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Windows Phone 7 : Building 2D Games with the XNA Framework - AlienShooter Game Play (part 2) - Game Object Class

5/22/2013 4:14:13 AM
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2. Game Object Class

We create a class named GameObject to be our base class for game assets, which includes the hero ship, the missiles, and the alien ships. The GameObject class handles the animation, Update, and Drawing for each object. We copied over the assets and sprite sheet logic from the SpriteAnimation sample project. Here is the GameObject constructor:

public GameObject(SpriteSheet loadedTexture, string spriteName, Rectangle screenRect)
{
  SpriteAnimationSpriteSheet = loadedTexture;
  SpriteCenter = new Vector2(
    SpriteAnimationSpriteSheet.SourceRectangle(spriteName + 0).Width / 2,
    SpriteAnimationSpriteSheet.SourceRectangle(spriteName + 0).Height / 2);
  //Used to access sprite in SpriteSheet
  //Assume starts at 0 so SpriteName+0 is first Sprite frame for animation
  //NumberOfAnimationFrames is how many sprite frames that are available
  SpriteName = spriteName;
  _screenRect = screenRect;

					  

//Default initialization
  FrameTime = TimeSpan.FromMilliseconds(100d);
  NumberOfAnimationFrames = 10;
  Position = Vector2.Zero;
  ElapsedFrameTime = TimeSpan.FromMilliseconds(0d);
  Velocity = Vector2.Zero;
  Rotation = 0f;
  Alive = false;
}

The constructor for GameObject takes the following parameters:

  • SpriteSheet loadedTexture

  • string spriteName

  • Rectangle screenRect

Just as with the SpriteAnimation sample, all of the individual object frames are combined into a single texture shared by all of the objects, which is more efficient than loading individual textures and switching textures when rendering. The loadedTexture parameter represents the single texture and is passed in to the constructor. The spriteName parameter is used by the animation code so that the correct object frames can be found in the loadedTexture. This code assumes a naming convention starting at spriteName+0 through spriteName+NumberOfAnimationFrames, which is hard-coded to 10 frames for all objects. The screenRect parameter is used to check when objects collide with screen bounds.

2.1. Properties

The GameObject class has quite a few public properties declared that are used to animate the sprite and to hold information on Sprite such as the center point, position, velocity and rotation. Here are the declarations:

public SpriteSheet SpriteAnimationSpriteSheet { get; set; }
public string SpriteName { get; private set; }
public int NumberOfAnimationFrames { get; set; }
public TimeSpan FrameTime { get; set; }
public TimeSpan ElapsedFrameTime { get; set; }
public Vector2 SpriteCenter { get; set; }
public bool Alive { get; set; }
public Vector2 Position { get; set; }
public Vector2 Velocity { get; set; }
public float Rotation { get; set; }

There is an additional property related to collision detection that we will cover next.

2.2. Collision Detection

The BoundingRect property of type Rectangle is used to return the Rectangle area that contains the sprite on screen. This property is used for collision detection. If you have ever played a video game where the objects seemed to touch but nothing happen, it is a result of imperfect collision detection, as shown in Figure 4 where a regular shaped object, like a rectangle, is used to define the area of an irregularly shaped object.

Figure 4. Bounding box collision detection

Probably the best answer lies somewhere in-between those extremes, such as using more than one bounding box or only performing point-by-point comparison on sides that could collide, and so on. For our purposes, we check for intersection using the BoundingRect property defined in GameObject:

public virtual Rectangle BoundingRect
{
  get
  {
    return new Rectangle((int)Position.X, (int)Position.Y,
      SpriteAnimationSpriteSheet.SourceRectangle(SpriteName + 0).Width,
      SpriteAnimationSpriteSheet.SourceRectangle(SpriteName + 0).Height);
  }
}

One trick to that you can use to adjust collision detection is to call the Inflate(horizontalAmount,verticalAmount) method to individually increase or decrease (with a negative value) the sides of the Rectangle to better match the object shape.

2.3. GameObject Methods

The rest of the GameObject class contain its individual methods to reset its position if the object flies off the screen, update the object state, and draw the object to screen. Methods are marked with the virtual keyword so that they can be overridden in inherited classes as needed:

public virtual void ResetGameObject()
{
  Position = Vector2.Zero;
  Velocity = Vector2.Zero;
  Alive = false;

}

public virtual void Update(GameTime GameTime)
{
  if (Alive)
  {
    Position += Velocity;
    //Check screen bounds
    if ((Position.X < 0) ||
        (Position.X > _screenRect.Width) ||
        (Position.Y < 0) ||
        (Position.Y > _screenRect.Height))
      ResetGameObject();
    //Update animation
    UpdateAnimation(GameTime);
  }
}

private void UpdateAnimation(GameTime gameTime)
{
  ElapsedFrameTime += gameTime.ElapsedGameTime;
  if (ElapsedFrameTime > FrameTime)
  {
    if (_spriteIndex < NumberOfAnimationFrames - 1)
      _spriteIndex++;
    else _spriteIndex = 0;
    ElapsedFrameTime = TimeSpan.FromMilliseconds(0d);
  }
}

public virtual void Draw(GameTime gameTime, SpriteBatch spriteBatch)
{
  if (Alive)
  {
    spriteBatch.Draw(SpriteAnimationSpriteSheet.Texture, Position - SpriteCenter,
      SpriteAnimationSpriteSheet.SourceRectangle(SpriteName + _spriteIndex.ToString()),
      Color.White);
  }
}

					  

The ResetGameObject is called when an object is "destroyed" or flies off the screen. The method is pretty simple in just setting Position and Velocity to a zero vector and Alive to false.

The Update method checks to see if the object is Alive before updating Position by adding Velocity to it and then checking screen bounds. The Update method also calls the UpdateAnimation method, which leverages the code we developed in the SpriteAnimation sample. Finally, the Draw method simply applies the same logic we used in the SpriteAnimation sample to draw the correct frame to screen as part of the animation sequence.

Now that we have our base class out of the way, we move on to cover the enemy alien class and then move on to the user controlled hero ship and missile classes.

 
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