Developing Game-Themed Applications with XNA Game Studio

Section III: The Block Breaker Game
b. The Ball class

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• Pre-requisite: this is the example (Section IIIa. Simple Circle) we are building off from.
• You can start with this template source file (resulting source code from previous example), and here is the result the source code for this example.
• Here is the mapping between keyboard and the controller.

Goals: In this tutorial we want to experience:

• Creation and bouncing animation of a circle (the ball class).
• Validity of object: remove when out of bound.

Library functions to notice:

• Application window bounds: 

Notice that these bounds are defined during the construction of the application window (please refer to the constructor). These bounds are not in pixel unit! Please refer to the library documentation for the details of programmer defined bounds. In this application, the width of the application window is defined to be 15 units wide.

• XnaAssignmentBase.WorldMax: a vector2 indicating the maximum bounds (top and left bounds) of the application window.

• XnaAssignmentBase.WorldMin: a vector2 indicating the minimum bounds (right and bottom) of the application window.

 
• Memory allocation: memory are allocated during initialization.

1. Game1.cs:
 add in the following code in the beginning of the file. This is the general utility for generating random number for our application.

#region __CODE_ADDED__ random number support      private static Random m_Rand = new Random();      static public float RandomNumber() {          return (float)m_Rand.NextDouble();        }     #endregion



2. Create a new file: Ball.cs. Here is the link to the file. Interesting features of this class includes:
   • Instance variables:
     • m_Position: position of the ball
     • m_Velocity: velocity of the ball
     • m_BounceCount: number of bounce (for fun).
     • m_Expired: is the ball still valid? This will be determined by if the ball is still within the bounds of the application window.
   • Constructor: Notice the randomness in initial velocity of the ball.
   • Bounds:
     • MinBound: lower-left corner of the ball.
     • MaxBound: upper-right corner of the ball.
   • Update:
     • Update ball position by velocity.
     • Bouncing with the bounds of the application bounds (notice the XnaAssignmentBase.WorldMax, and XnaAssignmentBase.WorldMin).
     • Notice we set the expiration to true when the ball is below the window.
   • Draw: draws a circle.
   
   
3. Changes to: Game1.cs
   • Instance variable:
     • Ball m_Ball: The ball!
   • InitializeWorld: set m_Ball to null. No ball until we press the "A"-button.
   • UpdateWorld:
     • Create ball: allocate memory for a ball if button-A is pressed.
     • Update Ball: if there is a ball, calls the update function, and check to remove the ball if already expired.
   • DrawWorld: Draws the ball if not null.

Most classes we create will have the following two functions:

• Update: this function should update the state of the object, typically changing position, or colliding with another object.
• Draw: draws the object.