Surface and shape in Android Kotlin

Introduction

Surfaces and shapes help you create and control the look of your app's screen and elements. They make your app look nice and easy to use.

When you want to draw custom shapes like circles or rectangles on the screen.

When you need to create a background with rounded corners or shadows.

When you want to control how your app's UI elements appear and respond visually.

When you want to use a Canvas to draw graphics in your app.

When you want to create a custom view with a specific shape or style.

Syntax

Android Kotlin

val surfaceView = SurfaceView(context)
surfaceView.holder.addCallback(object : SurfaceHolder.Callback {
  override fun surfaceCreated(holder: SurfaceHolder) {
    val canvas = holder.lockCanvas()
    canvas.drawColor(Color.WHITE)
    val paint = Paint().apply {
      color = Color.RED
      style = Paint.Style.FILL
    }
    canvas.drawCircle(100f, 100f, 50f, paint)
    holder.unlockCanvasAndPost(canvas)
  }
  override fun surfaceChanged(holder: SurfaceHolder, format: Int, width: Int, height: Int) {}
  override fun surfaceDestroyed(holder: SurfaceHolder) {}
})

SurfaceView provides a dedicated drawing surface embedded inside your app's view hierarchy.

You use SurfaceHolder.Callback to know when the surface is ready to draw on.

Examples

This draws a blue oval shape on the canvas.

Android Kotlin

val shapeDrawable = ShapeDrawable(OvalShape())
shapeDrawable.paint.color = Color.BLUE
shapeDrawable.setBounds(50, 50, 200, 200)
shapeDrawable.draw(canvas)

This draws a rectangle with rounded corners.

Android Kotlin

val rect = RectF(10f, 10f, 150f, 150f)
canvas.drawRoundRect(rect, 20f, 20f, paint)

Sample App

This custom SurfaceView draws a green rectangle on a white background when the surface is ready.

Android Kotlin

class MySurfaceView(context: Context) : SurfaceView(context), SurfaceHolder.Callback {
  private val paint = Paint().apply {
    color = Color.GREEN
    style = Paint.Style.FILL
  }

  init {
    holder.addCallback(this)
  }

  override fun surfaceCreated(holder: SurfaceHolder) {
    val canvas = holder.lockCanvas()
    canvas.drawColor(Color.WHITE)
    canvas.drawRect(50f, 50f, 250f, 150f, paint)
    holder.unlockCanvasAndPost(canvas)
  }

  override fun surfaceChanged(holder: SurfaceHolder, format: Int, width: Int, height: Int) {}
  override fun surfaceDestroyed(holder: SurfaceHolder) {}
}

Important Notes

Always unlock the canvas after drawing to display your changes.

SurfaceView is useful for animations or games where you need fast drawing.

Use Paint to set colors, styles, and effects for your shapes.

Summary

SurfaceView gives you a special area to draw graphics directly.

Use shapes and Paint to create colorful and styled drawings.

Remember to manage the surface lifecycle with SurfaceHolder.Callback.

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