Add `ActorStalker` and it's API method

sources/Unreal/ActorStalker.uc

@@ -0,0 +1,90 @@
+/**
+ *      An auxiliary actor class to help detect the exact moment when non-Acedia
+ *  `Actor`s get destroyed. This is accomplished by attaching to them like to
+ *  the base and then waiting for `BaseChange` event that will be called once
+ *  our base gets destroyed.
+ *      Copyright 2021 Anton Tarasenko
+ *------------------------------------------------------------------------------
+ * This file is part of Acedia.
+ *
+ * Acedia is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Acedia is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Acedia.  If not, see <https://www.gnu.org/licenses/>.
+ */
+class ActorStalker extends AcediaActor;
+
+var private bool initialized;
+
+//  Actor, whos destruction we want to detect
+var private Actor target;
+
+//  To notify that stalked target got destroyed
+var private SimpleSignal onActorDestructionSignal;
+
+protected function Constructor()
+{
+    onActorDestructionSignal =
+        SimpleSignal(_.memory.Allocate(class'SimpleSignal'));
+}
+
+protected function Finalizer()
+{
+    _.memory.Free(onActorDestructionSignal);
+}
+
+/**
+ *  Signal that will be emitted once stalked actor gets destroyed.
+ *
+ *  [Signature]
+ *  void <slot>()
+ */
+/* SIGNAL */
+public final function SimpleSlot OnActorDestruction(AcediaObject receiver)
+{
+    return SimpleSlot(onActorDestructionSignal.NewSlot(receiver));
+}
+
+/**
+ *  Initialized `ActorStalker` to stalk a particular target.
+ *
+ *  Cannot fail unless `none` is passed as an argument.
+ *
+ *  @param  initTarget  Target that new `ActorStalker` will stalk.
+ */
+public final function Initialize(Actor initTarget)
+{
+    if (initTarget == none) {
+        Destroy();
+        return;
+    }
+    target = initTarget;
+    SetBase(initTarget);
+    initialized = true;
+}
+
+event BaseChange()
+{
+    if (initialized && target == none)
+    {
+        onActorDestructionSignal.Emit();
+        Destroy();
+    }
+}
+
+defaultproperties
+{
+    RemoteRole      = ROLE_None
+    drawType        = DT_None
+    bCollideActors  = false
+    bCollideWorld   = false
+    bBlockActors    = false
+}

sources/Unreal/UnrealAPI.uc

@@ -22,7 +22,7 @@ class UnrealAPI extends AcediaObject;
 
 var public GameRulesAPI gameRules;
 
-var private LoggerAPI.Definition errNoService;
+var private LoggerAPI.Definition fatalNoStalker;
 
 protected function Constructor()
 {
@@ -46,13 +46,43 @@ public final function Unreal_OnTick_Slot OnTick(
     local Signal        signal;
     local UnrealService service;
     service = UnrealService(class'UnrealService'.static.Require());
-    if (service == none)
+    signal = service.GetSignal(class'Unreal_OnTick_Signal');
+    return Unreal_OnTick_Slot(signal.NewSlot(receiver));
+}
+
+/**
+ *  Signal that will be emitted when a passed `targetToStalk` is destroyed.
+ *
+ *  Passed parameter `targetToStalk` cannot be `none`, otherwise `none` will be
+ *  returned instead of a valid slot.
+ *
+ *  @param  receiver        Specify a receiver like for any other signal.
+ *  @param  targetToStalk   Actor whose destruction we want to detect.
+ *
+ *  [Signature]
+ *  void <slot>()
+ */
+/* SIGNAL */
+public final function SimpleSlot OnDestructionFor(
+    AcediaObject    receiver,
+    Actor           targetToStalk)
 {
-        _.logger.Auto(errNoService);
+    local ActorStalker stalker;
+    if (receiver == none)       return none;
+    if (targetToStalk == none)  return none;
+
+    //  Failing to spawn this actor without any collision flags is considered
+    //  completely unexpected and grounds for fatal failure on Acedia' part
+    stalker = ActorStalker(_.memory.Allocate(class'ActorStalker'));
+    if (stalker == none)
+    {
+        _.logger.Auto(fatalNoStalker);
         return none;
     }
-    signal = service.GetSignal(class'Unreal_OnTick_Signal');
-    return Unreal_OnTick_Slot(signal.NewSlot(receiver));
+    //  This will not fail, since we have already ensured that
+    //  `targetToStalk == none`
+    stalker.Initialize(targetToStalk);
+    return stalker.OnActorDestruction(receiver);
 }
 
 /**
@@ -236,5 +266,5 @@ public final function NativeActorRef ActorRef(optional Actor value)
 
 defaultproperties
 {
-    errNoService = (l=LOG_Error,m="`UnrealService` could not be reached.")
+    fatalNoStalker = (l=LOG_Fatal,m="Cannot spawn `PawnStalker`")
 }