Unity RectTransform 深度解析

public static void StretchToFill(RectTransform rt) {
    rt.anchorMin = Vector2.zero; // 左下
    rt.anchorMax = Vector2.one;  // 右上
    rt.pivot = new Vector2(0.5f, 0.5f);
    
    // 将边距设为0，贴合父级
    rt.offsetMin = Vector2.zero; // Left, Bottom = 0
    rt.offsetMax = Vector2.zero; // Right, Top = 0
    // 或者: rt.sizeDelta = Vector2.zero; 
}

public class HealthBarFollow : MonoBehaviour {
    public Transform target3D; // 3D 目标 (头顶)
    public RectTransform healthBarUI; // UI 元素
    public Canvas canvas; // 所在的 Canvas
    public Vector3 offset = new Vector3(0, 2.0f, 0); // 头顶偏移量

    void LateUpdate() {
        if (target3D == null) return;

        // 1. 3D 世界坐标 -> 屏幕坐标
        Vector3 screenPos = Camera.main.WorldToScreenPoint(target3D.position + offset);

        // 2. 屏幕坐标 -> UI 局部坐标
        // 处理 Canvas 缩放模式 (Scale With Screen Size) 的关键步骤
        Vector2 localPos;
        RectTransformUtility.ScreenPointToLocalPointInRectangle(
            canvas.transform as RectTransform, 
            screenPos, 
            canvas.worldCamera, // Overlay模式传null
            out localPos
        );

        // 3. 应用坐标
        healthBarUI.anchoredPosition = localPos;
    }
}

public class SafeAreaAdapter : MonoBehaviour {
    RectTransform panel;

    void Awake() {
        panel = GetComponent<RectTransform>();
        ApplySafeArea();
    }

    void ApplySafeArea() {
        Rect safeArea = Screen.safeArea;
        
        // 将 Safe Area 转换为归一化锚点 (0~1)
        Vector2 anchorMin = safeArea.position;
        Vector2 anchorMax = safeArea.position + safeArea.size;

        anchorMin.x /= Screen.width;
        anchorMin.y /= Screen.height;
        anchorMax.x /= Screen.width;
        anchorMax.y /= Screen.height;

        // 应用给全屏 Panel
        panel.anchorMin = anchorMin;
        panel.anchorMax = anchorMax;
    }
}

/// <summary>
/// 将 sourceRect (在 sourceCanvas 下) 的中心点位置，转换为 targetCanvas 下的局部坐标
/// </summary>
public static Vector2 ConvertUiPosition(RectTransform sourceRect, Canvas sourceCanvas, RectTransform targetParent, Canvas targetCanvas) {
    // 1. 获取源 UI 的屏幕坐标
    Vector3 screenPos;
    Camera sourceCam = sourceCanvas.renderMode == RenderMode.ScreenSpaceOverlay ? null : sourceCanvas.worldCamera;
    
    // WorldToScreenPoint 处理了 Camera 模式下的透视和深度
    // 如果是 Overlay，sourceRect.position 本身就是屏幕像素坐标，但用此 API 更安全通用
    screenPos = RectTransformUtility.WorldToScreenPoint(sourceCam, sourceRect.position);

    // 2. 将屏幕坐标转为目标 Canvas 的局部坐标
    Vector2 localPos;
    Camera targetCam = targetCanvas.renderMode == RenderMode.ScreenSpaceOverlay ? null : targetCanvas.worldCamera;

    RectTransformUtility.ScreenPointToLocalPointInRectangle(
        targetParent, 
        screenPos, 
        targetCam, 
        out localPos
    );

    return localPos;
}

// 使用示例: 金币从背包(Overlay)飞到世界UI(Camera)
void Update() {
    Vector2 targetPos = ConvertUiPosition(inventoryIcon, inventoryCanvas, worldEffectContainer, worldCanvas);
    flyingCoin.anchoredPosition = Vector2.Lerp(flyingCoin.anchoredPosition, targetPos, Time.deltaTime * 5);
}