FieldOfView

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

public class FieldOfView : MonoBehaviour {

    public float viewRadius;
    [Range(0, 360)]
    public float viewAngle;

    public LayerMask targetMask;
    public LayerMask obstacleMask;

    [HideInInspector]
    public List<Transform> visibleTargets = new List<Transform>();

    void Start() {
        StartCoroutine("FindTargetWithDelay", 0.2f);
    }

    IEnumerator FindTargetWithDelay(float delay) {
        while (true) {
            yield return new WaitForSeconds(delay);
            FindVisibleTargets();
        }
    }
    
    //Finds targets inside field of view not blocked by walls
    void FindVisibleTargets() {
        visibleTargets.Clear();
        //Adds targets in view radius to an array
        Collider2D[] targetsInViewRadius = Physics2D.OverlapCircleAll(transform.position, viewRadius, targetMask);
        //For every targetsInViewRadius it checks if they are inside the field of view
        for (int i = 0; i < targetsInViewRadius.Length; i++) {
            Transform target = targetsInViewRadius[i].transform;
            Vector3 dirToTarget = (target.position - transform.position).normalized;
            if (Vector3.Angle(transform.up, dirToTarget) < viewAngle / 2) {
                float dstToTarget = Vector3.Distance(transform.position, target.position);
                //If line draw from object to target is not interrupted by wall, add target to list of visible 
                //targets
                if (!Physics2D.Raycast(transform.position, dirToTarget, dstToTarget, obstacleMask)) {
                    visibleTargets.Add(target);
                }
            }
        }
    }


    public Vector3 DirFromAngle(float angleInDegrees, bool angleIsGlobal) {
        if (!angleIsGlobal) {
            angleInDegrees -= transform.eulerAngles.z;
        }
        return new Vector3(Mathf.Sin(angleInDegrees * Mathf.Deg2Rad), Mathf.Cos(angleInDegrees * Mathf.Deg2Rad), 0);
    }
}

FieldOfViewEditor

using UnityEngine;
using System.Collections;
using UnityEditor;

[CustomEditor (typeof (FieldOfView))]
public class FieldOfViewEditor : Editor {

    void OnSceneGUI() {
        FieldOfView fow = (FieldOfView)target;

        //Draws view reach
        Handles.color = Color.black;
        Handles.DrawWireArc(fow.transform.position, Vector3.forward, Vector3.up, 360, fow.viewRadius);

        //Draws cone of view
        Vector3 viewAngleA = fow.DirFromAngle(-fow.viewAngle / 2, false);
        Vector3 viewAngleB = fow.DirFromAngle(fow.viewAngle / 2, false);
        Handles.DrawLine(fow.transform.position, fow.transform.position + viewAngleA * fow.viewRadius);
        Handles.DrawLine(fow.transform.position, fow.transform.position + viewAngleB * fow.viewRadius);
    }
}