Every Libgdx application has a very well-defined lifecycle controlling the states it can be in at a given time. These states are: creation, pausing, resuming, rendering, and disposing. The lifecycle is modeled by the ApplicationListener interface, which we are required to implement as it will serve as the entrance to our game logic. In our recipe's example, the EnvironmentTest class in the core project carries out such roles.

Meet the ApplicationListener interface:

public interface ApplicationListener {
   public void create ();
   public void resize (int width, int height);
   public void render ();
   public void pause ();
   public void resume ();
   public void dispose ();
}

Your ApplicationListener interface implementation can handle each one of these events in the way it deems convenient. Here are the typical usages:

When do each of these methods get called? Well, that's a really good question! Before we start looking at cryptic diagrams, it is much better to investigate and find out for ourselves. Shall we? We will simply add some logging to know exactly how the flow works.

Take a look at the EnvironmentTest.java file:

public class EnvironmentTest implements ApplicationListener {
   private Logger logger;
   private boolean renderInterrupted = true;
   
   @Override
   public void create() {
      logger = new Logger("Application lifecycle", Logger.INFO);
      logger.info("create");
   }

   @Override
   public void dispose() {
      logger.info("dispose");
   }

   @Override
   public void render() {
      if (renderInterrupted) {
         logger.info("render");
         renderInterrupted = false;
      }
   }

   @Override
   public void resize(int width, int height) {
      logger.info("resize");
      renderInterrupted = true;
   }

   @Override
   public void pause() {
      logger.info("pause");
      renderInterrupted = true;
   }

   @Override
   public void resume() {
      logger.info("resume");
      renderInterrupted = true;
   }
}

The renderInterrupted member variable avoids printing render for every game loop iteration.

The Logger class helps us show useful debug information and errors on the console. Not only does it work on desktops but also on external devices, as long as they are connected to Eclipse. Remember this little new friend as it will be truly useful for as long as you work with Libgdx. The constructor receives a string that will be useful to identify the messages in the log as well as on a logging level.

In order of increasing severity, these are the available logging levels: Logger.INFO, Logger.DEBUG, Logger.ERROR, and Logger.NONE. Several methods can be used to log messages:

Logging levels can be retrieved and set with the getLevel() and setLevel() methods, respectively. Both the level and the method used to log a message will determine whether they will actually be printed on the console. For example, if the level is set to Logger.INFO, only messages sent through info() and error() will appear, and those sent through debug() will be ignored.

Now, run the application on all the platforms and pay attention to the console. Depending on how you play with the focus, the output will vary, but it should be similar to this:

Application lifecycle: create
Application lifecycle: resize
Application lifecycle: render
Application lifecycle: pause
Application lifecycle: render
Application lifecycle: resume
Application lifecycle: render
Application lifecycle: dispose

This should give you a pretty decent handle of how the application lifecycle works.

Placing breakpoints on each ApplicationListener overridden method is also a good way of discovering what is going on. Instruction breakpoints allow you to debug an application and stop the execution flow that reaches the said instruction. At this point, you can run the code instruction by instruction and examine the current state of the active variables. To set a breakpoint, double-click next to the corresponding line; a blue dot will confirm that the breakpoint is set. Once you are done, you can debug the application by right-clicking on the desired project and selecting the Debug As menu.

The Eclipse Debug view will then enter the stage with all its shiny panels. The Debug tab shows the current execution callstack, the Variables tab contains the current state of the variables within scope, and in the following screenshot, you can see the code with the current line highlighted. The arrow buttons in the upper toolbar can be used to step over the next instruction (F6) or move on to the next method (F5), where applicable, or out of the current method (F7), as shown in the following screenshot:

Every platform project consists of a starter class (or entry point). This class is responsible for constructing the platform-specific application backend. Each backend implements the Application interface. The starter class also passes a new instance of our ApplicationListener implementation to the application backend. This implementation typically lives in the core project and serves as an entry point to our cross-platform game code. Finally, it also submits a configuration object, and by doing so, it provides a mechanism to customize general parameters, as we will see later.

