#1,152 – A Custom “Pie Slice” Shape

We can build on the earlier custom Arc shape code to create a custom shape that draws a slice of a pie.  We only need to add a line segment back to the origin and set options to close the resulting curve and to fill in the middle.

Here is the resulting PieSlice class:

    public class PieSlice : Shape
    {
        // Angle that arc starts at
        public double StartAngle
        {
            get { return (double)GetValue(StartAngleProperty); }
            set { SetValue(StartAngleProperty, value); }
        }

        // DependencyProperty - StartAngle
        private static PropertyMetadata startAngleMetadata =
                new PropertyMetadata(
                    0.0,     // Default value
                    null,    // Property changed callback
                    new CoerceValueCallback(CoerceAngle));   // Coerce value callback

        public static readonly DependencyProperty StartAngleProperty =
            DependencyProperty.Register("StartAngle", typeof(double), typeof(PieSlice), startAngleMetadata);

        // Angle that arc ends at
        public double EndAngle
        {
            get { return (double)GetValue(EndAngleProperty); }
            set { SetValue(EndAngleProperty, value); }
        }

        // DependencyProperty - EndAngle
        private static PropertyMetadata endAngleMetadata =
                new PropertyMetadata(
                    90.0,     // Default value
                    null,    // Property changed callback
                    new CoerceValueCallback(CoerceAngle));   // Coerce value callback

        public static readonly DependencyProperty EndAngleProperty =
            DependencyProperty.Register("EndAngle", typeof(double), typeof(PieSlice), endAngleMetadata);

        private static object CoerceAngle(DependencyObject depObj, object baseVal)
        {
            double angle = (double)baseVal;
            angle = Math.Min(angle, 359.9);
            angle = Math.Max(angle, 0.0);
            return angle;
        }

        protected override Geometry DefiningGeometry
        {
            get
            {
                double maxWidth = Math.Max(0.0, RenderSize.Width - StrokeThickness);
                double maxHeight = Math.Max(0.0, RenderSize.Height - StrokeThickness);
                //Console.WriteLine(string.Format("* maxWidth={0}, maxHeight={1}", maxWidth, maxHeight));

                double xStart = maxWidth / 2.0 * Math.Cos(StartAngle * Math.PI / 180.0);
                double yStart = maxHeight / 2.0 * Math.Sin(StartAngle * Math.PI / 180.0);

                double xEnd = maxWidth / 2.0 * Math.Cos(EndAngle * Math.PI / 180.0);
                double yEnd = maxHeight / 2.0 * Math.Sin(EndAngle * Math.PI / 180.0);

                StreamGeometry geom = new StreamGeometry();
                using (StreamGeometryContext ctx = geom.Open())
                {
                    ctx.BeginFigure(
                        new Point((RenderSize.Width / 2.0) + xStart,
                                  (RenderSize.Height / 2.0) - yStart),
                        true,   // Filled
                        true);  // Closed
                    ctx.ArcTo(
                        new Point((RenderSize.Width / 2.0) + xEnd,
                                  (RenderSize.Height / 2.0) - yEnd),
                        new Size(maxWidth / 2.0, maxHeight / 2),
                        0.0,     // rotationAngle
                        (EndAngle - StartAngle) > 180,   // greater than 180 deg?
                        SweepDirection.Counterclockwise,
                        true,    // isStroked
                        false);
                    ctx.LineTo(new Point((RenderSize.Width / 2.0), (RenderSize.Height / 2.0)), true, false);
                }

                return geom;
            }
        }
    }

To use the PieSlice from XAML, we specify a stroke color (for the outline), a fill color, and start and end angles.

        <loc:PieSlice Stroke="Black" Fill="Black"
                 Height="100" Width="100" Margin="5"
                 StartAngle="0" EndAngle="60"
                 HorizontalAlignment="Center"/>

1152-001

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About Sean
Software developer in the Twin Cities area, passionate about software development and sailing.

3 Responses to #1,152 – A Custom “Pie Slice” Shape

  1. Pingback: Dew Drop – September 5, 2014 (#1849) | Morning Dew

  2. MikeKo says:

    Hi Sean,
    Very good explained example.

    Is there any option to add Click on PieSlice?

  3. Shujee says:

    To let binding work, you should define Start and End angle metadata as FrameworkPropertyMetadata with FrameworkPropertyMetadataOptions.AffectsRender flag set. Otherwise, setting the value at runtime will not affect the control.

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