Why Custom drawing (Canvas) in Android Kotlin? - Purpose & Use Cases

The Big Idea

What if you could paint anything on your app screen with just a few lines of code?

The Scenario

Imagine you want to create a unique shape or design in your app, like a colorful star or a special pattern. Without custom drawing, you might try to use many images or buttons to fake the look.

The Problem

This manual way is slow and clumsy. You have to find or create many images, arrange them perfectly, and it's hard to change colors or shapes later. It's also heavy on your app's memory and can cause glitches.

The Solution

Custom drawing with Canvas lets you draw shapes, lines, and colors directly on the screen with code. It's fast, flexible, and you can create any design you imagine without extra images.

Before vs After

✗ Before

val imageView = ImageView(this)
imageView.setImageResource(R.drawable.star)
layout.addView(imageView)

✓ After

override fun onDraw(canvas: Canvas) {
  val paint = Paint()
  paint.color = Color.RED
  canvas.drawCircle(100f, 100f, 50f, paint)
}

What It Enables

It enables you to create smooth, custom graphics that respond to user actions and fit perfectly with your app's style.

Real Life Example

Think of a drawing app where users can paint freely or a game that shows animated shapes and effects drawn directly on the screen.

Key Takeaways

Manual image use is slow and inflexible.

Canvas drawing lets you create any shape or design with code.

This makes your app faster, lighter, and more creative.

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