Add functionality to `Service`

Add ability to auto-launch and obtain instance of
a `Service` with a single command.

Add `Service`-specific events `Launch()` / `ShutDown()`

Add ability to auto-register required listeners on launch.

sources/Core/Service.uc

@@ -1,7 +1,7 @@
 /**
  *  Parent class for all services used in Acedia.
  *  Currently simply makes itself server-only.
- *      Copyright 2019 Anton Tarasenko
+ *      Copyright 2020 Anton Tarasenko
  *------------------------------------------------------------------------------
  * This file is part of Acedia.
  *
@@ -21,8 +21,61 @@
 class Service extends Singleton
     abstract;
 
+//  Listeners listed here will be automatically activated.
+var public const array< class<Listener> > requiredListeners;
+
+//  Enables feature of given class.
+public static final function Service Require()
+{
+    local Service newInstance;
+    if (IsRunning())
+    {
+        return Service(GetInstance());
+    }
+    default.blockSpawning = false;
+    newInstance = class'Acedia'.static.GetInstance().Spawn(default.class);
+    default.blockSpawning = true;
+    return newInstance;
+}
+
+//  Whether service is currently running is determined by
+public static final function bool IsRunning()
+{
+    return (GetInstance() != none);
+}
+
+protected function OnLaunch(){}
+protected function OnShutdown(){}
+
+protected function OnCreated()
+{
+    default.blockSpawning = true;
+    SetListenersActiveSatus(true);
+    OnLaunch();
+}
+
+protected function OnDestroyed()
+{
+    SetListenersActiveSatus(false);
+    OnShutdown();
+}
+
+//  Set listeners' status
+private static function SetListenersActiveSatus(bool newStatus)
+{
+    local int i;
+    for (i = 0; i < default.requiredListeners.length; i += 1)
+    {
+        if (default.requiredListeners[i] == none) continue;
+        default.requiredListeners[i].static.SetActive(newStatus);
+    }
+}
+
 defaultproperties
 {
-    remoteRole  = ROLE_None
+    DrawType        = DT_None
-    DrawType    = DT_None
+    //  Prevent spawning this feature by any other means than 'Launch()'.
+    blockSpawning   = true
+    //  Features are server-only actors
+    remoteRole      = ROLE_None
 }