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CustomPaint

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2019/12/11 Share

CustomPaint

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. [CustomPaint] 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.

    Sample code

    This sample extends the same code shown for [RadialGradient] to create a
    custom painter that paints a sky.
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    class Sky extends CustomPainter {
    @override
    void paint(Canvas canvas, Size size) {
    var rect = Offset.zero & size;
    var gradient = new RadialGradient(
    center: const Alignment(0.7, -0.6),
    radius: 0.2,
    colors: [const Color(0xFFFFFF00), const Color(0xFF0099FF)],
    stops: [0.4, 1.0],
    );
    canvas.drawRect(
    rect,
    new 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.
    var rect = Offset.zero & size;
    var width = size.shortestSide * 0.4;
    rect = const Alignment(0.8, -0.9).inscribe(new Size(width, width), rect);
    return [
    new CustomPainterSemantics(
    rect: rect,
    properties: new 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;
    }
    See also:
    • [Canvas], the class that a custom painter uses to paint.
    • [CustomPaint], the widget that uses [CustomPainter], and whose sample
      code shows how to use the above Sky class.
    • [RadialGradient], whose sample code section shows a different take
      on the sample code above.
CATALOG
  1. 1. CustomPaint
    1. 1.1. Sample code