Creating the GamePiece Class

.NET Framework (current version)

The GamePiece class encapsulates all the functionality required to load a Microsoft XNA game piece image, track the state of the mouse in relation to the game piece, capture the mouse, provide manipulation and inertia processing, and provide bouncing capability when the game piece reaches the limits of the view port.

At the top of the GamePiece class, several private members are declared.

#region PrivateMembers
// The sprite batch used for drawing the game piece.
private SpriteBatch spriteBatch;
// The position of the game piece.
private Vector2 position;
// The origin used for rendering the game piece.
// Gets set to be the center of the piece.
private Vector2 origin;
// The texture for the piece.
private Texture2D texture;
// The bounds of the game piece. Used for hit testing.
private Rectangle bounds;
// The rotation of the game piece, in radians.
private float rotation;
// The scale, in percentage of the actual image size. 1.0 = 100%.
private float scale;
// The view port, used to detect when to bounce.
private Viewport viewport;
// The manipulation processor for this game piece.
private ManipulationProcessor2D manipulationProcessor;
// The inertia processor for this game piece.
private InertiaProcessor2D inertiaProcessor;
// Flag to indicate that inertia processing should start or continue.
private bool processInertia;
// Flag to indicate whether this piece has captured the mouse.
private bool isMouseCaptured;
// Used during manipulation to indicate where the drag is occurring.
private System.Windows.Point dragPoint;
// The color of the game piece.
private Color pieceColor;
// Represents how spongy the walls act when the piece bounces.
// Must be <= 1.0 (if greater than 1.0, the piece will accelerate on bounce)
// 1.0 = no slowdown during a bounce.
// 0.0 (or less) = won't bounce.
private float spongeFactor = 0.925f;

Three of these private members are exposed through public properties. The Scale and PieceColor properties enable the application to specify the scale and the color of the piece, respectively. The Bounds property is exposed to enable one piece to use the bounds of another to render itself, such as when one piece should overlay another. The following code shows the declaration of the public properties.

#region PublicProperties
public float Scale
    get { return scale; }
        scale = value;
        bounds.Width = (int)(texture.Width * value);
        bounds.Height = (int)(texture.Height * value);
        // Setting X and Y (private properties) causes 
        // bounds.X and bounds.Y to adjust to the scale factor.
        X = X;
        Y = Y;

public Color PieceColor
    get { return pieceColor; }
    set { pieceColor = value; }

public Rectangle Bounds
    get { return bounds; }

The constructor for the GamePiece class accepts the following parameters:

  • A SpriteBatch type. The reference passed here is assigned to the private member spriteBatch, and is used to access the SpriteBatch.Draw method when the game piece renders itself. In addition, the GraphicsDevice property is used to create the Texture object associated with the game piece, and to obtain the size of the view port in order to detect when the game piece encounters a window boundary so that the piece can bounce.

  • A string that specifies the file name of the image to use for the game piece.

The constructor also creates a ManipulationProcessor2D object and an InertiaProcessor2D object, and establishes event handlers for their events.

The following code shows the constructor for the GamePiece class.

#region Constructor
public GamePiece(SpriteBatch spriteBatch, string fileName)
    // For brevity, omitting checking of null parameters.
    this.spriteBatch = spriteBatch;

    // Get the texture from the specified file.
    texture = Texture2D.FromFile(spriteBatch.GraphicsDevice, fileName);

    // Initial position set to 0,0.
    position = new Vector2(0);

    // Set the origin to be the center of the texture.
    origin = new Vector2(texture.Width / 2.0f, texture.Height / 2.0f);

    // Set bounds. bounds.X and bounds.Y are set as the position or scale changes.
    bounds = new Rectangle(0, 0, texture.Width, texture.Height);

    // Create manipulation processor.
    Manipulations2D enabledManipulations =
        Manipulations2D.Translate | Manipulations2D.Rotate;
    manipulationProcessor = new ManipulationProcessor2D(enabledManipulations);

    manipulationProcessor.Pivot = new ManipulationPivot2D();
    manipulationProcessor.Pivot.Radius = texture.Width / 2;

    manipulationProcessor.MinimumScaleRotateRadius = 10.0f;

    manipulationProcessor.Started += OnManipulationStarted;
    manipulationProcessor.Delta += OnManipulationDelta;
    manipulationProcessor.Completed += OnManipulationCompleted;

    // Create inertia processor.
    inertiaProcessor = new InertiaProcessor2D();
    inertiaProcessor.Delta += OnInertiaDelta;
    inertiaProcessor.Completed += OnInertiaCompleted;

    inertiaProcessor.TranslationBehavior.DesiredDeceleration = 0.0001F;
    inertiaProcessor.RotationBehavior.DesiredDeceleration = 1e-6F;
    inertiaProcessor.ExpansionBehavior.DesiredDeceleration = 0.0001F;

