Custom drawing (Canvas) in Android Kotlin - UI Render Trace

Component - Custom drawing (Canvas)

This UI component uses a Canvas to draw custom shapes directly on the screen. It allows drawing lines, circles, and other shapes by controlling pixels, giving full freedom to create unique visuals beyond standard UI widgets.

Widget Tree

FrameLayout
└── CustomView (extends View)
    └── Canvas (used internally for drawing)

The root layout is a FrameLayout that holds a CustomView. The CustomView overrides the onDraw method and uses a Canvas object internally to draw shapes like circles and lines. This Canvas is not a widget but a drawing surface provided by the system.

Render Trace - 3 Steps

Step 1: FrameLayout

Step 2: CustomView

Step 3: Canvas (inside CustomView.onDraw)

State Change - Re-render

Trigger:Calling invalidate() on CustomView to refresh drawing

Before

Canvas shows red circle and blue diagonal line

→

After

Canvas redraws the same red circle and blue diagonal line

Re-renders:Only the CustomView's onDraw method is called to redraw the canvas

UI Quiz - 3 Questions

What does the Canvas object do in the CustomView?

A It manages user input events like clicks and touches

B It provides a surface to draw shapes and lines directly on the screen

C It arranges child views inside the CustomView

D It loads images from the internet

Key Insight

Using Canvas for custom drawing lets you create unique visuals by controlling pixels directly. Remember to keep drawing code efficient and call invalidate() to refresh the view when needed. This approach is powerful for games, charts, or any custom graphics beyond standard UI widgets.

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