Custom drawing (Canvas) in Android Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the <code>Canvas</code> class in Android custom drawing?

The <code>Canvas</code> class provides the drawing surface on which you can draw shapes, text, and images in custom views.

beginner

Which method do you override to perform custom drawing in an Android View?

You override the onDraw(canvas: Canvas) method to perform custom drawing on the view's canvas.

beginner

What is the role of the Paint object when drawing on a Canvas?

The Paint object defines the style and color used for drawing shapes, text, and bitmaps on the canvas.

intermediate

Why should you avoid creating new objects inside the onDraw method?

Creating new objects inside onDraw can cause performance issues because this method is called frequently during rendering.

beginner

How can you trigger a redraw of a custom view in Android?

Call the invalidate() method on the view to request a redraw, which will call onDraw again.

Which method provides the canvas to draw on in a custom Android view?

A onCreate()

B onDraw(canvas: Canvas)

C onStart()

D onMeasure()

The onDraw(canvas: Canvas) method is where you receive the canvas to perform custom drawing.

What does the Paint object control when drawing on a canvas?

A The style and color of what is drawn

B The size of the canvas

C The layout of the view

D The touch events

Paint defines color, style, stroke width, and other drawing attributes.

Why is it important to avoid creating new objects inside onDraw?

A It slows down drawing performance

B It causes the app to crash

C It changes the canvas size

D It disables touch events

Creating objects in onDraw can slow down rendering because it is called many times.

How do you request a view to redraw itself?

A call <code>refresh()</code>

B call <code>draw()</code>

C call <code>start()</code>

D call <code>invalidate()</code>

Calling invalidate() tells the system to redraw the view by calling onDraw.

Which of these can you draw on a Canvas?

A Shapes like circles and rectangles

B Text

C All of the above

D Images (bitmaps)

Canvas supports drawing shapes, text, and images.

Explain how to create a simple custom view that draws a red circle using Canvas and Paint.

Think about what you do inside onDraw and how Paint affects drawing.

Describe why performance matters in custom drawing and how to optimize your onDraw method.

Consider how often onDraw is called and what slows it down.