using UnityEngine;

/// <summary>
/// 全屏背景/遮罩自适应(Cover)：保持原始宽高比，等比放大到覆盖整个根 Canvas，超出部分裁切。
/// 过覆盖 => 任意宽高比都无缝铺满、且 sizeDelta 永不为 0；对纯色遮罩与艺术背景都正确、不失真。
/// 仅调整 sizeDelta(表现层)，不改锚点/位置/pivot，不影响居中布局与位移动画。
/// 16:9 屏下结果等于设计尺寸，与原始逐像素一致。由 UIResolutionAutoFitter 自动附加。
/// </summary>
[RequireComponent(typeof(RectTransform))]
[DisallowMultipleComponent]
public class UIBackgroundCover : MonoBehaviour
{
    public float sourceAspect = 16f / 9f;

    private RectTransform rt;
    private RectTransform coverTarget;
    private float lastW = -1f, lastH = -1f;
    private bool aspectResolved;
    // 重入锁：设置 sizeDelta 会同步触发 OnRectTransformDimensionsChange，
    // 若不拦截会无限递归导致 StackOverflow 崩溃。
    private bool _isFitting;

    private void OnEnable()
    {
        rt = GetComponent<RectTransform>();
        ResolveAspectFromInitialSize();
        ResolveTarget();
        Fit(true);
    }

    private void LateUpdate()
    {
        // 每帧检测 coverTarget 实际尺寸变化（而非只看 Screen）：Unity 布局延迟更新，
        // Screen 变化后 CanvasScaler 可能过几帧才把新尺寸写入 rootCanvas.rect。
        // Fit(false) 内部按 lastW/lastH 判重，尺寸未变时零开销，无需额外节流。
        if (coverTarget == null) ResolveTarget();
        if (coverTarget != null)
        {
            float pw = coverTarget.rect.width;
            float ph = coverTarget.rect.height;
            if (!Mathf.Approximately(pw, lastW) || !Mathf.Approximately(ph, lastH))
            {
                Fit(false);
            }
        }
    }

    /// <summary>用元素初始 sizeDelta 推断原始宽高比，避免写死。</summary>
    private void ResolveAspectFromInitialSize()
    {
        if (aspectResolved) return;
        if (rt == null) rt = GetComponent<RectTransform>();
        Vector2 s = rt.sizeDelta;
        if (s.x > 1f && s.y > 1f) sourceAspect = s.x / s.y;
        aspectResolved = true;
    }

    private void OnRectTransformDimensionsChange()
    {
        if (_isFitting) return; // 拦截自身写入触发的递归回调
        Fit(false);
    }

    private void ResolveTarget()
    {
        Canvas canvas = GetComponentInParent<Canvas>();
        if (canvas != null)
        {
            Canvas root = canvas.rootCanvas != null ? canvas.rootCanvas : canvas;
            coverTarget = root.GetComponent<RectTransform>();
        }
    }

    private void Fit(bool force)
    {
        if (_isFitting) return;
        if (rt == null) rt = GetComponent<RectTransform>();
        if (coverTarget == null) ResolveTarget();
        if (coverTarget == null || sourceAspect <= 0f) return;

        float pw = coverTarget.rect.width;
        float ph = coverTarget.rect.height;
        if (pw <= 0f || ph <= 0f) return;
        if (!force && Mathf.Approximately(pw, lastW) && Mathf.Approximately(ph, lastH)) return;
        lastW = pw; lastH = ph;

        float targetAspect = pw / ph;
        float w, h;
        if (targetAspect > sourceAspect) { w = pw; h = pw / sourceAspect; }
        else { h = ph; w = ph * sourceAspect; }

        Vector2 newSize = new Vector2(w, h);
        if (rt.sizeDelta != newSize)
        {
            _isFitting = true;
            rt.sizeDelta = newSize;
            _isFitting = false;
        }
    }
}