0.17 Xna0132-a simple game program with on-screen text  (Page 2/13)

General background information

When we finished the previous module, we were missing one critical element needed to write a simple game program: user input . We were also lacking some other tools that would make it possible for us tocreate more interesting 2D arcade-style games including the ability to create on-screen text.

In this module, you will learn how to program for user input with the keyboard and the mouse. You will also learn how to create on-screen text.

Preview

The program that I will explain in this module is a simple game in which ten spiders attempt to traverse their web from top to bottom. Two largeladybugs are there to eat them if possible. (See Figure 1 .) The objective of the game is to prevent as many of the spiders as possible from making it tothe bottom of the game window.

Figure 1 shows a reduced view of the game window shortly after the game is started and before any spiders make it to the bottom.

Figure 1 . Reduced view of the game window.

Spider behavior

The spiders move at different speeds in different directions but generally towards the southeast. If a spider makes it to the bottom, it is counted ashaving crossed the finish line. That spider wraps back up to the top and starts the journey again. If a spider hits the right edge of the window, it wrapsaround to the left edge and continues the journey.

The ladybugs

The movements of the ladybugs are under the control of the player. If the player causes a ladybug to collide with a spider, the spider is eaten andremoved from the spider population.

Using the keyboard

The player can move the ladybugs using either the keyboard or the mouse. Pressing the four arrow keys causes one of the ladybugs to move in thecorresponding directions. Pressing two arrow keys at the same time causes the ladybug to move in a diagonal direction.

Holding down the A key and pressing the arrow keys causes the other ladybug to move.

Using the mouse

Pressing the left mouse button makes it possible to drag a ladybug with the mouse. Pressing the right mouse button makes it possible to drag the otherladybug with the mouse. Pressing both mouse buttons makes it possible to drag both ladybugs with the mouse.

Difficulty level

The game has only one level. I personally find it very difficult to prevent all ten spiders from making it to the bottom using the arrow keys only. On theother hand, it is easy to defeat the spiders by dragging a ladybug with the mouse.

A useful student project

A useful student project would be to upgrade the program to provide varying levels of difficulty. This could be accomplished by changing the speed of the spiders, changing the number of spiders, or a combination ofthe two.

Alternatives to the arrow keys

Another useful student project would be to define alternatives to the arrow keys, such as the keys labeled 4, 8, 6, and 2 on the keypad, or four keys in adiamond pattern on the left side of the keyboard.

Discussion and sample code

As usual, I will break this program down and explain it in fragments. However, I will only explain those portions of the code that are substantiallydifferent from code that I have explained in earlier modules.