Unity frame selection

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


public class SelectByRectangle1 : MonoBehaviour
{

    public Color rectColor = Color.green;

    public GameObject characterPrefab;
    public static List<GameObject> characters = new List<GameObject>();
    private List<GameObject> selectedCharacters = new List<GameObject>();

    [HideInInspector]
    public static bool MouseOverGUI = false;

    private Vector3 downPosition = Vector3.zero;
    public Material rectMat = null;
    private bool drawRectangle = false;
    private bool moved = false;
    private float farPlane = 100;

    // Use this for initialization
    void Start()
    {
        rectMat.hideFlags = HideFlags.HideAndDontSave;
        rectMat.shader.hideFlags = HideFlags.HideAndDontSave;

        for (int i = 0; i < 3; i++)
        {
            GameObject obj = (GameObject)Instantiate(characterPrefab);
            characters.Add(obj);
            obj.transform.Rotate(0, Random.Range(-180, 180), 0);
            obj.transform.Translate(Random.Range(-5, 5), 0, Random.Range(-5, 5));
        }
    }

    //框选
    void drawSelectionRectangle(Vector3 start, Vector3 end)
    {

        GL.PushMatrix();

        if (!rectMat)
            return;

        rectMat.SetPass(0);
        GL.LoadPixelMatrix();
        GL.Begin(GL.QUADS);
        GL.Color(new Color(rectColor.r, rectColor.g, rectColor.b, 0.1f));
        GL.Vertex3(start.x, start.y, 0);
        GL.Vertex3(end.x, start.y, 0);
        GL.Vertex3(end.x, end.y, 0);
        GL.Vertex3(start.x, end.y, 0);
        GL.End();
        GL.Begin(GL.LINES);
        GL.Color(rectColor);
        GL.Vertex3(start.x, start.y, 0);
        GL.Vertex3(end.x, start.y, 0);
        GL.Vertex3(end.x, start.y, 0);
        GL.Vertex3(end.x, end.y, 0);
        GL.Vertex3(end.x, end.y, 0);
        GL.Vertex3(start.x, end.y, 0);
        GL.Vertex3(start.x, end.y, 0);
        GL.Vertex3(start.x, start.y, 0);
        GL.End();
        GL.PopMatrix();
    }

    void Update()
    {
        if (MouseOverGUI)
            return;
        if (Input.GetMouseButtonDown(0))
        {
            downPosition = Input.mousePosition;
            moved = false;
            return;
        }
        else if (Input.GetMouseButtonUp(0))
        {
            moved = false;

            downPosition = Vector3.zero;
            drawRectangle = false;

            for(int i = 0;i < selectedCharacters.Count;i++)
            {
                print(selectedCharacters[i].name);
            }
        }
        else if (Input.GetMouseButton(0) && !moved)
        {
            moved = true;
            clearSelection();
            drawRectangle = true;
            return;
        }
    }

    void addItem(GameObject obj)
    {
        if (!selectedCharacters.Contains(obj))
        {
            selectedCharacters.Add(obj);
            //obj.SendMessage("doSelect");
            //或者改变材质以及颜色
        }
    }

    void removeItem(GameObject obj)
    {
        if (selectedCharacters.Contains(obj))
        {
            selectedCharacters.Remove(obj);
            //obj.SendMessage("doDisselect");
            //或者恢复材质以及颜色
        }

    }

    void clearSelection()
    {
        for (int i = selectedCharacters.Count - 1; i >= 0; i--)
        {
            removeItem(selectedCharacters[i]);
        }
    }

    void checkSelection(Vector3 start, Vector3 end)
    {
        Vector3 p1 = Vector3.zero;
        Vector3 p2 = Vector3.zero;
        if (start.x > end.x)
        {
            p1.x = end.x;
            p2.x = start.x;
        }
        else
        {
            p1.x = start.x;
            p2.x = end.x;
        }
        if (start.y > end.y)
        {
            p1.y = end.y;
            p2.y = start.y;
        }
        else
        {
            p1.y = start.y;
            p2.y = end.y;
        }
        foreach (GameObject obj in characters)
        {
            Vector3 location = GetComponent<Camera>().WorldToScreenPoint(obj.transform.position);
            if (location.x < p1.x || location.x > p2.x || location.y < p1.y || location.y > p2.y
                || location.z < 0 || location.z > farPlane)
            {
                removeItem(obj);
                //print("---" + obj.name);
            }
            else
            {
                addItem(obj);
                //print("+++" + obj.name);
            }
        }
    }

    //程序每一帧渲染之后执行，GL相关的在这里调用
    void OnPostRender()
    {
        if (drawRectangle)
        {
            Vector3 end = Input.mousePosition;
            drawSelectionRectangle(downPosition, end);
            checkSelection(downPosition, end);
        }

    }
}