Custom painters (CustomPaint) in Flutter - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the CustomPaint widget in Flutter?

The CustomPaint widget lets you draw graphics directly on the screen by using a CustomPainter. It is used when you want to create custom shapes, designs, or animations beyond standard widgets.

beginner

What method must you override when creating a CustomPainter?

You must override the paint(Canvas canvas, Size size) method. This method contains the drawing instructions using the canvas and the available size.

intermediate

What does the shouldRepaint method do in a CustomPainter?

The shouldRepaint method tells Flutter whether it needs to redraw the painting when the widget updates. Returning true means the painter should repaint, false means no repaint is needed.

beginner

How do you use CustomPaint to add a custom drawing behind a widget?

You wrap the widget inside CustomPaint and provide your CustomPainter to the painter property. The custom drawing appears behind the child widget.

intermediate

Why is it important to keep the paint method efficient in CustomPainter?

Because paint can be called many times during animations or screen updates, slow or complex drawing can cause the app to lag or drop frames. Keeping it efficient ensures smooth performance.

Which method do you override to draw on the canvas in a CustomPainter?

A build(BuildContext context)

B paint(Canvas canvas, Size size)

C draw(Canvas canvas)

D render(Canvas canvas)

The paint(Canvas canvas, Size size) method is where you write your drawing code.

What does the shouldRepaint method control?

A Whether the painter should repaint when the widget updates

B The color of the paint

C The size of the canvas

D The position of the widget

It tells Flutter if the painting needs to be redrawn when the widget changes.

How do you add a custom drawing behind a button widget?

A Use <code>Stack</code> without <code>CustomPaint</code>

B Put the button inside a <code>Container</code>

C Wrap the button with <code>CustomPaint</code> and set the painter

D Use <code>Text</code> widget

Wrapping the button with CustomPaint and providing a painter draws behind the button.

Which object do you use inside paint to draw shapes?

A Widget

B State

C BuildContext

D Canvas

The Canvas object provides methods to draw shapes, lines, and paths.

Why should you avoid heavy computations inside the paint method?

A It can cause slow UI and dropped frames

B It will crash the app immediately

C It changes the widget tree

D It disables animations

Heavy computations slow down drawing and make the app feel laggy.

Explain how CustomPaint and CustomPainter work together to create custom drawings in Flutter.

Think about how the widget uses the painter to draw on the screen.

Describe best practices to keep your custom painting efficient and smooth in a Flutter app.

Consider what happens if painting is slow during animations.