ArcMesh.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

[RequireComponent (typeof(MeshFilter)), ExecuteInEditMode]
public class ArcMesh : MonoBehaviour
{
	[SerializeField]
	float outerRadius = 1.0f;

	[SerializeField]
	float innerRadius = 0.5f;

	[SerializeField, Range (0.0f, 360.0f)]
	float beginAngle = 0.0f;

	[SerializeField, Range (0.0f, 360.0f)]
	float endAngle = 360.0f;

	[SerializeField]
	int numVertices = 32;

	Mesh mesh;
	Vector3[] vertices;
	int[] triangles;

	void Start ()
	{
		var filter = GetComponent<MeshFilter> ();
		mesh = new Mesh ();
		mesh.MarkDynamic ();
		filter.mesh = mesh;

		vertices = new Vector3[numVertices * 2];
		triangles = new int[numVertices * 6];
	}

	void OnValidate ()
	{
		if (numVertices < 3) {
			numVertices = 3;
		}
		vertices = new Vector3[numVertices * 2];
		triangles = new int[(numVertices - 1) * 6];
	}

	void Update ()
	{
		mesh.Clear ();

		var beginRadian = beginAngle / 360.0f * 2.0f * Mathf.PI;
		var endRadian = endAngle / 360.0f * 2.0f * Mathf.PI;
		var radianLength = Mathf.Abs (endRadian - beginRadian);

		for (int i = 0; i < vertices.Length; i += 2) {
			float phase = beginRadian + i / 2 * radianLength / (numVertices - 1);
			var unitCirclePoint = new Vector3 (Mathf.Cos (phase), Mathf.Sin (phase));
			vertices [i] = innerRadius * unitCirclePoint;
			vertices [i + 1] = outerRadius * unitCirclePoint;
		}

		for (int i = 0; i < numVertices - 1; i++) {
			triangles [i * 6] = i * 2;
			triangles [i * 6 + 1] = (i * 2 + 3) % (vertices.Length);
			triangles [i * 6 + 2] = i * 2 + 1;
			triangles [i * 6 + 3] = i * 2;
			triangles [i * 6 + 4] = (i * 2 + 2) % (vertices.Length);
			triangles [i * 6 + 5] = (i * 2 + 3) % (vertices.Length);
		}

		mesh.vertices = vertices;
		mesh.triangles = triangles;
	}
}