MakeScreenColliders

using UnityEngine;
using System.Collections;

public class MakeScreenColliders : MonoBehaviour {

    [Tooltip("What camera to poll viewport information from. Uses Camera.main when this is not specified.")]
    public Camera cam;
    [Tooltip("How far the colliders are scaled in the camera's forward direction. Has no effect when 'use3DPhysics' is set to 'false'.")]
    public float depth = 100f;
    [Tooltip("When camera is in perspective mode, what distance to sample at to get the correct bounding box. Has no effect when camera is in orthographic mode."), Range(0f, 1f)]
    public float distanceQuery = 1f;
    [SerializeField, Tooltip("Changes between 2D and 3D collider physics. Can not be adjusted at runtime.")]
    bool use3DPhysics = false;

    bool _use3DPhysics = false;
    float _distanceQuery = 1000f;
    float _skin2D = 0.05f;

    GameObject top, bottom, left, right;
    GameObject[] barriers;

    void Start() {
        //Allows overriding on what camera to use for positioning
        //Will use the main camera when one isn't provided.
        if(cam == null)
            cam = Camera.main;

        _use3DPhysics = use3DPhysics;

        if(_use3DPhysics)
            Create3DBarriers();
        else
            Create2DBarriers();
    }

    private void Create2DBarriers() {
        top = GameObject.CreatePrimitive(PrimitiveType.Cube);
        top.name = "Top";
        top.transform.localScale = new Vector3(0f, _skin2D, 0f);

        bottom = GameObject.CreatePrimitive(PrimitiveType.Cube);
        bottom.name = "Bottom";
        bottom.transform.localScale = new Vector3(0f, _skin2D, 0f);

        left = GameObject.CreatePrimitive(PrimitiveType.Cube);
        left.name = "Left";
        left.transform.localScale = new Vector3(_skin2D, 0f, 0f);

        right = GameObject.CreatePrimitive(PrimitiveType.Cube);
        right.name = "Right";
        right.transform.localScale = new Vector3(_skin2D, 0f, 0f);

        barriers = new GameObject[] { top, bottom, left, right };

        foreach(var b in barriers) {
            DestroyImmediate(b.collider);
            //no need to render these.
            DestroyImmediate(b.renderer);

            b.transform.parent = cam.transform;

            var bc = b.AddComponent<BoxCollider2D>();
            var rb = b.AddComponent<Rigidbody2D>();
            rb.isKinematic = true;
        }
    }

    private void Create3DBarriers() {
        top = GameObject.CreatePrimitive(PrimitiveType.Cube);
        top.name = "Top";
        top.transform.localScale = new Vector3(100f, 0f, 0f);

        bottom = GameObject.CreatePrimitive(PrimitiveType.Cube);
        bottom.name = "Bottom";
        bottom.transform.localScale = new Vector3(100f, 0f, 0f);

        left = GameObject.CreatePrimitive(PrimitiveType.Cube);
        left.name = "Left";
        left.transform.localScale = new Vector3(0f, 100f, 0f);

        right = GameObject.CreatePrimitive(PrimitiveType.Cube);
        right.name = "Right";
        right.transform.localScale = new Vector3(0f, 100f, 0f);

        barriers = new GameObject[] { top, bottom, left, right };

        foreach(var b in barriers) {
            b.transform.parent = cam.transform;
            var rb = b.AddComponent<Rigidbody>();
            rb.isKinematic = true;

            //no need to render these.
            DestroyImmediate(b.renderer);
        }

        SetScales();
        SetPositions();
    }

    private void SetScales() {
        var verticalSize = (cam.isOrthoGraphic) ? cam.orthographicSize * 2.0f : 2.0f * _distanceQuery * Mathf.Tan(cam.fieldOfView * 0.5f * Mathf.Deg2Rad);
        var horizontalSize = (cam.isOrthoGraphic) ? verticalSize * ((float)Screen.width / (float)Screen.height) : verticalSize * cam.aspect;

        top.transform.localScale = (_use3DPhysics) ? new Vector3(horizontalSize, 0f, depth) : new Vector3(horizontalSize, _skin2D, 0f);
        bottom.transform.localScale = (_use3DPhysics) ? new Vector3(horizontalSize, 0f, depth) : new Vector3(horizontalSize, _skin2D, 0f);
        left.transform.localScale = (_use3DPhysics) ? new Vector3(0f, verticalSize, depth) : new Vector3(_skin2D, verticalSize, 0f);
        right.transform.localScale = (_use3DPhysics) ? new Vector3(0f, verticalSize, depth) : new Vector3(_skin2D, verticalSize, 0f);
    }

    private void SetPositions() {
        //Camera rotations mess up the positions
        var camRot = cam.transform.rotation;
        cam.transform.rotation = Quaternion.identity;

        top.transform.position = cam.ViewportToWorldPoint(new Vector3(0.5f, 1f, _distanceQuery));
        bottom.transform.position = cam.ViewportToWorldPoint(new Vector3(0.5f, 0f, _distanceQuery));
        left.transform.position = cam.ViewportToWorldPoint(new Vector3(0f, 0.5f, _distanceQuery));
        right.transform.position = cam.ViewportToWorldPoint(new Vector3(1f, 0.5f, _distanceQuery));

        foreach(var b in barriers)
            b.transform.localPosition = ZeroZ(b.transform);

        //reset to the real rotation.
        cam.transform.rotation = camRot;
    }

    //Make each cube's position start at the camera's z position and extend outwards.
    private Vector3 ZeroZ(Transform t) {
        var pos = t.localPosition;
        pos.z = (_use3DPhysics) ? t.localScale.z / 2f : 0f;

        return pos;
    }

    void FixedUpdate() {
        if(cam == null)
            cam = Camera.main;

        distanceQuery = Mathf.Clamp01(distanceQuery);
        _distanceQuery = cam.farClipPlane * distanceQuery;
        _distanceQuery = Mathf.Clamp(_distanceQuery, cam.nearClipPlane, cam.farClipPlane);

        SetScales();
        SetPositions();
    }
}