From 1dbfcb4dc1ea0ea0a0fdb0d481c00b7dd9b8dd8f Mon Sep 17 00:00:00 2001
From: David Saltares <david.saltares@gmail.com>
Date: Tue, 9 Jun 2015 19:27:09 +0100
Subject: [PATCH] engine.addSystem() replaces the old system if there was one
 of the same class #175

---
 .../src/com/badlogic/ashley/core/Engine.java  | 28 +++++++++++++------
 .../com/badlogic/ashley/core/EngineTests.java | 19 +++++++++++++
 2 files changed, 38 insertions(+), 9 deletions(-)

diff --git a/ashley/src/com/badlogic/ashley/core/Engine.java b/ashley/src/com/badlogic/ashley/core/Engine.java
index a8bd2130..bd874ead 100644
--- a/ashley/src/com/badlogic/ashley/core/Engine.java
+++ b/ashley/src/com/badlogic/ashley/core/Engine.java
@@ -108,6 +108,8 @@ private long obtainEntityId() {
 
 	/**
 	 * Adds an entity to this Engine.
+	 * This will throw an IllegalArgumentException if the given entity
+	 * was already registered with an engine.
 	 */
 	public void addEntity(Entity entity){
 		if (entity.uuid != 0L) {
@@ -174,17 +176,23 @@ public ImmutableArray<Entity> getEntities() {
 
 	/**
 	 * Adds the {@link EntitySystem} to this Engine.
+	 * If the Engine already had a system of the same class,
+	 * the new one will replace the old one.
 	 */
 	public void addSystem(EntitySystem system){
 		Class<? extends EntitySystem> systemType = system.getClass();
-
-		if (!systemsByClass.containsKey(systemType)) {
-			systems.add(system);
-			systemsByClass.put(systemType, system);
-			system.addedToEngineInternal(this);
-
-			systems.sort(comparator);
+		
+		EntitySystem oldSytem = getSystem(systemType);
+		
+		if (oldSytem != null) {
+			removeSystem(oldSytem);
 		}
+		
+		systems.add(system);
+		systemsByClass.put(systemType, system);
+		system.addedToEngineInternal(this);
+
+		systems.sort(comparator);
 	}
 
 	/**
@@ -305,7 +313,7 @@ else if (belongsToFamily && !matches) {
 	}
 
 	protected void removeEntityInternal(Entity entity) {
-		boolean removed;
+		boolean removed = false;
 		
 		entity.scheduledForRemoval = false;
 		entities.removeValue(entity, true);
@@ -340,7 +348,9 @@ protected void removeEntityInternal(Entity entity) {
 		entityListeners.end();
 		notifying = false;
 		
-		entity.uuid = 0L;
+		if (removed) {
+			entity.uuid = 0L;
+		}
 	}
 
 	protected void addEntityInternal(Entity entity) {
diff --git a/ashley/tests/com/badlogic/ashley/core/EngineTests.java b/ashley/tests/com/badlogic/ashley/core/EngineTests.java
index fd1865c4..af907d88 100644
--- a/ashley/tests/com/badlogic/ashley/core/EngineTests.java
+++ b/ashley/tests/com/badlogic/ashley/core/EngineTests.java
@@ -214,6 +214,25 @@ public void getSystems () {
 
 		assertEquals(2, engine.getSystems().size());
 	}
+	
+	@Test
+	public void addTwoSystemsOfSameClass () {
+		Engine engine = new Engine();
+		EntitySystemMockA system1 = new EntitySystemMockA();
+		EntitySystemMockA system2 = new EntitySystemMockA();
+
+		assertEquals(0, engine.getSystems().size());
+
+		engine.addSystem(system1);
+		
+		assertEquals(1, engine.getSystems().size());
+		assertEquals(system1, engine.getSystem(EntitySystemMockA.class));
+		
+		engine.addSystem(system2);
+
+		assertEquals(1, engine.getSystems().size());
+		assertEquals(system2, engine.getSystem(EntitySystemMockA.class));
+	}
 
 	@Test
 	public void systemUpdate () {