Add prediction for standing states and mob states (#1937)

Content.Shared/GameObjects/EntitySystems/SharedStandingStateSystem.cs

@@ -0,0 +1,50 @@
+using Robust.Shared.GameObjects.Systems;
+using Robust.Shared.Interfaces.GameObjects;
+
+namespace Content.Shared.GameObjects.EntitySystems
+{
+    public abstract class SharedStandingStateSystem : EntitySystem
+    {
+        protected abstract bool OnDown(IEntity entity, bool playSound = true, bool dropItems = true,
+            bool force = false);
+
+        protected abstract bool OnStand(IEntity entity);
+
+        /// <summary>
+        ///     Set's the mob standing state to down.
+        /// </summary>
+        /// <param name="entity">The mob in question</param>
+        /// <param name="playSound">Whether to play a sound when falling down or not</param>
+        /// <param name="dropItems">Whether to make the mob drop all the items on his hands</param>
+        /// <param name="force">Whether or not to check if the entity can fall.</param>
+        /// <returns>False if the mob was already downed or couldn't set the state</returns>
+        public bool Down(IEntity entity, bool playSound = true, bool dropItems = true, bool force = false)
+        {
+            if (dropItems)
+            {
+                DropAllItemsInHands(entity, false);
+            }
+
+            if (!force && !EffectBlockerSystem.CanFall(entity))
+            {
+                return false;
+            }
+
+            return OnDown(entity, playSound, dropItems, force);
+        }
+
+        /// <summary>
+        ///     Sets the mob's standing state to standing.
+        /// </summary>
+        /// <param name="entity">The mob in question.</param>
+        /// <returns>False if the mob was already standing or couldn't set the state</returns>
+        public bool Standing(IEntity entity)
+        {
+            return OnStand(entity);
+        }
+
+        public virtual void DropAllItemsInHands(IEntity entity, bool doMobChecks = true)
+        {
+        }
+    }
+}