using UnityEngine;

[ExecuteAlways]
[RequireComponent(typeof(Camera))]
public class CameraLetterbox : MonoBehaviour
{
    public float targetAspect = 16f / 9f;

    // 关闭时相机填满整个屏幕（跟随用户分辨率，无黑边）；开启时回退到旧的 16:9 锁定加黑边行为。
    // 透视相机填满屏幕会按 Hor+ 水平扩展视野，不会拉伸变形。
    public bool lockAspect = false;

    private int lastWidth;
    private int lastHeight;

    void OnEnable()
    {
        ApplyLetterbox();
        lastWidth = Screen.width;
        lastHeight = Screen.height;
    }

    void Update()
    {
        if (Screen.width != lastWidth || Screen.height != lastHeight)
        {
            ApplyLetterbox();
            lastWidth = Screen.width;
            lastHeight = Screen.height;
        }
    }

    void ApplyLetterbox()
    {
        Camera cam = GetComponent<Camera>();

        if (!lockAspect)
        {
            cam.rect = new Rect(0f, 0f, 1f, 1f);
            return;
        }

        float screenAspect = (float)Screen.width / Screen.height;
        float scaleHeight = screenAspect / targetAspect;

        if (scaleHeight < 1.0f)
        {
            cam.rect = new Rect(
                0,
                (1f - scaleHeight) / 2f,
                1,
                scaleHeight
            );
        }
        else
        {
            float scaleWidth = 1f / scaleHeight;
            cam.rect = new Rect(
                (1f - scaleWidth) / 2f,
                0,
                scaleWidth,
                1
            );
        }
    }
}