All 7 Custom Painters & CustomPaint questions
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What is a CustomPainter class, and how does its paint method let you actually issue the specific drawing instructions rendered by a CustomPaint widget? Why is properly implementing the shouldRepaint method on a CustomPainter important for performance, and what happens if it always incorrectly returns true? How do you draw more complex custom shapes using the Path class within a CustomPainter, rather than being limited to Flutter's simpler built in shapes like circles and rectangles? How can you combine a CustomPainter with an AnimationController to create a smoothly animated custom drawing that changes over time? How can clipping and layering multiple CustomPaint widgets together help achieve more visually complex custom graphics, such as a custom chart with several distinct visual layers? What performance considerations become important when using CustomPaint for a complex, frequently redrawn custom drawing, such as a real time data visualization updating many times per second?
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What is the CustomPaint widget in Flutter, and when would you need to use it instead of relying on Flutter's existing built in widgets?

Beginner

Answer

The CustomPaint widget lets you draw completely custom graphics directly onto the screen using low level drawing commands, such as lines, shapes, arcs, and text, giving you full pixel level control over exactly what gets rendered, and you would reach for it whenever a design requires a visual element that cannot reasonably be built by simply composing Flutter's existing built in widgets together, such as a custom chart, a unique progress indicator shape, or a specialized data visualization not covered by any standard widget.
CustomPaint(
  size: Size(200, 200),
  painter: MyCirclePainter(),
)
Real-world example A fitness app draws a custom circular progress ring showing a user's daily step count progress using CustomPaint, achieving a precise visual design that would have been very difficult to recreate using only standard built in widgets.

Common follow-ups: What is the difference between the painter and foregroundPainter properties of CustomPaint?;How does CustomPaint compare to simply using an Image or an SVG asset for a similar visual effect?

Flutter Widgets Fundamentals (Stateless & Stateful);Flutter Animations Basics
All 7 Custom Painters & CustomPaint questions