CustomPainter class Null safety

The interface used by CustomPaint (in the widgets library) and RenderCustomPaint (in the rendering library).

To implement a custom painter, either subclass or implement this interface to define your custom paint delegate. CustomPainter subclasses must implement the paint and shouldRepaint methods, and may optionally also implement the hitTest and shouldRebuildSemantics methods, and the semanticsBuilder getter.

The paint method is called whenever the custom object needs to be repainted.

The shouldRepaint method is called when a new instance of the class is provided, to check if the new instance actually represents different information.

The most efficient way to trigger a repaint is to either:

Extend this class and supply a repaint argument to the constructor of the CustomPainter, where that object notifies its listeners when it is time to repaint.
Extend Listenable (e.g. via ChangeNotifier) and implement CustomPainter, so that the object itself provides the notifications directly.

In either case, the CustomPaint widget or RenderCustomPaint render object will listen to the Listenable and repaint whenever the animation ticks, avoiding both the build and layout phases of the pipeline.

The hitTest method is called when the user interacts with the underlying render object, to determine if the user hit the object or missed it.

The semanticsBuilder is called whenever the custom object needs to rebuild its semantics information.

The shouldRebuildSemantics method is called when a new instance of the class is provided, to check if the new instance contains different information that affects the semantics tree.

This sample extends the same code shown for RadialGradient to create a custom painter that paints a sky.

class Sky extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    final Rect rect = Offset.zero & size;
    const RadialGradient gradient = RadialGradient(
      center: Alignment(0.7, -0.6),
      radius: 0.2,
      colors: <Color>[Color(0xFFFFFF00), Color(0xFF0099FF)],
      stops: <double>[0.4, 1.0],
    );
    canvas.drawRect(
      rect,
      Paint()..shader = gradient.createShader(rect),
    );
  }

  @override
  SemanticsBuilderCallback get semanticsBuilder {
    return (Size size) {
      // Annotate a rectangle containing the picture of the sun
      // with the label "Sun". When text to speech feature is enabled on the
      // device, a user will be able to locate the sun on this picture by
      // touch.
      Rect rect = Offset.zero & size;
      final double width = size.shortestSide * 0.4;
      rect = const Alignment(0.8, -0.9).inscribe(Size(width, width), rect);
      return <CustomPainterSemantics>[
        CustomPainterSemantics(
          rect: rect,
          properties: const SemanticsProperties(
            label: 'Sun',
            textDirection: TextDirection.ltr,
          ),
        ),
      ];
    };
  }

  // Since this Sky painter has no fields, it always paints
  // the same thing and semantics information is the same.
  // Therefore we return false here. If we had fields (set
  // from the constructor) then we would return true if any
  // of them differed from the same fields on the oldDelegate.
  @override
  bool shouldRepaint(Sky oldDelegate) => false;
  @override
  bool shouldRebuildSemantics(Sky oldDelegate) => false;
}

Constructors

CustomPainter({Listenable? repaint}): Creates a custom painter.
const

Properties

hashCode → int: The hash code for this object.
read-onlyinherited
runtimeType → Type: A representation of the runtime type of the object.
read-onlyinherited
semanticsBuilder → SemanticsBuilderCallback?: Returns a function that builds semantic information for the picture drawn by this painter.
read-only

Methods

addListener(VoidCallback listener) → void: Register a closure to be notified when it is time to repaint.
override
hitTest(Offset position) → bool?: Called whenever a hit test is being performed on an object that is using this custom paint delegate.
noSuchMethod(Invocation invocation) → dynamic: Invoked when a non-existent method or property is accessed.
inherited
paint(Canvas canvas, Size size) → void: Called whenever the object needs to paint. The given Canvas has its coordinate space configured such that the origin is at the top left of the box. The area of the box is the size of the size argument.
removeListener(VoidCallback listener) → void: Remove a previously registered closure from the list of closures that the object notifies when it is time to repaint.
override
shouldRebuildSemantics(covariant CustomPainter oldDelegate) → bool: Called whenever a new instance of the custom painter delegate class is provided to the RenderCustomPaint object, or any time that a new CustomPaint object is created with a new instance of the custom painter delegate class (which amounts to the same thing, because the latter is implemented in terms of the former).
shouldRepaint(covariant CustomPainter oldDelegate) → bool: Called whenever a new instance of the custom painter delegate class is provided to the RenderCustomPaint object, or any time that a new CustomPaint object is created with a new instance of the custom painter delegate class (which amounts to the same thing, because the latter is implemented in terms of the former).
toString() → String: A string representation of this object.
override

Operators

operator ==(Object other) → bool: The equality operator.
inherited