Triangle Prefab Script

DrawTriangle.cs

public class DrawTriangle : DrawShape 
{
    // ...
    
    private PolygonCollider2D _polygonCollider2D;

    // Triangle vertices (in absolute coordinates)
    private readonly List<Vector2> _vertices = new List<Vector2>(3);
    
    public override bool ShapeFinished { get { return _vertices.Count >= 3; } }

    // ...

    private void Awake()
    {
        // ...
        _polygonCollider2D = GetComponent<PolygonCollider2D>();
    }

    // ...

    public override void UpdateShape(Vector2 newVertex)
    {
        if (_vertices.Count < 2) {
            return;
        }
        
        _vertices[_vertices.Count - 1] = newVertex;
        
        // Set the gameobject's position to be the center of mass
        var center = _vertices.Centroid();
        transform.position = center;

        // Update the mesh relative to the transform
        var relativeVertices = _vertices.Select(v => v - center).ToArray();
        _meshFilter.mesh = PolygonMesh(relativeVertices, FillColor);
        
        // ...
		
        // Update the collider
        _polygonCollider2D.points = relativeVertices;
    }

    
    /// <summary>
    /// Creates and returns a polygon mesh given a list of its vertices.
    /// </summary>
    private static Mesh PolygonMesh(Vector2[] vertices, Color fillColor)
    {
        // Find all the triangles in the shape
        var triangles = new Triangulator(vertices).Triangulate();
		
        // Assign each vertex the fill color
        var colors = Enumerable.Repeat(fillColor, vertices.Length).ToArray();

        var mesh = new Mesh {
            name = "Triangle",
            vertices = vertices.ToVector3(),
            triangles = triangles,
            colors = colors
        };
		
        mesh.RecalculateNormals();
        mesh.RecalculateBounds();

        return mesh;
    }
}