Visualize melee weapon cooldowns in HUD.

Content.Shared/GameObjects/Components/Items/ItemCooldownComponent.cs

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+using System;
+using Robust.Shared.GameObjects;
+using Robust.Shared.Serialization;
+using Robust.Shared.ViewVariables;
+
+namespace Content.Shared.GameObjects.Components.Items
+{
+    /// <summary>
+    ///     Stores a visual "cooldown" for items, that gets displayed in the hands GUI.
+    /// </summary>
+    [RegisterComponent]
+    public sealed class ItemCooldownComponent : Component
+    {
+        public override string Name => "ItemCooldown";
+        public override Type StateType => typeof(ItemCooldownComponentState);
+        public override uint? NetID => ContentNetIDs.ITEMCOOLDOWN;
+
+        private TimeSpan? _cooldownEnd;
+        private TimeSpan? _cooldownStart;
+
+        /// <summary>
+        ///     The time when this cooldown ends.
+        /// </summary>
+        /// <remarks>
+        ///     If null, no cooldown is displayed.
+        /// </remarks>
+        [ViewVariables]
+        public TimeSpan? CooldownEnd
+        {
+            get => _cooldownEnd;
+            set
+            {
+                _cooldownEnd = value;
+                Dirty();
+            }
+        }
+
+        /// <summary>
+        ///     The time when this cooldown started.
+        /// </summary>
+        /// <remarks>
+        ///     If null, no cooldown is displayed.
+        /// </remarks>
+        [ViewVariables]
+        public TimeSpan? CooldownStart
+        {
+            get => _cooldownStart;
+            set
+            {
+                _cooldownStart = value;
+                Dirty();
+            }
+        }
+
+        public override ComponentState GetComponentState()
+        {
+            return new ItemCooldownComponentState
+            {
+                CooldownEnd = CooldownEnd,
+                CooldownStart = CooldownStart
+            };
+        }
+
+        public override void HandleComponentState(ComponentState curState, ComponentState nextState)
+        {
+            var cast = (ItemCooldownComponentState) curState;
+
+            CooldownStart = cast.CooldownStart;
+            CooldownEnd = cast.CooldownEnd;
+        }
+
+        [Serializable, NetSerializable]
+        private sealed class ItemCooldownComponentState : ComponentState
+        {
+            public TimeSpan? CooldownStart { get; set; }
+            public TimeSpan? CooldownEnd { get; set; }
+
+            public ItemCooldownComponentState() : base(ContentNetIDs.ITEMCOOLDOWN)
+            {
+            }
+        }
+    }
+}