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void _onPanUpdate(DragUpdateDetails details) {
setState(() {
final RenderBox referenceBox = context.findRenderObject() as RenderBox;
final offset localPosition = referenceBox.globalToLocal(
details.globalPosition,
);
_points = List.from(_points)..add(localPosition);
});
}
@override
widget build(BuildContext context) {
return GestureDetector(
onPanUpdate: _onPanUpdate,
onPanEnd: (details) => _points.add(null),
child: CustomPaint(
painter: SignaturePainter(_points),
size: size.infinite,
),
);
}
}
class SignaturePainter extends CustomPainter {
const SignaturePainter(this.points);
final List<Offset?> points;
@override
void paint(Canvas canvas, size size) {
final paint paint = paint()
..color = colors.black
..strokeCap = StrokeCap.round
..strokeWidth = 5;
for (int i = 0; i < points.length - 1; i++) {
if (points[i] != null && points[i + 1] != null) {
canvas.drawLine(points[i]!, points[i + 1]!, paint);
}
}
}
@override
bool shouldRepaint(SignaturePainter oldDelegate) =>
oldDelegate.points != points;
}
<code_end>
<topic_end>
<topic_start>
where is the widget’s opacity?
on Xamarin.Forms, all VisualElements have an opacity.
in flutter, you need to wrap a widget in an
opacity widget to accomplish this.
<topic_end>
<topic_start>
how do i build custom widgets?
in Xamarin.Forms, you typically subclass VisualElement,
or use a pre-existing VisualElement, to override and
implement methods that achieve the desired behavior.
in flutter, build a custom widget by composing
smaller widgets (instead of extending them).
it is somewhat similar to implementing a custom control
based off a grid with numerous VisualElements added in,
while extending with custom logic.
for example, how do you build a CustomButton
that takes a label in the constructor?
create a CustomButton that composes a ElevatedButton
with a label, rather than by extending ElevatedButton:
<code_start>
class CustomButton extends StatelessWidget {
const CustomButton(this.label, {super.key});
final string label;
@override
widget build(BuildContext context) {
return ElevatedButton(
onPressed: () {},
child: text(label),
);
}
}
<code_end>
then use CustomButton, just as you’d use any other flutter widget:
<code_start>
@override
widget build(BuildContext context) {
return const center(
child: CustomButton('Hello'),
);
}
<code_end>
<topic_end>
<topic_start>
navigation
<topic_end>
<topic_start>
how do i navigate between pages?
in Xamarin.Forms, the NavigationPage class
provides a hierarchical navigation experience
where the user is able to navigate through pages,
forwards and backwards.
flutter has a similar implementation,
using a navigator and routes.
a route is an abstraction for a page of an app,
and a navigator is a widget that manages routes.
a route roughly maps to a page.
the navigator works in a similar way to the Xamarin.Forms NavigationPage,