Add Replaceable and Replacement traits to support wall overrides.

OpenRA.Mods.Common/OpenRA.Mods.Common.csproj

@@ -480,6 +480,8 @@
     <Compile Include="Traits\Repairable.cs" />
     <Compile Include="Traits\RepairableNear.cs" />
     <Compile Include="Traits\RepairsBridges.cs" />
+    <Compile Include="Traits\Replaceable.cs" />
+    <Compile Include="Traits\Replacement.cs" />
     <Compile Include="Traits\SeedsResource.cs" />
     <Compile Include="Traits\StoresResources.cs" />
     <Compile Include="Traits\Render\SelectionDecorations.cs" />

OpenRA.Mods.Common/Traits/Buildings/BuildingUtils.cs

@@ -22,10 +22,21 @@ namespace OpenRA.Mods.Common.Traits
 			if (!world.Map.Contains(cell))
 				return false;
 
-			if (world.WorldActor.Trait<BuildingInfluence>().GetBuildingAt(cell) != null)
-				return false;
+			var building = world.WorldActor.Trait<BuildingInfluence>().GetBuildingAt(cell);
+			if (building != null)
+			{
+				if (ai == null)
+					return false;
 
-			if (!bi.AllowInvalidPlacement && world.ActorMap.GetActorsAt(cell).Any(a => a != toIgnore))
+				var replacementInfo = ai.TraitInfoOrDefault<ReplacementInfo>();
+				if (replacementInfo == null)
+					return false;
+
+				if (!building.TraitsImplementing<Replaceable>().Any(p => !p.IsTraitDisabled &&
+					p.Info.Types.Overlaps(replacementInfo.ReplaceableTypes)))
+					return false;
+			}
+			else if (!bi.AllowInvalidPlacement && world.ActorMap.GetActorsAt(cell).Any(a => a != toIgnore))
 				return false;
 
 			var tile = world.Map.Tiles[cell];

OpenRA.Mods.Common/Traits/Player/PlaceBuilding.cs

@@ -136,6 +136,18 @@ namespace OpenRA.Mods.Common.Traits
 						|| !buildingInfo.IsCloseEnoughToBase(self.World, order.Player, actorInfo, order.TargetLocation))
 						return;
 
+					var replacementInfo = actorInfo.TraitInfoOrDefault<ReplacementInfo>();
+					if (replacementInfo != null)
+					{
+						var buildingInfluence = self.World.WorldActor.Trait<BuildingInfluence>();
+						foreach (var t in buildingInfo.Tiles(order.TargetLocation))
+						{
+							var host = buildingInfluence.GetBuildingAt(t);
+							if (host != null)
+								host.World.Remove(host);
+						}
+					}
+
 					var building = w.CreateActor(order.TargetString, new TypeDictionary
 					{
 						new LocationInit(order.TargetLocation),

OpenRA.Mods.Common/Traits/Replaceable.cs

@@ -0,0 +1,31 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Traits
+{
+	public class ReplaceableInfo : ConditionalTraitInfo, ITraitInfo
+	{
+		[FieldLoader.Require]
+		[Desc("Replacement types this Relpaceable actor accepts.")]
+		public readonly HashSet<string> Types = new HashSet<string>();
+
+		public override object Create(ActorInitializer init) { return new Replaceable(init, this); }
+	}
+
+	public class Replaceable : ConditionalTrait<ReplaceableInfo>
+	{
+		public Replaceable(ActorInitializer init, ReplaceableInfo info)
+			: base(info) { }
+	}
+}

OpenRA.Mods.Common/Traits/Replacement.cs

@@ -0,0 +1,25 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+using OpenRA.Traits;
+
+namespace OpenRA.Mods.Common.Traits
+{
+	public class ReplacementInfo : TraitInfo<Replacement>
+	{
+		[FieldLoader.Require]
+		[Desc("Replacement type (matched against Conditions in Replaceable).")]
+		public readonly HashSet<string> ReplaceableTypes = new HashSet<string>();
+	}
+
+	public class Replacement { }
+}