0.14 Xna0126-using oop - a simple sprite class  (Page 2/8)

General background information

The three pillars of OOP

An object-oriented programming language like C# supports encapsulation , inheritance , and polymorphism . In this module, you will learn how to take advantage of encapsulation.

Wanted, lots of sprites

Assume that you are writing a game program in which you need to have several dozen similar sprites on the screen at the same time. Creating and controllingthat many sprites without using encapsulation would require you to write a lot of program code.

Encapsulation to the rescue

However, by encapsulating the characteristics of a sprite into a class, which is a blueprint for an object, you can instantiate sprite objects just like cutting out cookies with a cookie cutter.

In this module, you will learn to define and use a very simple Sprite class. In future modules, you will learn how to modify the Sprite class to make it more sophisticated by making use of inheritance andpolymorphism in addition to encapsulation

Preview

The XNA project that I will explain in this module is named XNA0126Proj . This project demonstrates how to design and use a very simple version of a Sprite class. An object instantiated from the Sprite class has the following general characteristics:

• It can be instantiated without an image.
• It can be instantiated with an image.
• It can have its image set.
• It can have its position within the game window set.
• It can cause itself to be drawn.

Demonstrate how to use the Sprite class

Methods are overridden in the standard XNA Game1 class that demonstrate the use of the Sprite class.

One Sprite object is instantiated in the overridden LoadContent method of the Game1 class. The object's reference is saved in a generic List object. Twenty-three more Sprite objects are instantiated in the overridden Update method while the game loop is running.

Instantiate Sprite objects on the move

A new Sprite object is instantiated in the Update method every 8th iteration of the game loop until twenty-four Sprite objects have been instantiated. The object's references are saved in the samegeneric List object mentioned above.

An image of a blue ball is stored in 12 of the objects and an image of a red ball is stored in the other 12 objects. The red and blue balls alternate and the Sprite objects are drawn in a diagonal line as shown in Figure 1 .

Figure 1 . Seven Sprite objects.

Seven Sprite objects

Figure 1 shows the game window after seven of the twenty-four Sprite objects have been instantiated and drawn in the game window.

Move to the right and down

The line of Sprite objects moves across the game window from upper left to lower right as the Sprite objects are being instantiated. They stop moving when they reach the bottom right corner of thegame window.

When the objects stop moving, the image in the topmost Sprite object is changed from a blue ball to a green ball as shown in Figure 2 .

Figure 2 . Twenty-four Sprite objects with a green one at the top.

Discussion and sample code

As usual, I will explain the program code in fragments. A complete listing of the class named Sprite is provided in Listing 14 and a complete listing of the class named Game1 is provided in Listing 15 .