Creating Game Components in .NET framework

Drawer QR Code in .NET framework Creating Game Components

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The LoseLife method is part of the BreadAndCheeseGame class and is how game sprites tell the game that a life has been lost. You use a similar method when you score points. Sprites need to have this form of coupling so that the sprites can affect the game where required. However, this means that they are tightly linked with the BreadAndCheeseGame class and can be used only with it. This is actually a serious restriction. We would like to use the same set of sprites in an Alien Wars game that we are also working on, but because this is held in a class called AlienWarsGame, our sprites can t talk to it. We can t use abstract classes to solve this problem, because the game classes are children of the XNA Game class, whereas our sprites are all children of the BaseSprite class.

However, it turns out that we can use another C# feature to solve this problem and turn our sprites into genuine components. This feature is called an interface. Note One po nt I shou d make here s that we are not ta k ng about the user nterface to our game The user nterface s the way a person us ng a program wou d make t work for them These are usua y e ther text-based (that s, the user types n commands and gets responses) or graph ca (that s, the user c cks buttons on a screen us ng the mouse) In programm ng terms, an nterface just spec fies how a software component cou d be used by another software component You can think of interfaces in terms of plugs and sockets. When you plug your computer into the wall socket, you are actually using an interface. The power company has created a standard that describes the shape of the outlet on the wall and the voltage and frequency of the power that comes out of it. This interface lets you plug in anything built to use that connection, whether it is a computer, a toaster, or an Xbox 360. You can create a software interface to specify the connection between your game and the sprite you would like to plug into it. You can design this interface by deciding what a sprite needs to be able to do with the game that it is part of:

Update the lives Update the score Get the current score in the game Get the gamepad state Start the game End the game

Anything which provides these behaviors can act as a "host" for our sprites, in that it can do anything that they need.

I worked out the contents of the interface by looking at the existing classes and deciding which methods the sprites actually needed. These methods can be put into a C# interface as follows:

public interface ISpriteBasedGame { void UpdateLives(int update); void UpdateScore(int update); int GetScore(); GamePadState GetGamePad(); void StartGame(); void EndGame(); }

A C# interface looks a lot like an abstract class. It is a collection of method specifications. The idea is that rather than using a reference to a particular class, you can instead use a reference to a class that can implement that interface. In other words, rather than thinking of the host of a sprite as a BreadAndCheeseGame, we think of it as a class that implements the ISpriteBasedGame interface. The BreadAndCheeseGame class can indicate that it implements the interface, as shown in bold here:

/// <summary> /// This is the main type for your game /// </summary> public class BreadAndCheeseGame : Microsoft.Xna.Framework.Game, ISpriteBasedGame { // All of the game class code goes here. // This must include implementations of UpdateLives, // UpdateScore, GetScore, GetGamePad, StartGame; // and EndGame }

When you declare a class, you can state that it extends a parent (in this case, the Microsoft.XNA.Framework.Game class) and also give a list of any interfaces that it implements (in this case, the ISpriteBasedGame interface). A class can implement many interfaces, depending on the number of things you want to be able to ask it to do. Note The name of the nterfaces I have created s ISpriteBasedGame There s a convent on

n C# that nterfaces have names that start w th I Th s s so that a programmer can te whether a g ven tem s an nterface or an object You do not have to use th s convent on, but the Great Programmer has to d me that she w hunt you down f you don t