Custom drawing (Canvas) in Android Kotlin

Introduction

Custom drawing lets you create your own shapes and designs on the screen. It helps make your app look unique and show things that built-in widgets can't.

You want to draw a simple shape like a circle or rectangle that changes color.

You need to create a custom graph or chart that shows data visually.

You want to make a game with moving objects drawn on the screen.

You want to add decorations or patterns that are not standard UI elements.

Syntax

Android Kotlin

class MyView(context: Context) : View(context) {
  override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
    // Your drawing code here
  }
}

Override the onDraw() method to do your custom drawing.

Use the Canvas object to draw shapes, text, or images.

Examples

This draws a red circle on a white background.

Android Kotlin

override fun onDraw(canvas: Canvas) {
  super.onDraw(canvas)
  canvas.drawColor(Color.WHITE) // Fill background
  val paint = Paint().apply {
    color = Color.RED
    style = Paint.Style.FILL
  }
  canvas.drawCircle(100f, 100f, 50f, paint)
}

This draws a blue rectangle outline.

Android Kotlin

override fun onDraw(canvas: Canvas) {
  super.onDraw(canvas)
  val paint = Paint().apply {
    color = Color.BLUE
    strokeWidth = 5f
    style = Paint.Style.STROKE
  }
  canvas.drawRect(50f, 50f, 200f, 200f, paint)
}

Sample App

This custom view fills the background with light gray and draws a big magenta circle in the center of the screen.

Android Kotlin

import android.content.Context
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.view.View

class CustomDrawingView(context: Context) : View(context) {
  private val paint = Paint().apply {
    color = Color.MAGENTA
    style = Paint.Style.FILL
  }

  override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
    canvas.drawColor(Color.LTGRAY) // Light gray background
    canvas.drawCircle(width / 2f, height / 2f, 150f, paint) // Big magenta circle in center
  }
}

Important Notes

Always call super.onDraw(canvas) first to keep default drawing behavior.

Use Paint to set color, style, and stroke width for your shapes.

Coordinates and sizes are in pixels as floats, with (0,0) at top-left.

Summary

Custom drawing uses the onDraw() method and Canvas to draw shapes.

Paint controls how your shapes look (color, fill, stroke).

This lets you create unique visuals beyond standard UI components.

Practice

1. What is the primary purpose of the onDraw() method in Android custom views?

easy

A. To manage view layout and positioning

B. To draw shapes and graphics on the screen using a Canvas

C. To handle user touch events

D. To load images from resources

Solution

1. Step 1: Understand the role of onDraw()

   The onDraw() method is called when the view needs to display its content.

2. Step 2: Identify what onDraw() uses

   It uses a Canvas object to draw shapes, text, or images directly on the view.

3. Final Answer:

   To draw shapes and graphics on the screen using a Canvas -> Option B

4. Quick Check:

   onDraw() draws on Canvas [OK]

Hint: Remember: onDraw() is for drawing only [OK]

Common Mistakes:

• Confusing onDraw() with event handlers
• Thinking onDraw() manages layout
• Assuming onDraw() loads images automatically

2. Which of the following is the correct way to create a Paint object with red color and stroke style in Kotlin?

easy

A. val paint = Paint.setColor(Color.RED).setStyle(Paint.Style.STROKE)

B. val paint = Paint(Color.RED, Paint.Style.STROKE)

C. val paint = Paint().apply { color = Color.RED; style = Paint.Style.STROKE }

D. val paint = Paint(Color.RED).style(Paint.Style.STROKE)

Solution

1. Step 1: Check Paint object creation syntax

   In Kotlin, Paint is created with default constructor and properties set via setters or apply block.

2. Step 2: Verify correct property assignments

   Using apply allows setting color and style properly.

3. Final Answer:

   val paint = Paint().apply { color = Color.RED; style = Paint.Style.STROKE } -> Option C

4. Quick Check:

   Use apply to set Paint properties [OK]

Hint: Use apply block to set Paint properties cleanly [OK]

Common Mistakes:

• Trying to pass parameters to Paint constructor
• Using chained setters that don't exist
• Confusing method calls with property assignments

3. Given the following onDraw() code snippet, what shape will be drawn?

override fun onDraw(canvas: Canvas) {
  super.onDraw(canvas)
  val paint = Paint().apply { color = Color.BLUE; style = Paint.Style.FILL }
  canvas.drawCircle(100f, 100f, 50f, paint)
}

medium

A. A blue filled circle centered at (100, 100) with radius 50

B. A blue outlined circle centered at (100, 100) with radius 50

C. A red filled circle centered at (100, 100) with radius 50

D. A blue filled rectangle at (100, 100) with size 50

Solution

1. Step 1: Analyze Paint properties

   Paint color is blue and style is FILL, so shape will be filled blue.

