added code from unity

Runtime/PopupCategory.cs

@@ -0,0 +1,48 @@
+using System;
+using UnityEngine;
+
+namespace Jovian.PopupSystem {
+    /// <summary>
+    /// Value type identifying a popup channel. Each category gets its own popup view instance.
+    /// Compared by string ID using ordinal comparison. Define custom categories as static fields.
+    /// </summary>
+    [Serializable]
+    public struct PopupCategory : IEquatable<PopupCategory> {
+        [SerializeField] string id;
+
+        public string Id => id;
+
+        public PopupCategory(string id) {
+            this.id = id;
+        }
+
+        public static readonly PopupCategory Character = new("Character");
+        public static readonly PopupCategory Item = new("Item");
+        public static readonly PopupCategory Skill = new("Skill");
+        public static readonly PopupCategory General = new("General");
+
+        public bool Equals(PopupCategory other) {
+            return string.Equals(id, other.id, StringComparison.Ordinal);
+        }
+
+        public override bool Equals(object obj) {
+            return obj is PopupCategory other && Equals(other);
+        }
+
+        public override int GetHashCode() {
+            return id != null ? id.GetHashCode() : 0;
+        }
+
+        public override string ToString() {
+            return id ?? string.Empty;
+        }
+
+        public static bool operator ==(PopupCategory left, PopupCategory right) {
+            return left.Equals(right);
+        }
+
+        public static bool operator !=(PopupCategory left, PopupCategory right) {
+            return !left.Equals(right);
+        }
+    }
+}