Learning XNA 3.0

Let's Move

Drawing different sort modes and layer depths is fun and all, but it really isn't all that exciting. Now, let's make the two images that you've drawn move and bounce off the edges of the screen. To move the objects, you are going to need to change the positions at which they are drawn. Right now, they are both being drawn at constant positions, one with its top-left corner at the exact center of the window and the other offset from the center so that the image itself is centered in the middle of the screen.

Tip

The code for this section of the chapter is available with the source code for the book under Chapter 2 and is entitled MovingSprites.

To move something around the screen, you have to modify the position of that object between frames. Therefore, the first thing you need to do is start using a variable in place of the constant values you've been using to specify the objects' positions. Add two class-level Vector2 variable definitions (called pos1 and pos2) at the top of your class, and initialize both objects to Vector2.Zero:

Vector2 pos1 = Vector2.Zero;
Vector2 pos2 = Vector2.Zero;

You'll also need to have a speed variable for each object. This variable will be used to determine how far you move each object between frames. Add two float variables (called speed1 and speed2) below the Vector2 variables you just added:

float speed1 = 2f;
float speed2 = 3f;

Now, change the position parameters in each Draw method to use pos1 and pos2, respectively. Just to clean things up, ...