Beginning Windows® Phone 7 Application Development: Building Windows® Phone Applications Using Silverlight® and XNA®

UPDATING GAME STATE

As discussed earlier, the Update() method of the Tank game is responsible for updating the state of the game's world in response to various types of inputs, especially the passing of time. But what do we mean by “the state of the world”? Software uses variables to hold state, and fields (that is, class-level variables) represent more persistent state. But none of the fields you've used so far represent the conceptual world that is the Tank game.

Here are some examples of conceptual things that a game could include, along with potential types that could hold the state for these variables in an XNA game:

CONCEPT	TYPE
Position	Vector2
Speed	Vector2
Acceleration	Vector2
Angle/direction	float
Alive/dead	bool
Score	int
Health points/lives	int
The last time something occurred	DateTime
How long something might take	TimeSpan

The Versatility of Vectors

As it turns out, that's the hard way to translate vectors. The Vector2 class in XNA has all sorts of nifty methods. Mathematically, you use it to add one vector to another. As it turns out, Vector2 overloads the plus operator. This means that even though an instance of Vector2 isn't a number, you can simply add two Vector2s together. And you can use x += y as shorthand for x = x+y. Here's how you could refactor the code to make use of this versatility:

protected override void Update(GameTime gameTime) { // Allows the game to exit GamePadState gamePadState = GamePad.GetState(PlayerIndex.One);  ...