    // Save the view port. Used to detect when the piece needs to bounce.
    viewport = spriteBatch.GraphicsDevice.Viewport;

    // Set the piece in a random location.
    Random random = new Random((int)Timestamp);
    X = random.Next(viewport.Width);
    Y = random.Next(viewport.Height);

    // Set a random orientation.
    rotation = (float)(random.NextDouble() * Math.PI * 2.0);

    dragPoint = new System.Windows.Point(double.NaN, double.NaN);
    pieceColor = Color.White;

    // Set scale to normal (100%)
    Scale = 1.0f;

The UpdateFromMouse method is responsible for detecting when a mouse button is pressed while the mouse is within the boundaries of the game piece, and for detecting when the mouse button has been released.

When the left mouse button is pressed (while the mouse is inside the piece boundaries), this method sets a flag to indicate that this game piece has captured the mouse, and begins manipulation processing.

Manipulation processing is started by creating an array of Manipulator2D objects and passing them to the ManipulationProcessor2D object. This causes the manipulation processor to evaluate the manipulators (in this case a single manipulator), and raise manipulation events.

In addition, the point at which the drag is occurring is saved. This is used later during the Delta event to adjust the delta translation values so that the game piece swings into line behind the drag point.

Finally, this method returns the state of the mouse capture. This enables the GamePieceCollection object to manage capturing when there are multiple game pieces.

The following code shows the UpdateFromMouse method.

#region UpdateFromMouse
public bool UpdateFromMouse(MouseState mouseState)
    if (mouseState.LeftButton == ButtonState.Released)
        if (isMouseCaptured)
        isMouseCaptured = false;

    if (isMouseCaptured ||
       (mouseState.LeftButton == ButtonState.Pressed &&
       bounds.Contains(mouseState.X, mouseState.Y)))
        isMouseCaptured = true;

        Manipulator2D[] manipulators = new Manipulator2D[] 
            new Manipulator2D(0, mouseState.X, mouseState.Y)

        dragPoint.X = mouseState.X;
        dragPoint.Y = mouseState.Y;
        manipulationProcessor.ProcessManipulators(Timestamp, manipulators);

    // If the right button is pressed, stop the piece and move it to the center.
    if (mouseState.RightButton == ButtonState.Pressed)
        processInertia = false;
        X = viewport.Width / 2;
        Y = viewport.Height / 2;
        rotation = 0;
    return isMouseCaptured;

When manipulation begins, the Started event is raised. The handler for this event stops inertia processing if it is occurring, and sets the processInertia flag to false.

#region OnManipulationStarted
private void OnManipulationStarted(object sender, Manipulation2DStartedEventArgs e)
    if (inertiaProcessor.IsRunning)
    processInertia = false;

As the values associated with the manipulation change, the Delta event is raised. The handler for this event uses the delta values passed in the event arguments to make changes to the position and rotation values of the game piece.

#region OnManipulationDelta
private void OnManipulationDelta(object sender, Manipulation2DDeltaEventArgs e)
    //// Adjust the position and rotation of the game piece.
    float deltaX = e.Delta.TranslationX;
    float deltaY = e.Delta.TranslationY;
    if (dragPoint.X != double.NaN || dragPoint.Y != double.NaN)
        // Single-manipulator-drag-rotate mode. Adjust for drag / rotation
        System.Windows.Point center = new System.Windows.Point(position.X, position.Y);
        System.Windows.Vector toCenter = center - dragPoint;
        double sin = Math.Sin(e.Delta.Rotation);
        double cos = Math.Cos(e.Delta.Rotation);
        System.Windows.Vector rotatedToCenter =
            new System.Windows.Vector(
                toCenter.X * cos - toCenter.Y * sin,
                toCenter.X * sin + toCenter.Y * cos);
        System.Windows.Vector shift = rotatedToCenter - toCenter;
        deltaX += (float)shift.X;
        deltaY += (float)shift.Y;

    X += deltaX;
    Y += deltaY;
    rotation += e.Delta.Rotation;

When all of the manipulators (in this case, a single manipulator) that are associated with a manipulation are removed, the manipulation processor raises the Completed event. The handler for this event begins inertia processing by setting the initial velocities of the inertia processor to those reported by the event arguments, and sets the processInertia flag to true.

