Add active flag to the IUpdateable interface

This allows to exclude Updatable implementations from the update mechanism if they overwrite the default implementation of the isActive() method and define an appropriate condition (e.g. game is paused/freezed/...)

Issue #267

src/de/gurkenlabs/litiengine/IUpdateable.java

@@ -1,15 +1,30 @@
 package de.gurkenlabs.litiengine;
 
+import de.gurkenlabs.litiengine.configuration.ClientConfiguration;
+
 /**
- * The Interface IUpdateable provides the functionality to update an instance
- * from the game loop. The instance needs to register itself.
+ * The Interface IUpdateable provides the functionality to automatically update an instance that implements it
+ * from the game loop. The instance needs to be registered on the loop.
+ * 
+ * @see ILoop#attach(IUpdateable)
+ * @see ILoop#detach(IUpdateable)
  */
 public interface IUpdateable {
 
   /**
-   * This method is called by the game loop on all objects that need to update
-   * their attributes. It is called on every tick, means, it is called
-   * Game.GameLoop.TICKS_PER_SECOND times per second.
+   * This method is called by the game loop on all objects that are attached to the loop.
+   * It's called on every tick of the loop and the frequency can be configured using the <code>ClientConfiguration</code>.
+   *
+   * @see ClientConfiguration#setUpdaterate(int)
    */
   public void update();
+
+  /**
+   * This flag controls whether this instance is currently active and thereby needs to be updated by the game loop.
+   * 
+   * @return True if this instance should be updated; otherwise false.
+   */
+  public default boolean isActive() {
+    return true;
+  }
 }

src/de/gurkenlabs/litiengine/UpdateLoop.java

@@ -135,7 +135,7 @@ protected long getExpectedDelta() {
   protected void update() {
     for (IUpdateable updatable : this.getUpdatables()) {
       try {
-        if (updatable != null) {
+        if (updatable != null && updatable.isActive()) {
           updatable.update();
         }
       } catch (final Exception e) {

src/de/gurkenlabs/litiengine/configuration/ClientConfiguration.java

@@ -92,6 +92,10 @@ public void setShowGameMetrics(final boolean showGameMetrics) {
   /**
    * Sets the updaterate. On a very good machine the max update rate is sth.
    * around 500 but such a high value will never be beneficial for the player.
+   * 
+   * <p>
+   * This defaults to a value of 60.
+   * </p>
    *
    * @param updaterate
    *          the new updaterate