Running with Code Like with scissors, only more dangerous

15May/080

My First XNA Application: The Bouncing Ball

My first application in XNA!  It’s a… well, it’s a ball that bounces.

It's bouncing!!

I created a new Windows XNA 2.0 application project.  I decided to abstract away a Ball object, as well as a GravitySource object.  There’s a lot of cross-talk — I’m not sure if this is good or not — but it’s happy enough for me. :-)

Gravity seems to be pretty straightforward to implement; the applied speed is going to be a vector direction with a constant speed as the magnitude.  Here’s the code for the gravity source class:

   1: public class GravitySource
   2: {
   3:     private const float GRAVITY = 9.8f;
   4:  
   5:     private Vector2 m_gravitySpeed;
   6:  
   7:     public void ApplyGravity(ref Vector2 currentSpeed)
   8:     {
   9:         currentSpeed += m_gravitySpeed;
  10:     }
  11:  
  12:     public void ResetToDirection(Direction target)
  13:     {
  14:         switch (target)
  15:         {
  16:             case Direction.Up:
  17:                 m_gravitySpeed = new Vector2(0f, -GRAVITY);
  18:                 break;
  19:             case Direction.Left:
  20:                 m_gravitySpeed = new Vector2(-GRAVITY, 0f);
  21:                 break;
  22:             case Direction.Right:
  23:                 m_gravitySpeed = new Vector2(GRAVITY, 0f);
  24:                 break;
  25:             case Direction.Down:
  26:                 m_gravitySpeed = new Vector2(0f, GRAVITY);
  27:                 break;
  28:             case Direction.DownLeft:
  29:                 m_gravitySpeed = Vector2.Normalize(new Vector2(-1, 1)) * GRAVITY;
  30:                 break;
  31:             case Direction.DownRight:
  32:                 m_gravitySpeed = Vector2.Normalize(new Vector2(1, 1)) * GRAVITY;
  33:                 break;
  34:             case Direction.UpLeft:
  35:                 m_gravitySpeed = Vector2.Normalize(new Vector2(-1, -1)) * GRAVITY;
  36:                 break;
  37:             case Direction.UpRight:
  38:                 m_gravitySpeed = Vector2.Normalize(new Vector2(1, -1)) * GRAVITY;
  39:                 break;
  40:         }
  41:     }
  42: }

Note that I added a Direction enumeration to make it nice to read.

The game class has a reference to the ball object, and it handles the keyboard input to change the gravity direction all within the Update method:

   1: protected override void Update(GameTime gameTime)
   2: {
   3:     // Allows the game to exit
   4:     if (GamePad.GetState(PlayerIndex.One).Buttons.Back == ButtonState.Pressed)
   5:         this.Exit();
   6:  
   7:     KeyboardState keybstate = Keyboard.GetState();
   8:     if (keybstate.IsKeyDown(Keys.Down))
   9:     {
  10:         if (keybstate.IsKeyDown(Keys.Left))
  11:         {
  12:             m_grav.ResetToDirection(Direction.DownLeft);
  13:         }
  14:         else if (keybstate.IsKeyDown(Keys.Right))
  15:         {
  16:             m_grav.ResetToDirection(Direction.DownRight);
  17:         }
  18:         else
  19:         {
  20:             m_grav.ResetToDirection(Direction.Down);
  21:         }
  22:     }
  23:     else if (keybstate.IsKeyDown(Keys.Up))
  24:     {
  25:         if (keybstate.IsKeyDown(Keys.Left))
  26:         {
  27:             m_grav.ResetToDirection(Direction.UpLeft);
  28:         }
  29:         else if (keybstate.IsKeyDown(Keys.Right))
  30:         {
  31:             m_grav.ResetToDirection(Direction.UpRight);
  32:         }
  33:         else
  34:         {
  35:             m_grav.ResetToDirection(Direction.Up);
  36:         }
  37:     }
  38:     else if (keybstate.IsKeyDown(Keys.Left))
  39:     {
  40:         m_grav.ResetToDirection(Direction.Left);
  41:     }
  42:     else if (keybstate.IsKeyDown(Keys.Right))
  43:     {
  44:         m_grav.ResetToDirection(Direction.Right);
  45:     }
  46:  
  47:     m_ball.Update(gameTime, graphics.GraphicsDevice.Viewport);
  48:  
  49:     base.Update(gameTime);
  50: }

The game’s drawing method is actually quite simple:

   1: protected override void Draw(GameTime gameTime)
   2: {
   3:     graphics.GraphicsDevice.Clear(Color.Black);
   4:  
   5:     spriteBatch.Begin(SpriteBlendMode.AlphaBlend);
   6:     m_ball.Render(gameTime, spriteBatch);
   7:     spriteBatch.End();
   8:  
   9:     base.Draw(gameTime);
  10: }

And that gets implemented in the Ball class; here are the meat and potatoes:

   1: public void Render(GameTime time, SpriteBatch spriteBatch)
   2: {
   3:     spriteBatch.Draw(m_texture, m_position, Color.White);
   4: }
   5:  
   6: public void Update(GameTime time, Viewport bounds)
   7: {
   8:     m_position += m_speed * (float)time.ElapsedGameTime.TotalSeconds;
   9:  
  10:     int maxX = bounds.Width - m_texture.Width;
  11:     int maxY = bounds.Height - m_texture.Height;
  12:  
  13:     if (m_position.X > maxX)
  14:     {
  15:         m_speed.X *= -1.0f;
  16:         m_position.X = maxX;
  17:     }
  18:     else if (m_position.X < 0)
  19:     {
  20:         m_speed.X *= -1.0f;
  21:         m_position.X = 0;
  22:     }
  23:     else
  24:     {
  25:         // else we're in a freefall to the right!
  26:         m_gravity.ApplyGravity(ref m_speed);
  27:     }
  28:  
  29:     if (m_position.Y > maxY)
  30:     {
  31:         m_speed.Y *= -1.0f;
  32:         m_position.Y = maxY;
  33:     }
  34:     else if (m_position.Y < 0)
  35:     {
  36:         m_speed.Y *= -1.0f;
  37:         m_position.Y = 0;
  38:     }
  39:     else
  40:     {
  41:         // else we're in a freefall
  42:         // v(t) = at + k
  43:         // in this case a = -9.8
  44:         // Inverted because we're going down which is positive Y ;-)
  45:         m_gravity.ApplyGravity(ref m_speed);
  46:     }
  47: }

One kind of odd part is that it’s designed to be simply a perfectly elastic ball (m_speed.Y *= -1.0f) but it actually bounces higher over time.

Source code is downloadable here.  Thanks to Betsy Aoki’s samples online – they were incredibly helpful!

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