From 012b17b974966ba21a1d7be08965308cf38f113c Mon Sep 17 00:00:00 2001
From: Paul Chote <pchote@users.noreply.github.com>
Date: Sat, 2 Feb 2019 11:18:50 +0000
Subject: [PATCH] Implement a secondary target-of-opportunity for AttackFollow.

---
 .../Traits/Attack/AttackFollow.cs             | 25 +++++++++++++------
 OpenRA.Mods.Common/Traits/AutoTarget.cs       | 24 +++---------------
 2 files changed, 21 insertions(+), 28 deletions(-)

diff --git a/OpenRA.Mods.Common/Traits/Attack/AttackFollow.cs b/OpenRA.Mods.Common/Traits/Attack/AttackFollow.cs
index 517fdf48c4..49f0280a53 100644
--- a/OpenRA.Mods.Common/Traits/Attack/AttackFollow.cs
+++ b/OpenRA.Mods.Common/Traits/Attack/AttackFollow.cs
@@ -28,6 +28,8 @@ namespace OpenRA.Mods.Common.Traits
 		protected Target requestedTarget;
 		protected bool requestedForceAttack;
 		protected int requestedTargetLastTick;
+		protected Target opportunityTarget;
+		protected bool opportunityForceAttack;
 		Mobile mobile;
 
 		public AttackFollow(Actor self, AttackFollowInfo info)
@@ -59,7 +61,7 @@ namespace OpenRA.Mods.Common.Traits
 		protected override void Tick(Actor self)
 		{
 			if (IsTraitDisabled)
-				requestedTarget = Target.Invalid;
+				requestedTarget = opportunityTarget = Target.Invalid;
 
 			if (requestedTargetLastTick != self.World.WorldTick)
 			{
@@ -79,6 +81,12 @@ namespace OpenRA.Mods.Common.Traits
 				if (IsAiming)
 					DoAttack(self, requestedTarget);
 			}
+			else if (opportunityTarget.Type != TargetType.Invalid)
+			{
+				IsAiming = CanAimAtTarget(self, opportunityTarget, opportunityForceAttack);
+				if (IsAiming)
+					DoAttack(self, opportunityTarget);
+			}
 			else
 				IsAiming = false;
 
@@ -105,18 +113,13 @@ namespace OpenRA.Mods.Common.Traits
 
 		public override void OnStopOrder(Actor self)
 		{
-			requestedTarget = Target.Invalid;
+			requestedTarget = opportunityTarget = Target.Invalid;
 			base.OnStopOrder(self);
 		}
 
-		public bool HasReachableTarget(bool allowMove)
-		{
-			return IsReachableTarget(requestedTarget, allowMove);
-		}
-
 		void INotifyOwnerChanged.OnOwnerChanged(Actor self, Player oldOwner, Player newOwner)
 		{
-			requestedTarget = Target.Invalid;
+			requestedTarget = opportunityTarget = Target.Invalid;
 		}
 
 		class AttackActivity : Activity
@@ -157,7 +160,13 @@ namespace OpenRA.Mods.Common.Traits
 			public override Activity Tick(Actor self)
 			{
 				if (IsCanceled)
+				{
+					// Cancel the requested target, but keep firing on it while in range
+					attack.opportunityTarget = attack.requestedTarget;
+					attack.opportunityForceAttack = attack.requestedForceAttack;
+					attack.requestedTarget = Target.Invalid;
 					return NextActivity;
+				}
 
 				// Check that AttackFollow hasn't cancelled the target by modifying attack.Target
 				// Having both this and AttackFollow modify that field is a horrible hack.
diff --git a/OpenRA.Mods.Common/Traits/AutoTarget.cs b/OpenRA.Mods.Common/Traits/AutoTarget.cs
index 59ea75b303..16190d62ed 100644
--- a/OpenRA.Mods.Common/Traits/AutoTarget.cs
+++ b/OpenRA.Mods.Common/Traits/AutoTarget.cs
@@ -112,7 +112,6 @@ namespace OpenRA.Mods.Common.Traits
 	public class AutoTarget : ConditionalTrait<AutoTargetInfo>, INotifyIdle, INotifyDamage, ITick, IResolveOrder, ISync, INotifyCreated, INotifyOwnerChanged
 	{
 		readonly IEnumerable<AttackBase> activeAttackBases;
-		readonly AttackFollow[] attackFollows;
 		[Sync] int nextScanTime = 0;
 
 		public UnitStance Stance { get { return stance; } }
@@ -161,7 +160,6 @@ namespace OpenRA.Mods.Common.Traits
 				stance = self.Owner.IsBot || !self.Owner.Playable ? info.InitialStanceAI : info.InitialStance;
 
 			PredictedStance = stance;
-			attackFollows = self.TraitsImplementing<AttackFollow>().ToArray();
 		}
 
 		void INotifyCreated.Created(Actor self)
@@ -221,9 +219,8 @@ namespace OpenRA.Mods.Common.Traits
 
 			Aggressor = attacker;
 
-			bool allowMove;
-			if (ShouldAttack(out allowMove))
-				Attack(self, Target.FromActor(Aggressor), allowMove);
+			var allowMove = Info.AllowMovement && Stance > UnitStance.Defend;
+			Attack(self, Target.FromActor(Aggressor), allowMove);
 		}
 
 		void INotifyIdle.TickIdle(Actor self)
@@ -231,21 +228,8 @@ namespace OpenRA.Mods.Common.Traits
 			if (IsTraitDisabled || Stance < UnitStance.Defend)
 				return;
 
-			bool allowMove;
-			if (ShouldAttack(out allowMove))
-				ScanAndAttack(self, allowMove);
-		}
-
-		bool ShouldAttack(out bool allowMove)
-		{
-			allowMove = Info.AllowMovement && Stance > UnitStance.Defend;
-
-			// PERF: Avoid LINQ.
-			foreach (var attackFollow in attackFollows)
-				if (!attackFollow.IsTraitDisabled && attackFollow.HasReachableTarget(allowMove))
-					return false;
-
-			return true;
+			var allowMove = Info.AllowMovement && Stance > UnitStance.Defend;
+			ScanAndAttack(self, allowMove);
 		}
 
 		void ITick.Tick(Actor self)