CustomPainter
- Flutter course
- Advanced
- Lesson 52/69
- 4 min
Learn paint and shouldRepaint with CustomPainter.
The CustomPainter class
CustomPainter is the abstract class that does the drawing; you must override two methods. The paint method does the actual drawing, and shouldRepaint decides whether to redraw. The CustomPaint widget expects an instance of this class.
class LinePainter extends CustomPainter {
@override
void paint(Canvas canvas, Size size) {
// drawing here
}
@override
bool shouldRepaint(covariant CustomPainter old) => false;
}When shouldRepaint should return true
If shouldRepaint returns true, paint is called again. Returning false for a fixed drawing protects performance. If the drawing depends on state, you can compare old and new state and return true.
@override
bool shouldRepaint(covariant CustomPainter old) =>
this.color != (old as MyPainter).color;Practice A painter that always returns true
Define MyPainter; leave paint empty, and have shouldRepaint always return true. In main, take an instance and print its shouldRepaint result.
Starter code
import 'package:flutter/material.dart';
// MyPainter sınıfını tanımla
void main() {
// shouldRepaint sonucunu yazdır
}Sample solution
import 'package:flutter/material.dart';
class MyPainter extends CustomPainter {
@override
void paint(Canvas canvas, Size size) {}
@override
bool shouldRepaint(covariant CustomPainter oldDelegate) => true;
}
void main() {
final painter = MyPainter();
print(painter.shouldRepaint(painter));
}Expected output
true
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which two parameters does the paint method signature have?
- Color and Offset
- BuildContext and Widget
- Paint and Path
- Canvas and Size Correct answer
Explanation: The paint method signature is void paint(Canvas canvas, Size size); you draw on the canvas, and size gives the drawing area.
2 Override the paint method
class MyPainter extends CustomPainter {
@override
void ____(Canvas canvas, Size size) {}
@override
bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}class MyPainter extends CustomPainter {
@override
void paint(Canvas canvas, Size size) {}
@override
bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}Explanation: The paint method is where CustomPainter does the drawing; its signature must be void paint(Canvas canvas, Size size).
3 CustomPainter parts
| Term | Meaning |
|---|---|
CustomPainter |
Abstract painting class |
paint |
Method that draws |
shouldRepaint |
Repaint decision |
Canvas |
Drawing surface |