Knowing What the Player Is Doing - Learning C# by Programming Games - page 70

5.3.3 A Constructor Describes How an Object Is Created
When a SpriteBatch object is created using the new keyword, we need to provide
some information so that this object can be created, namely the graphics device.
This information is passed as a parameter to a special method called the construc-
tor method. Inside this method, we put all the instructions needed to initialize our
object. A constructor method has a special header: it carries the same name as the
class. The Balloon class contains a constructor method, which looks as follows:
public Balloon()
{
Content.RootDirectory = "Content";
graphics = new GraphicsDeviceManager( this );
}
Just like a normal method, the constructor method has parentheses which may con-
tain parameters (in this case there are no parameters). And just like a normal method,
a constructor method has a header and a body.
5.3.4 this : The Object We Are Currently Manipulating
When we create the instance of our Balloon class in the Main method, the Balloon
constructor method will be called. Inside the constructor method, we set the root
directory where all assets are placed, and we initialize the graphics device. The first
line looks like we are accessing a property RootDirectory from the class Content .How-
ever, this is not what happens. In fact, Content is a property of the Game class, and
because Balloon is a special version of that class, it also has the Content property.
But if properties need an object, which object is this property manipulating? It ma-
nipulates the object that is currently created. So, in the Main method, we create an
instance of the Balloon class and store it in the variable game . The instructions in the
constructor method then manipulate this object.
This object has a variable graphics . We assign the graphics device to that variable,
which we create by constructing an instance of the GraphicsDeviceManager class. The
constructor method of this class needs a variable of type Game , which we need to
pass as a parameter. Of course, we want to pass our variable game to this constructor.
However, if we would write the following line in the constructor, we would get a
compiler error:
graphics = new GraphicsDeviceManager(game);
The reason for this is that the variable game is declared in the Main method, and
because of the variable scope rule discussed in the previous chapter, we can only
access this variable in the method is was declared in . What we in fact want to do is to