2. Step 2: Identify Canvas drawing method

   drawCircle draws a circle at (100f, 100f) with radius 50f.

3. Final Answer:

   A blue filled circle centered at (100, 100) with radius 50 -> Option A

4. Quick Check:

   drawCircle + FILL + blue = filled blue circle [OK]

Hint: Look at Paint style and Canvas method to identify shape [OK]

Common Mistakes:

• Confusing FILL with STROKE style
• Mixing up circle and rectangle drawing methods
• Ignoring Paint color property

4. Identify the error in this custom view's onDraw() method:

override fun onDraw(canvas: Canvas) {
  val paint = Paint()
  paint.color = Color.GREEN
  canvas.drawRect(50f, 50f, 150f, 150f)

medium

A. Missing Paint parameter in drawRect call

B. Paint color should be set after drawRect

C. drawRect requires integer coordinates, not floats

D. onDraw should not create Paint objects

Solution

1. Step 1: Check drawRect method parameters

   The drawRect method requires a Paint object as the last parameter to know how to draw.

2. Step 2: Identify missing argument

   The code calls drawRect with only coordinates, missing the Paint parameter.

3. Final Answer:

   Missing Paint parameter in drawRect call -> Option A

4. Quick Check:

   drawRect needs Paint argument [OK]

Hint: Always pass Paint to Canvas draw methods [OK]

Common Mistakes:

• Forgetting Paint parameter in draw calls
• Thinking Paint color order matters
• Assuming floats are invalid for coordinates

5. You want to draw a custom view that shows a red circle with a black stroke border of 5 pixels thickness. Which of the following onDraw() implementations achieves this correctly?

hard

A. override fun onDraw(canvas: Canvas) {
super.onDraw(canvas)
val paint = Paint().apply { color = Color.RED; style = Paint.Style.FILL; strokeWidth = 5f }
canvas.drawCircle(100f, 100f, 50f, paint)
}

B. override fun onDraw(canvas: Canvas) {
super.onDraw(canvas)
val paint = Paint().apply { color = Color.RED; style = Paint.Style.STROKE; strokeWidth = 5f }
canvas.drawCircle(100f, 100f, 50f, paint)
}

C. override fun onDraw(canvas: Canvas) {
super.onDraw(canvas)
val paint = Paint().apply { color = Color.BLACK; style = Paint.Style.FILL }
canvas.drawCircle(100f, 100f, 50f, paint)
}

D. override fun onDraw(canvas: Canvas) {
super.onDraw(canvas)
val fillPaint = Paint().apply { color = Color.RED; style = Paint.Style.FILL }
val strokePaint = Paint().apply { color = Color.BLACK; style = Paint.Style.STROKE; strokeWidth = 5f }
canvas.drawCircle(100f, 100f, 50f, fillPaint)
canvas.drawCircle(100f, 100f, 50f, strokePaint)
}

Solution

1. Step 1: Understand the requirement

   We need a red filled circle with a black stroke border of 5 pixels thickness.

2. Step 2: Check each option's drawing logic

   override fun onDraw(canvas: Canvas) { super.onDraw(canvas) val fillPaint = Paint().apply { color = Color.RED; style = Paint.Style.FILL } val strokePaint = Paint().apply { color = Color.BLACK; style = Paint.Style.STROKE; strokeWidth = 5f } canvas.drawCircle(100f, 100f, 50f, fillPaint) canvas.drawCircle(100f, 100f, 50f, strokePaint) } draws the filled circle first, then draws the stroke circle on top with correct colors and stroke width.

3. Step 3: Identify why others fail

   override fun onDraw(canvas: Canvas) { super.onDraw(canvas) val paint = Paint().apply { color = Color.RED; style = Paint.Style.STROKE; strokeWidth = 5f } canvas.drawCircle(100f, 100f, 50f, paint) } only draws a red stroke, no fill. override fun onDraw(canvas: Canvas) { super.onDraw(canvas) val paint = Paint().apply { color = Color.BLACK; style = Paint.Style.FILL } canvas.drawCircle(100f, 100f, 50f, paint) } draws a black filled circle only. override fun onDraw(canvas: Canvas) { super.onDraw(canvas) val paint = Paint().apply { color = Color.RED; style = Paint.Style.FILL; strokeWidth = 5f } canvas.drawCircle(100f, 100f, 50f, paint) } sets strokeWidth but style is FILL, so no stroke border.

4. Final Answer:

   Uses two Paint objects: red FILL then black STROKE with 5f width -> Option D

5. Quick Check:

   Draw fill then stroke with separate Paints [OK]

Hint: Use two Paints: one FILL red, one STROKE black [OK]

Common Mistakes:

• Using strokeWidth with FILL style only
• Drawing stroke before fill causing overlap issues
• Using one Paint for both fill and stroke