#1,153 – Handling Custom Dependency Properties that Affect Rendering

In our example of a custom “pie slice” shape, we defined StartAngle and EndAngle properties that dictate where the pie shape starts and ends.  We then define how the shape is rendered by overriding the DefiningGeometry property.

One problem with the earlier code example was that the shape wasn’t automatically re-rendered if the start or end angles were changed.  Since the values of these properties are used in rendering the shape, we want the shape to automatically redraw itself whenever either property changes.

We can force a re-render by specifying the AffectsRender flag when defining the metadata for the StartAngle and EndAngle properties.  Below is updated code for defining these properties.

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

        // DependencyProperty - StartAngle
        private static FrameworkPropertyMetadata startAngleMetadata =
                new FrameworkPropertyMetadata(
                    0.0,     // Default value
                    FrameworkPropertyMetadataOptions.AffectsRender,
                    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 FrameworkPropertyMetadata endAngleMetadata =
                new FrameworkPropertyMetadata(
                    90.0,     // Default value
                    FrameworkPropertyMetadataOptions.AffectsRender,
                    null,    // Property changed callback
                    new CoerceValueCallback(CoerceAngle));   // Coerce value callback

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