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The appearance of this empty game window matches the second item in the above list titled default behavior . In particular, the default behavior of the Slick2D game engine is to display an empty game window with an active FPS counter in the upper-leftcorner..

Discussion and sample code

A service provider program

Many game engines, (and the Slick2D game engine is no exception) , are "service provider" programs. They provide services that make it easier to write game programs than would otherwise be the case if you were to "start from scratch" to write a game program.

Different game engines provide different services. However, most game engines, including Slick2D, provideat least a minimum set of services, which includes the following:

  • An opportunity to initialize the game state.
  • Overall flow control that includes a game loop, which alternates in some fashion between
    • an update phase, in which the game state is updated, and
    • a rendering phase in which portions of the game state may be displayed for the benefit of the player.

These services are provided in different ways in different game engines. You will learn how they are provided by the Slick2D game engine in future modules.

Two primary objects

At a minimum, a Slick2D game that runs as an application (not an applet) consists of at least two cooperating objects :

  1. An object instantiated from a subclass of the Slick2D class named GameContainer .
  2. An object instantiated from a class that implements the Slick2D interface named Game .

(For the remainder of this and future modules, unless I specifically indicatethat I am discussing a Slick2D game applet, you can assume that I am talking about a Slick2D game program that runs as an application.)

Behavior of an object of the AppGameContainer class

The GameContainer object ( item 1 above ) , and the behavior of its methods, manages the game play after the game program starts running. For example, this is the object that managesthe game loop.

For the program shown in Listing 1 , this object is an object of the class named AppGameContainer , which extends the class named GameContainer .

The AppGameContainer class provides many public methods by which you can manipulate the behavior ofthe container object. However, the authors of the Slick2D library did not intend for you to physically modify the source code in the GameContainer class or the AppGameContainer class.

Behavior of an object thatimplements the Game interface

The Game object ( item 2 above ) , and the behavior of its methods is what distinguishes one Slick2Dgame from another Slick2D game. The authors of the Slick2D library did intend for you to physically modify the source code in the class that implements the Game interface. This is how you distinguish your game from games written by others.

(Clarification: See a later discussion of a class named BasicGame , which implements the Game interface. The authors of the Slick2D library did not intend for you to modify the source code in the BasicGame class. Instead, they intended for you to subclass that class and to modify thebehavior of the Game object by overriding inherited abstract methods.)

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Source:  OpenStax, Anatomy of a game engine. OpenStax CNX. Feb 07, 2013 Download for free at https://legacy.cnx.org/content/col11489/1.13
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