Add health overlay and a command to toggle it (#3278)

* Add health overlay bar and a command to toggle it

* Remove empty line

Content.Client/GameObjects/EntitySystems/HealthOverlay/HealthOverlaySystem.cs

@@ -0,0 +1,106 @@
+#nullable enable
+using System.Collections.Generic;
+using Content.Shared.GameObjects.Components.Damage;
+using Content.Shared.GameObjects.Components.Mobs.State;
+using Content.Shared.GameTicking;
+using JetBrains.Annotations;
+using Robust.Client.GameObjects;
+using Robust.Client.Graphics;
+using Robust.Shared.GameObjects;
+using Robust.Shared.IoC;
+
+namespace Content.Client.GameObjects.EntitySystems.HealthOverlay
+{
+    [UsedImplicitly]
+    public class HealthOverlaySystem : EntitySystem, IResettingEntitySystem
+    {
+        [Dependency] private readonly IEyeManager _eyeManager = default!;
+
+        private readonly Dictionary<EntityUid, HealthOverlayGui> _guis = new();
+        private IEntity? _attachedEntity;
+        private bool _enabled;
+
+        public bool Enabled
+        {
+            get => _enabled;
+            set
+            {
+                if (_enabled == value)
+                {
+                    return;
+                }
+
+                _enabled = value;
+
+                foreach (var gui in _guis.Values)
+                {
+                    gui.SetVisibility(value);
+                }
+            }
+        }
+
+        public override void Initialize()
+        {
+            base.Initialize();
+
+            SubscribeLocalEvent<PlayerAttachSysMessage>(HandlePlayerAttached);
+        }
+
+        public void Reset()
+        {
+            foreach (var gui in _guis.Values)
+            {
+                gui.Dispose();
+            }
+
+            _guis.Clear();
+            _attachedEntity = null;
+        }
+
+        private void HandlePlayerAttached(PlayerAttachSysMessage message)
+        {
+            _attachedEntity = message.AttachedEntity;
+        }
+
+        public override void FrameUpdate(float frameTime)
+        {
+            base.Update(frameTime);
+
+            if (!_enabled)
+            {
+                return;
+            }
+
+            if (_attachedEntity == null || _attachedEntity.Deleted)
+            {
+                return;
+            }
+
+            var viewBox = _eyeManager.GetWorldViewport().Enlarged(2.0f);
+
+            foreach (var (mobState, _) in ComponentManager.EntityQuery<IMobStateComponent, IDamageableComponent>())
+            {
+                var entity = mobState.Owner;
+
+                if (_attachedEntity.Transform.MapID != entity.Transform.MapID ||
+                    !viewBox.Contains(entity.Transform.WorldPosition))
+                {
+                    if (_guis.TryGetValue(entity.Uid, out var oldGui))
+                    {
+                        oldGui.Dispose();
+                    }
+
+                    continue;
+                }
+
+                if (_guis.ContainsKey(entity.Uid))
+                {
+                    continue;
+                }
+
+                var gui = new HealthOverlayGui(entity);
+                _guis.Add(entity.Uid, gui);
+            }
+        }
+    }
+}