A complete two-anchor trend line, end to end — metadata, render, hit-test — showing how little code a real drawing tool needs once the base class and render context do their part.

SIMPLE A trend line

The simplest useful tool has two anchors and draws one segment between them. The shape of every drawing tool is the same three steps: declare the [DrawingToolMetadata] attribute, pass the anchor count to the base constructor, then fill in OnRender and HitTest.

In OnRender you convert both anchors to pixels with AnchorToPixel, draw the line with the tool's Color and LineWidthPx, and — only when selected — paint a handle at each end. In HitTest you convert the same anchors and ask DrawingGeometry whether the cursor is within the tolerance of the segment. That is the whole tool.

TrendLine.csusing TradeStrike.Pipeline.Drawing;

[DrawingToolMetadata("Trend Line",
    Category = "Lines", Hotkey = "Alt+T", IconKey = "TrendLine",
    AnchorCount = 2, Order = 0, Description = "Straight line between two points.")]
public sealed class TrendLine : DrawingToolBase
{
    public TrendLine() : base(anchorCount: 2) { }

    public override void OnRender(IDrawingToolRenderContext ctx)
    {
        var (x1, y1) = ctx.AnchorToPixel(Anchors[0]);
        var (x2, y2) = ctx.AnchorToPixel(Anchors[1]);
        ctx.Renderer.DrawLine(x1, y1, x2, y2, Color, LineWidthPx);

        if (ctx.IsSelected)
        {
            ctx.DrawHandle(x1, y1);
            ctx.DrawHandle(x2, y2);
        }
    }

    public override bool HitTest(double px, double py, IDrawingToolRenderContext ctx, double tol = 6)
    {
        var (x1, y1) = ctx.AnchorToPixel(Anchors[0]);
        var (x2, y2) = ctx.AnchorToPixel(Anchors[1]);
        return DrawingGeometry.DistanceToSegment(px, py, x1, y1, x2, y2) <= tol;
    }
}

The metadata attribute

[DrawingToolMetadata] is what makes the tool discoverable. The catalog reads it at assembly-scan time to populate the toolbar menu, hotkeys, icons and anchor counts — no registration boilerplate. Exactly one attribute per tool class.