Math Operators: Using Constant Values in C#

References:

• Pre-requisite: it is assumed that you have read through the prior tutorials, and are familiar with the concepts covered in those tutorials.

Goals:

• In this tutorial, we will:
  • Add several more blocks to the screen
  • Add a new background image for 'Expert' mode

1. Obtain the example code

• Here is the zip file to the source files and compiled executable of this example.
• Here is the keyboard mapping for the XBOX 360 controller.  
• Here is the main tutorial page for working with XNACS1Lib, and here is the complete documentation of the library: html-page or compiled html help file.

When the game starts, you'll see a screen that looks similar to this:

The rectangle on the far left is a 'paddle', which the soccer ball will bounce off of (if you move the paddle in the way of the soccer ball).  You can move it up and down by pushing the left thumbstick up and down.  The rectangle on the far right is a 'paddle', which the soccer ball will bounce off of if you move the paddle in the way of the soccer ball.  You can move it up and down by pushing the right thumbstick up and down.  The other, labeled rectangles are placed randomly in the middle of the screen.  The soccer ball starts off in the middle of the screen, moving in a random direction at a random speed. 

If the soccer ball hits the top or bottom of the screen, it will bounce off - it continue to move at the same speed, but the vertical component of it's velocity will be reversed (i.e., when it hits the top of the screen, instead of going upwards, it will switch and go downwards).  If it hits one of the labeled rectangles, it will bounce off (reversing both it's horizontal & vertical speeds).  If the soccer ball reaches the left or right edge of the screen then you "missed" it, and it is put back into the middle of the screen, with a new random direction.  Missing the ball results in the CurrentBounces number going back to zero, and the "Balls Missed" number going up by one.

Your goal in this game is to bounce the soccer ball off the paddles as many times as you can.  Once you've bounced it 5 times in a row (without missing it), then you go to expert mode, where things move a little bit faster.  If you can bounce it 10 times in a row, and you've never missed more than two balls, then you win the game.  If you can get to 10 bounces, but you've missed it more than twice, then each additional bounce beyond 10 will decrease your 'missed balls' count.  What this means is that you can still win, although it will be more difficult the more you have missed the soccer ball.  Notice that at the bottom of the screen, the game tells you whether you're in Novice or Expert mode, how many times (in a row) you've bounced the ball since the last time you missed it, the longest run of bounces you've managed to do, and the number of soccer balls that you've missed.  At the top, the game tells you which labeled rectangle the soccer ball has most recently collided with.

Once you reach expert mode, the screen should look something like this:

Once you've won the game, the screen should look something like this:

2. Examining The Program:

Let's examine the C# source code that produces the behavior we see on-screen

• Declaring the instance variables

  public class Game1 : XNACS1Base {      #region random blocks and their positions     private XNACS1Rectangle m_ABlock;     private XNACS1Rectangle m_BBlock;     private XNACS1Rectangle m_CBlock;     private XNACS1Rectangle m_DBlock;     private XNACS1Rectangle m_EBlock;     private const float BLOCK_A_X_POSITION = 40.0f;     private const float BLOCK_X_OFFSET = 10.0f;

  • Since we want to add two more blocks to the game, we will need to add instance variables to keep track of them.
• InitializeWorld():

  m_CBlock.Label = "C"; blockXPos = blockXPos + BLOCK_X_OFFSET; m_DBlock = InitializeRectangle(blockXPos,RandomFloat(BLOCK_POSITION_MIN_Y, BLOCK_POSITION_MAX_Y),         BLOCK_WIDTH, BLOCK_HEIGHT); m_DBlock.Label = "D"; blockXPos = blockXPos + BLOCK_X_OFFSET; m_EBlock = InitializeRectangle(blockXPos,RandomFloat(BLOCK_POSITION_MIN_Y, BLOCK_POSITION_MAX_Y),         BLOCK_WIDTH, BLOCK_HEIGHT); m_EBlock.Label = "E"; #endregion // initialize statistics InitializeStats(); }

  • Similarly, we will extend the initialization code to create and initialize the two new blocks, following the same pattern that we used for the others.
• UpdateWorld():

  // Should we move from Notice to Expert mode? if (m_MaxBounces > EXPERT_LEVEL_BOUNCES) {     // String equality test     if (m_SkillLevel == "Novice")     {         BackgroundTexture = "ExpertImage";         PlayACue("Victory");         m_TheSoccer.VelocityX = m_TheSoccer.VelocityX + Math.Sign(m_TheSoccer.VelocityX) * BALL_VELOCITY_EXPERT_BOOST_X;         m_TheSoccer.VelocityY = m_TheSoccer.VelocityY + Math.Sign(m_TheSoccer.VelocityY) * BALL_VELOCITY_EXPERT_BOOST_Y;     }     m_SkillLevel = "Expert"; }

  • UpdateWorld is almost identical, except that we've added in a line (BackgroundTexture = "ExpertImage";) to change the background from the blank "Novice" background to the "ExpertImage" background, but only once - when we first switch from Novice mode to Expert mode.
• BounceOffBlocks():

  private void BounceOffBlocks() {     if (m_ABlock.Collided(m_TheSoccer))     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;         EchoToTopStatus("Last block collision: A-Block");         PlayACue("Break");     }     else if (m_BBlock.Collided(m_TheSoccer))     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;         EchoToTopStatus("Last block collision: B-Block");         PlayACue("Break");     }     else if (m_CBlock.Collided(m_TheSoccer))     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;         EchoToTopStatus("Last block collision: C-Block");         PlayACue("Break");     }     else if (m_DBlock.Collided(m_TheSoccer))     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;         EchoToTopStatus("Last block collision: D-Block");         PlayACue("Break");     }     else if (m_EBlock.Collided(m_TheSoccer))     {         m_TheSoccer.VelocityX = -m_TheSoccer.VelocityX;         EchoToTopStatus("Last block collision: E-Block");         PlayACue("Break");     } }

  • Similarly, we extend the pattern that BounceOffBlocks utilizes, so that we will now, also check for collision with the two new blocks.

FURTHER EXERCISES:: 

1. Start from a blank starter project (1000.201, if you need it), and re-do the code from memory as much as possible.  On your first try, do what you can, and keep the above code open so that when you get stuck, you can quickly look up what you forgot (and that after you finish a line, so that you can compare your line to the 'correct' line).  On the next try, do the same thing, but try to use the finished code less.  Repeat this until you can type everything, without refering the tutorial's code.
   • Repeat this exercise daily for several days, so that you really get the hang of this.  As you go on, periodically review this by re-doing this exercise.

	This work is supported in part by a grant from Microsoft Research under the Computer Gaming Curriculum in Computer Science RFP, Award Number 15871 and 16531.
2/8/2010