceHolder.unlockCanvasAndPost(canvas)

D. val canvas = surfaceHolder.lockCanvas()
val paint = Paint().apply { color = Color.BLACK; style = Paint.Style.FILL }
canvas.drawCircle(100f, 100f, 50f, paint)
surfaceHolder.unlockCanvasAndPost(canvas)

Solution

1. Step 1: Understand fill and stroke styles

   To draw a filled shape with border, use Paint.Style.FILL for fill and Paint.Style.STROKE for border.

2. Step 2: Check colors and order

   Fill should be red, stroke black with strokeWidth set. Drawing fill first then stroke overlays border correctly.

3. Final Answer:

   Code with red fill and black stroke paints drawn in order -> Option C

4. Quick Check:

   Fill + Stroke paints = red circle with black border [OK]

Hint: Use two Paints: fill red, stroke black with strokeWidth [OK]

Common Mistakes:

• Using stroke style for fill color
• Drawing stroke before fill causing border hidden
• Not setting strokeWidth for border thickness