Developing Programming Assignments on the XBOX 360 Console

Section I: Introduction and Overview
d. Interactive control of the triangle

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• Pre-requisite: this is the example (Section 1c. Drawing with XNA) we are building off from.
• You can start with this template source file (resulting source code from the above example), and here is the source code we will generate at the end of this tutorial.

Goals:

• To experience with and examine simple user interaction with XNA.

Notice:

• If we change a variable during the Update() function and the result in the Draw() function then we can create very smooth control of objects.
• In the Update() function, we poll user's actions (either controller or keyboard) and changes instance variables. We do not draw in the Update() function.
• In the Draw() function, we draw based on instance variables. We do not change any instance variables in the Draw() function.

1. Instance variables: for defining the position of the triangle: Vector2.
2. Initialize():  allocate and initialize the position of the triangle.
3. Update(): change the reference position based on user input.
4. Draw():  draws the triangle based on the reference position.

Here are the details, each of the following changes are enclosed by regions with label: __CODE_ADDED__:

1. Declare new instance variables (in the beginning of the file) for reference position of the triangle:

   #region __CODE_ADDED__    ... // EffectPool and BasicEffect exactly the same as previously    Vector2 pos; // Center position of the triangle #endregion



2. Initialize the triangle position  (in Initialize() function):

   #region __CODE_ADDED__ // Initialize the graphics rendering state   ... // code exactly the same as previously                    // Triangle position   pos = new Vector2();   pos.X = 0.0f;       // Initial position is at the center of the application window   pos.Y = 0.0f; #endregion



3. Updates the position based on user input (in Update() function):

   #region __CODE_ADDED__    if (GamePad.GetState(PlayerIndex.One).IsConnected)  { // either from controller ...        Vector2 delta = GamePad.GetState(PlayerIndex.One).ThumbSticks.Left;        pos = delta;    } else {    // or from the keyboard        float dx = 0.0f, dy = 0.0f;        if (Keyboard.GetState().IsKeyDown(Keys.Right)) dx += 0.01f;        if (Keyboard.GetState().IsKeyDown(Keys.Left))  dx -= 0.01f;        if (Keyboard.GetState().IsKeyDown(Keys.Up))    dy += 0.01f;        if (Keyboard.GetState().IsKeyDown(Keys.Down))  dy -= 0.01f;        pos.X += dx;        pos.Y += dy;    } #endregion

    
4. Draw a triangle (in Draw() function):

   #region __CODE_ADDED__    // exactly the same as previously, the only difference is the position ...    ...    // Triangle vertex position based on center at current position    Vector3 p = new Vector3(pos.X, pos.Y, 0);    ...    // rest of the code is identical to as previously #endregion