Custom drawing (Canvas) in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What will this custom view draw on the screen?

Consider this Kotlin custom view code that overrides onDraw. What shape and color will appear?

Android Kotlin

class MyView(context: Context) : View(context) {
  private val paint = Paint().apply { color = Color.RED }
  override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
    canvas.drawCircle(width / 2f, height / 2f, 100f, paint)
  }
}

A A blue square filling the entire view

B Nothing is drawn because onDraw is empty

C A red rectangle at the top-left corner

D A red circle centered in the view with radius 100 pixels

💡 Hint

Look at the paint color and the drawCircle parameters.

The code sets paint color to red and draws a circle at the center with radius 100 pixels.

❓ lifecycle

intermediate

When is the best time to initialize Paint objects in a custom view?

In Android custom views, when should you create and configure Paint objects for drawing?

A Inside the constructor or as class properties, before onDraw is called

B Inside the onDraw method every time it is called

C Inside onMeasure method

D Inside onDetachedFromWindow

💡 Hint

Think about performance and how often onDraw runs.

Paint objects are expensive to create, so they should be initialized once in the constructor or as properties, not repeatedly in onDraw.

📝 Syntax

advanced

What error does this Kotlin code cause in a custom view?

Examine this onDraw override code snippet. What error will it cause?

Android Kotlin

override fun onDraw(canvas: Canvas) {
  canvas.drawRect(0f, 0f, width.toFloat(), height.toFloat(), Paint())
  super.onDraw(canvas)
}

A Runtime error because super.onDraw must be called before drawing

B No error, draws a rectangle and then calls super

C No visible drawing because Paint() has default color transparent

D Compilation error because super.onDraw must be first line

💡 Hint

What is the default color and style of a new Paint object?

No error. A new Paint() has default opaque black color (alpha=255) and FILL style, so it draws a black rectangle filling the entire view, then calls super.onDraw.

❓ navigation

advanced

How to trigger a redraw of a custom view after data changes?

You updated data that affects your custom view's drawing. Which method should you call to refresh the view?

A call setVisibility(View.VISIBLE)

B call requestLayout() on the view

C call invalidate() on the view

D call onDraw() directly

💡 Hint

Which method tells Android to redraw the view?

invalidate() schedules a redraw by calling onDraw later. requestLayout() triggers layout pass, not redraw.

🔧 Debug

expert

Why does this custom view not show the expected blue circle?

Given this code, why is the blue circle not visible on screen?

Android Kotlin

class BlueCircleView(context: Context) : View(context) {
  private val paint = Paint().apply {
    color = Color.BLUE
    style = Paint.Style.STROKE
    strokeWidth = 0f
  }
  override fun onDraw(canvas: Canvas) {
    super.onDraw(canvas)
    canvas.drawCircle(width / 2f, height / 2f, 50f, paint)
  }
}

A The strokeWidth is zero, so the circle's outline is invisible

B The paint color is blue but the style is fill, so no outline is drawn

C The circle radius is too small to see

D The onDraw method is missing a call to invalidate() to refresh

💡 Hint

Check the strokeWidth value and what it means for drawing.

A strokeWidth of 0 means the outline has no thickness, so the circle is not visible even though color is set.