Creating a Game World - Learning C# by Programming Games - page 50

4.2.9 Scope of Variables
The place where we declare a variable has consequences on where we are allowed
to use this variable. Take a look again at the variable red in the DiscoWorld program.
This variable is declared (and assigned a value) in the Update method. Because it is
declared in the Update method, we are only allowed to use it in this method. We are
not allowed for example to use this variable again in the Draw method. Of course, we
could declare another variable called red inside the Draw method, but it is important
to realize that the red variable declared in Update would not be the same red variable
declared in the Draw method. Alternatively, if we declare a variable at the class body
level, then we can use it in the entire class . We need to use the background color
in both the Update and Draw methods, because in the Update method, we update this
background color, and in the Draw method, we use the updated color to paint the
window using the Clear method. Therefore, the variable needs to be declared at the
class level, so that all methods in the class can use this variable.
The places where a variable can be used are also called the scope of a variable. In
this example, the scope of the variable red is the Update method, and the scope of the
variable background is the DiscoWorld class. More officially, the scope of a variable is
determined by the braces that enclose the declaration. In the case of the variable red ,
these braces are the delimiters of the Update method body. In the case of the variable
background , the braces are the delimiters of the class body .
4.2.10 Setting the Background Color
After we have updated our color in the Update method, we can pass it as a parameter
to the Clear method, which we call in the Draw method:
GraphicsDevice.Clear(background);
Try executing the program by pressing F5. You will see that the color changes from
black to red, stays red for a while, then turns from red to black again, and so on.
The Update and Draw methods are executed a number of times per second, and every
time we look at what the current passed game time is, and we use the milliseconds
component to create a color. This number of milliseconds starts with the value 0 (in
the beginning of the game) and then rises to 1000. Once this value has been reached,
the seconds counter is increased, and the milliseconds counter is reset to 0. Since
we use the milliseconds counter to create the R value of the background color, we
see the color changing on the screen.
You have probably noticed that the color stays red for a while, before starting
from black again. Why is this? Remember that the range of RGB value is between
0 and 255? Since the milliseconds counter goes from 0 to 999, we are probably
creating colors like new Color(260, 0, 0) , new Color(378, 0, 0) ,or new Color(956, 0, 0) .
What happens when the color is created is that if the R value exceeds its maximum