画面の端にUIを表示させる【Unity, uGui】

using UnityEngine;
using System.Collections;

public class UGuiPositionSetter : MonoBehaviour {
    public enum PositionX {
        LEFT,
        CENTER,
        RIGHT,
        FREE
    };
    public enum PositionY {
        TOP,
        MIDDLE,
        BOTTOM,
        FREE
    };

    [SerializeField]
    public PositionX positionX = PositionX.FREE;
    [SerializeField]
    public PositionY positionY = PositionY.FREE;
    public bool isUpdate = false;

    public Vector2 margin;

    private RectTransform _rt;

    void Start () {
        _rt =  GetComponent<RectTransform>();
    }

    void Update () {
        if (isUpdate)
            setPosition();
    }

    private void setPosition() {
        Vector2 pos = _rt.anchoredPosition;
        switch (positionX) {
        case PositionX.CENTER:
            pos.x = 0f;
            break;
        case PositionX.LEFT:
            pos.x = - Screen.width * 0.5f + _rt.rect.width * 0.5f + margin.x;
            break;
        case PositionX.RIGHT:
            pos.x = Screen.width * 0.5f - _rt.rect.width * 0.5f - margin.x;
            break;
        }

        switch (positionY) {
        case PositionY.MIDDLE:
            pos.y = 0f;
            break;
        case PositionY.BOTTOM:
            pos.y = - Screen.height * 0.5f + _rt.rect.height * 0.5f + margin.y;
            break;
        case PositionY.TOP:
            pos.y = Screen.height * 0.5f - _rt.rect.height * 0.5f - margin.y;
            break;
        }
        _rt.anchoredPosition = pos;
    }
}