Making your own shapes

You can add new shapes or decorations without changing Ambit. The sample does this with CircleRegion + CountBadgeDecoration registered only in RegionKindsPage.cs (now under samples/Ambit.Sample/Extensibility/), take a look there for a full example. DirectionIndicatorDecoration is also sample-only — LabelDecoration remains the single built-in decoration; this proves the registry is truly open.

A new shape

Say you want a triangle.

1. Make the shape

public class TriangleRegion : IEditableRegion
{
    public const string TriangleTypeId = "triangle";
    public Guid Id { get; } = Guid.NewGuid();
    public string TypeId => TriangleTypeId;
    public IReadOnlyList<NormalizedPoint> Vertices { get; }
    public IReadOnlyList<IDecoration> Decorations { get; } = Array.Empty<IDecoration>();
    public RegionStyle Style { get; }
    public string? Label { get; }
    public NormalizedBounds Bounds => GeometryUtilities.GetBounds(Vertices);
    // ... HitTestBody, MoveHandle, Translate, GetHandles
}

You'll also need a small factory to save/load it (IRegionFactory).

2. Make the renderer

public class TriangleRenderer : IRegionRenderer
{
    public string TypeId => TriangleRegion.TriangleTypeId;
    public void Render(SKCanvas canvas, IRegion region, RegionRenderState state, ICoordinateTransform t, SkiaRenderResources r)
    {
        // draw with canvas.DrawPath(...)
    }
}

3. Plug it in

var types = new RegionTypeRegistry().RegisterBuiltInTypes();
types.Register(new TriangleFactory());

var renders = new RegionRenderRegistry().RegisterBuiltInRenderers();
renders.Register(new TriangleRenderer());

var editor = new RegionEditorControl(controller, new RegionOverlayRenderer(renders));
controller.RegionTypeRegistry = types;
controller.ActiveDrawTypeId = TriangleRegion.TriangleTypeId;

That's it, no library changes needed.

A new decoration

Decorations are little extras you attach to a shape, like an arrow or a badge.

1. Make a class with IDecoration
2. Make a factory and a renderer
3. Register both, same as above

You can put two badges on one LineRegion to get independent arrows at each end. The line itself doesn't need to know anything about arrows.