public class DesktopLauncher {
   public static void main (String[] arg) {
      LwjglApplicationConfiguration config = new LwjglApplicationConfiguration();
      new LwjglApplication(new EnvironmentTest(), config);
   }
}

public class AndroidLauncher extends AndroidApplication {
   @Override
   protected void onCreate (Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      AndroidApplicationConfiguration config = new AndroidApplicationConfiguration();
      initialize(new EnvironmentTest(), config);
   }

public class GwtLauncher extends GwtApplication {
   @Override
   public GwtApplicationConfiguration getConfig () {
      GwtApplicationConfiguration cfg = new GwtApplicationConfiguration(480, 320);
      return cfg;
   }

   @Override
   public ApplicationListener getApplicationListener () {
      return new EnvironmentTest();
   }
}

public class IOSLauncher extends IOSApplication.Delegate {
    @Override
    protected IOSApplication createApplication() {
        IOSApplicationConfiguration config = new IOSApplicationConfiguration();
        return new IOSApplication(new EnvironmentTest(), config);
    }

    public static void main(String[] argv) {
        NSAutoreleasePool pool = new NSAutoreleasePool();
        UIApplication.main(argv, null, IOSLauncher.class);
        pool.drain();
    }
}

As you already know, every Libgdx game needs to have an ApplicationListener interface implementation in its core project for the launchers to use. We also saw how the developer is forced to implement the create(), dispose(), render(), resize(), pause(), and resume() methods of such an interface. However, these overridden methods might end up completely empty. What a waste of digital ink, and more importantly, our precious time!

Luckily enough, Libgdx provides a useful ApplicationAdapter class that already contains an empty implementation for each ApplicationListener interface method. This means that you can simply inherit from ApplicationAdapter and only override the methods you really need. This comes particularly in handy when writing small tests rather than big games. These small adapter classes are quite common within the API, and they are really comfortable to use as long as we do not need to inherit from anything else. Remember that Java does not support multiple inheritance.

The following will be perfectly valid if we want a completely empty application:

public class MyGame extends ApplicationAdapter {}

Most games are made out of several screens the player can navigate through. The main menu, level selection settings, or levels are some of the most common examples. Though this completely depends on the nature of each project, most of them definitely share the structure. Libgdx comes with an utterly minimalistic screen system built-in, which might just be enough for your requirements, so why not use it? Reinventing the wheel is rarely a good idea.

The two main components of this system are the Game abstract class and Screen interface. Game implements the well-known ApplicationListener interface, so you will only need your main class to inherit from Game.

The Game class holds a reference to the current Screen and provides the getter and setter methods for it. Game requires you to implement the create() method, but already provides implementations for the rest of the application lifecycle methods. Be aware that if you override any of the other methods, you will need to call the parent version so as to maintain screen behavior correctness. The helpful bit comes with the render() method, which will automatically update and render the active Screen reference, as long as it is not null.

What follows is an UML class diagram illustrating a sample game architecture based on the Game/Screen model. The user implemented MyGame as a Game derived class, and the SettingsScreen, GameScreen, LevelSelectionScreen, and MainMenuScreen classes were derived from Screen:

The Game public API looks like this. Note that method implementation has been omitted for space reasons:

public abstract class Game implements ApplicationListener {
   public void dispose ();
   public void pause ();
   public void resume ();
   public void render ();
   public void resize (int width, int height);
   public void setScreen (Screen screen);
   public Screen getScreen ();
}

The Screen interface is quite similar to the ApplicationListener interface, but its equivalent methods will only be called when it is the active screen. It also adds the hide() and show() methods that will be called when changed to and from a screen, respectively. In the following code, you will find an overview of the interface:

public interface Screen {
   public void render (float delta);
   public void resize (int width, int height);
   public void show ();
   public void hide ();
   public void pause ();
   public void resume ();
   public void dispose ();
}