Game Development Reference
In-Depth Information
not written by us, but we can still use it because we have indicated in the beginning
of our program that we need things from the library Microsoft.Xna.Framework.Graphics .
So, a compilation unit consists of a number of using instructions followed by a
number of top level declarations. We can again use a syntax diagram to describe
this:
3.3.9 Subclasses: A Special Version of Another Class
Let us look at the class header again:
class BasicGame : Game
Game is in fact also a class, and it comes from the library Microsoft.Xna.Framework ,
hence the need for the using statement in the first line of the program. So why is it
placed after the BasicGame class name? Although for now it is not yet so important
to know what it means exactly, you can read this as: ' BasicGame is a special version
of Game '. Game is a class, so it contains a group of methods as well. BasicGame is a
special version of the Game class, so it inherits all of the methods that were in the
Game class. We can then add our own methods to this class, or rewrite some of the
methods that were in the original Game class, such as the Update method or the Draw
method. If this seems a lot to take in, do not worry about it, we will discuss this in
much more detail later on.
3.3.10 Calling a Method
When the instruction GraphicsDevice.Clear(Color.Olive); is executed, we can also say
that we call the Clear method. In other words: we want the program to execute the
instructions grouped in the method Clear . This group of instructions does exactly
what we need for this example, namely clearing the screen and setting a background
color. However, we need to give some extra information to this method, because it
needs to know which color to set the background to. This extra information is also
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