Add Empty and IfElse

src/main/java/net/citizensnpcs/api/ai/tree/Decorator.java

@@ -11,6 +11,9 @@
  * A decorator is a wrapper over a {@link Behavior}, which can add functionality
  * such as filtering {@link BehaviorStatus}es, conditions, timer loops and more
  * without knowing the internals of the behavior it wraps.
+ * 
+ * Note that there are often simpler alternatives to a full-blown decorator,
+ * which has to be generic for many different scenarios.
  */
 public class Decorator extends BehaviorGoalAdapter {
     private final Collection<Runnable> resetCallbacks;

src/main/java/net/citizensnpcs/api/ai/tree/Empty.java

@@ -0,0 +1,22 @@
+package net.citizensnpcs.api.ai.tree;
+
+public class Empty extends BehaviorGoalAdapter {
+    private Empty() {
+    }
+
+    @Override
+    public void reset() {
+    }
+
+    @Override
+    public BehaviorStatus run() {
+        return null;
+    }
+
+    @Override
+    public boolean shouldExecute() {
+        return false;
+    }
+
+    public static Empty INSTANCE = new Empty();
+}

src/main/java/net/citizensnpcs/api/ai/tree/IfElse.java

@@ -0,0 +1,45 @@
+package net.citizensnpcs.api.ai.tree;
+
+public class IfElse extends BehaviorGoalAdapter {
+    private final Condition condition;
+    private Behavior executing;
+    private final Behavior ifBehavior, elseBehavior;
+
+    public IfElse(Condition condition, Behavior ifBehavior, Behavior elseBehavior) {
+        this.condition = condition;
+        this.ifBehavior = ifBehavior;
+        this.elseBehavior = elseBehavior;
+    }
+
+    @Override
+    public void reset() {
+        if (executing != null) {
+            executing.reset();
+            executing = null;
+        }
+    }
+
+    @Override
+    public BehaviorStatus run() {
+        return executing.run();
+    }
+
+    @Override
+    public boolean shouldExecute() {
+        boolean cond = condition.get();
+        if (cond) {
+            executing = ifBehavior;
+        } else {
+            executing = elseBehavior;
+        }
+        if (executing == null || !executing.shouldExecute()) {
+            executing = null;
+            return false;
+        }
+        return true;
+    }
+
+    public static IfElse create(Condition condition, Behavior ifBehavior, Behavior elseBehavior) {
+        return new IfElse(condition, ifBehavior, elseBehavior);
+    }
+}

src/main/java/net/citizensnpcs/api/ai/tree/Precondition.java

@@ -54,7 +54,7 @@ public boolean shouldExecute() {
     /**
      * Creates a {@link Precondition} that returns either
      * {@link BehaviorStatus#SUCCESS} or {@link BehaviorStatus#FAILURE}
-     * depending on the underlying {@link Condition}s return status.
+     * depending on the underlying {@link Condition}'s return status.
      * 
      * @param condition
      *            The condition to check while executing