Circle Prefab Script

DrawCircle.cs

public class DrawCircle : DrawShape 
{
    private CircleCollider2D _circleCollider2D;

    // ...
	
    private void Awake()
    {
        _circleCollider2D = GetComponent<CircleCollider2D>();
        
	// ...
    }

    // ...
	
    public override void UpdateShape(Vector2 newVertex)
    {
        // ...

        // Update the mesh relative to the transform
        var v0Relative = Vector2.zero;
        var v1Relative = _vertices[1] - _vertices[0];
        _meshFilter.mesh = CircleMesh(v0Relative, v1Relative, FillColor);
		
        // Update the collider
        _circleCollider2D.radius = Vector2.Distance(_vertices[0], _vertices[1]);
    }

    /// <summary>
    /// Creates and returns a circle mesh given two vertices on its center 
    /// and any outer edge point.
    /// </summary>
    private static Mesh CircleMesh(Vector2 v0, Vector2 v1, Color fillColor)
    {
        var radius = Vector2.Distance(v0, v1);
		
        // We want to make sure that the circle appears to be curved.
        // This can be approximated by drawing a regular polygon with lots of segments.
        // The number of segments can be increased based on the radius so that large circles also appear curved.
        // We use an offset and multiplier to create a tunable linear function.
        const float segmentOffset = 40f;
        const float segmentMultiplier = 2 * Mathf.PI;
        var numSegments = (int) (radius * segmentMultiplier + segmentOffset);

        // Create an array of points arround a cricle
        var circleVertices = Enumerable.Range(0, numSegments)
            .Select(i => {
                var theta = 2 * Mathf.PI * i / numSegments;
                return new Vector2(Mathf.Cos(theta), Mathf.Sin(theta)) * radius;
            })
            .ToArray();
		
        // Find all the triangles in the shape
        var triangles = new Triangulator(circleVertices).Triangulate();
		
        // Assign each vertex the fill color
        var colors = Enumerable.Repeat(fillColor, circleVertices.Length).ToArray();

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

        return mesh;
    }
}