5. The Block and Paddle Classes

• Pre-requisite: this is the example (4. Ball class) 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:

• Interaction of objects: the ball and the paddle

Library functions to notice:

• DrawRectangle: simple function to draw a rectangle.

• EchoToBottomStatus(also EchoToTopStatus): Echo status text for the user.

1. Create a new file: Block.cs. This is a simple rectangular block that knows how to bounce a Ball. Here is the link to the file. Interesting features of this class includes:
   • Instance variables:
     • m_Position: center position of a rectangular block.
   • Constructor: sets the center position of the block.
   • Position and Bounds: access the position and returns the left/right/top/bottom bounds of the block.
   • CollideWithBall: determines if a collision has occurred. If so, computes a new velocity for the ball.
     • Bound-to-Bound check: collides the the ball by checking the x/y bounds of the block with the x/y bounds of the ball. Notice this is only a pretty good approximation of actually intersecting a circle with a rectangle.
     • Velocity of Ball: If a collision has occurred, check, if collision at the x-boundaries: flip the X-component of the velocity otherwise, flip the Y component.
     • Update the ball's bounces
   • Draw: draws a rectangle.
   
   
2. Create a new file: Paddle.cs. This will be the paddle at the bottom of the application window. Notice that the paddle is a special type of rectangular Block, so subclass we will subclass from the Block class. Here is the link to the source file. Interesting features of this class include:
   • Instance variable: we do not need any new instance variables.
   • MovePaddle: allow horizontal movement of the paddle. We check to make sure the center of the paddle does not move off the application window.
   • BounceBall: Calls super class's CollideWithBall function. If a collision indeed occurs:
     • Ball velocity: make sure the ball travel's upwards after the collision.
     • Ball position: make sure the ball is above the paddle right after collision.
   
   
3. Changes to: Game1.cs
   • Instance variable:
     • Paddle m_Paddle: The paddle!
   • InitializeWorld: allocate memory and instantiate m_Paddle.
   • UpdateWorld:
     • Move paddle: pass along the left-thumbstick horizontal movement as controls of the paddle horizontal position.
     • Paddle bounces ball: if there is a ball, compute the interaction between the ball and the paddle by calling the paddle's BounceBall() function.
   • DrawWorld:
     • Paddle: always draws the paddle.
     • Ball: if present draws the ball, and echo the number of bounces.

We worked with two objects in the application: the ball and the paddle. During the Game1::UpdateWorld() function we:

• Update the ball: create or update the ball position. Notice, the ball is updated independent from the paddle, we don't need to know anything about the paddle, we can create the ball and collide the ball against the application window boundary.
• Update the paddle: here we move the paddle horizontally. Similar to the ball update, the paddle's update can be carried out independent from the ball.
• Interaction of the objects: after both objects are properly updated, we then interact the two. In this case, we call the paddle's bounce ball function to bounce the ball.

In general, when our application has multiple objects, we can usually update each object independently, and then be concern about interaction between the objects.