#region OnManipulationCompleted
private void OnManipulationCompleted(object sender, Manipulation2DCompletedEventArgs e)
    inertiaProcessor.TranslationBehavior.InitialVelocityX = e.Velocities.LinearVelocityX;
    inertiaProcessor.TranslationBehavior.InitialVelocityY = e.Velocities.LinearVelocityY;
    inertiaProcessor.RotationBehavior.InitialVelocity = e.Velocities.AngularVelocity;
    processInertia = true;

As inertia processing extrapolates new values for angular and linear velocities, position (translation) coordinates, and rotation, the Delta event is raised. The handler for this event uses the delta values passed in the event arguments to modify the position and rotation of the game piece.

If the new coordinates result in the game piece moving beyond the view port boundaries, the velocity of the inertia processing is reversed. This causes the game piece to bounce off the view port boundary that it has encountered.

You cannot change the properties of an InertiaProcessor2D object while it is running extrapolation. Therefore, when reversing the X or Y velocity, the event handler first stops inertia by calling the M:System.Windows.Input.Manipulations.InertiaProcessor2D.Complete method. It then assigns the new initial velocity values to be the current velocity values (adjusted for sponge behavior), and sets the processInertia flag to true.

The following code shows the event handler for the Delta event.

#region OnInertiaDelta
private void OnInertiaDelta(object sender, Manipulation2DDeltaEventArgs e)
    // Adjust the position of the game piece.
    X += e.Delta.TranslationX;
    Y += e.Delta.TranslationY;
    rotation += e.Delta.Rotation;

    // Check to see if the piece has hit the edge of the view port.
    bool reverseX = false;
    bool reverseY = false;

    if (X > viewport.Width)
        reverseX = true;
        X = viewport.Width;

    else if (X < viewport.X)
        reverseX = true;
        X = viewport.X;

    if (Y > viewport.Height)
        reverseY = true;
        Y = viewport.Height;

    else if (Y < viewport.Y)
        reverseY = true;
        Y = viewport.Y;

    if (reverseX || reverseY)
        // Get the current velocities, reversing as needed.
        // If reversing, apply sponge factor to slow the piece slightly.
        float velocityX = e.Velocities.LinearVelocityX * ((reverseX) ? -spongeFactor : 1.0f);
        float velocityY = e.Velocities.LinearVelocityY * ((reverseY) ? -spongeFactor : 1.0f);
        // Must stop inertia processing before changing parameters.
        if (inertiaProcessor.IsRunning)
        // Assign the new velocities.
        inertiaProcessor.TranslationBehavior.InitialVelocityX = velocityX;
        inertiaProcessor.TranslationBehavior.InitialVelocityY = velocityY;
        // Set flag so that inertia processing will continue.
        processInertia = true;

When inertia processing is complete, the inertia processor raises the Completed event. The handler for this event sets the processInertia flag to false.

#region OnInertiaCompleted
private void OnInertiaCompleted(object sender, Manipulation2DCompletedEventArgs e)
    processInertia = false;

None of the logic presented so far actually causes inertia extrapolation to occur. This is accomplished in the ProcessInertia method. This method, which is called repeatedly from the game update loop (the Game.Update method) checks to see if the processInertia flag is set to true, and if so, calls the M:System.Windows.Input.Manipulations.InertiaProcessor2D.Process method. Calling this method causes extrapolation to occur, and raises the Delta event.

#region ProcessInertia
public void ProcessInertia()
    if (processInertia)

The game piece is not actually rendered until one of the Draw method overloads is called. The first overload of this method is called repeatedly from the game draw loop (the Game.Draw method). This renders the game piece with the current position, rotation, and scale factors.

#region Draw
public void Draw()
        texture, position,
        null, pieceColor, rotation,
        origin, scale,
        SpriteEffects.None, 1.0f);

public void Draw(Rectangle bounds)
    spriteBatch.Draw(texture, bounds, pieceColor);

Three private properties are used by the GamePiece class.

  1. Timestamp – Gets a timestamp value to be used by the manipulation and inertia processors.

  2. X – Gets or sets the X coordinate of the game piece. When setting, adjusts the bounds used for hit testing and the pivot location of the manipulation processor.

  3. Y – Gets or sets the Y coordinate of the game piece. When setting, adjusts the bounds used for hit testing and the pivot location of the manipulation processor.

#region PrivateProperties
private long Timestamp
        // Get timestamp in 100-nanosecond units.
        double nanosecondsPerTick = 1000000000.0 / System.Diagnostics.Stopwatch.Frequency;
        return (long)(System.Diagnostics.Stopwatch.GetTimestamp() / nanosecondsPerTick / 100.0);

private float X
    get { return position.X; }
        position.X = value;
        manipulationProcessor.Pivot.X = value;
        bounds.X = (int)(position.X - (origin.X * scale));

private float Y
    get { return position.Y; }
        position.Y = value;
        manipulationProcessor.Pivot.Y = value;
        bounds.Y = (int)(position.Y - (origin.Y * scale));