Visual Basic .NET BASICS

1
Visual Basic .NET BASICS

• Lesson 16
• Case Study Snake Game

2
Objectives

• Run the Snake Game.
• Draw with individual pixels and use the Paint
  event.
• Describe how the Snake Game draws and controls
  the direction of the snake.
• Describe how the code of the Snake Game functions.

3
Running the Snake Game

• The Snake Game that you created in Lesson 1 is an
  example of how a simple game can be created in
  Visual Basic .NET with very little code.
• Writing a program like the Snake Game is based on
  a very simple principle.

4
Drawing with Pixels and Using the Paint Event
Handler

• There are only four Visual Basic .NET features
  that the Snake Game uses that you have not
  learned
• The feature that allows you to create an image
  and draw it on your form
• The method that allows you to draw on an image
  one pixel at a time
• A feature that allows you to retrieve the color
  of a specific pixel
• A paint event that prevents the graphics you draw
  from being erased if another window covers the
  graphic you have already drawn

5
Creating a Graphic Image

• A Bitmap object is an object used to work with
  images defined by pixel data.
• A Bitmap object and a Graphics object are created
  using Dim statements.
• The DrawImage method can be called directly after
  any change has been made to the image.
• The Paint method is called whenever the form
  needs to be drawn.

6
Using the SetPixel Method

• The SetPixel method of the Bitmap object is an
  easy-to-use feature that allows you to change the
  color of individual pixels on a bitmap.
• To use SetPixel, you identify the X,Y coordinates
  and specify a color for the pixel.

7
Using the GetPixel Method

• The GetPixel method returns the Color structure
  of a pixel at the specified pixel.
• Use SetPixel to change the color of a pixel and
  use GetPixel to determine the current color of a
  pixel.
• The ToArgb method is used to convert this
  structure into a more easily compared integer
  value.

8
Handling the Paint Event

• The Paint event is called whenever an object is
  resized, covered, uncovered, minimized, and then
  restored or when the Refresh event is called.
• This event can make sure that the board is
  redrawn or repainted to prevent from losing data
  from the form.

9
How the Snake Game Draws the Snake

• The snake that appears on the Snake Game is drawn
  one pixel at a time using SetPixel.
• There are four variables that are key in
  controlling the snake.

10
Analyzing the Snake Game Code

• The primary work of the game is done in the Start
  Game buttons Click event procedure.
• The Application.DoEvents function allows other
  events to be processed as the Start Game buttons
  Click event procedure is being executed.
• The Do Events function also allows the user to
  click the direction buttons.

11
Form-Level Variables

• There are three form-level variables necessary.
• First, we need a way to stop the program if the
  user clicks the Stop Game button.
• The intXFactor and intYFactor variables must be
  form-level because the factors will be changed by
  the direction buttons and used by the Start Game
  button.

12
The Start Game Button

• The Start Game Click event procedure is the
  primary code for the game.
• At the heart of the event procedure is a loop.
• First six local variables are declared to be used
  in this procedure.
• Now that all the initial work is done, the
  DrawBoard method is called to draw a box with a
  border around it.

13
The Start Game Buttons Loop

• The loop adds one pixel to the snake with each
  interaction.
• The intXFactor and intYFactor determine where the
  next pixel is added.
• The GetPixel method looks at the color of the
  next pixel that the program would write to and
  makes sure that it is not already clrSnakeColor.
• Clicking the End Game button would change the
  value of blnStop, which would end the game.

14
Changing the Coordinate Factors

• The intXFactor and intYFactor variables are
  adjusted through the Click event procedures of
  the four direction buttons.
• Because intXFactor and intYFactor are form-level
  variables, the new values for these factors will
  remain set until one of the other direction
  buttons changes the values.

15
The Stop Game Button

• The code for the Stop Game button simply changes
  the blnStop to True.
• If the Stop Game button is pressed while the game
  is in play and blnStop is changed to True, the
  loop will stop.

16
The Quit button

• The Quit button differs from the Stop Game
  button.
• The Quit button ends the program.

17
Summary

• The Snake Game is an example of a simple game
  created in Visual Basic. NET.
• The SetPixel method allows you to change the
  color of individual pixels on a bitmap.
• The GetPixel method retrieves the color of the
  specified pixel on a bitmap.
• The Refresh method causes the Paint event to
  execute.

18
Summary (continued)

• The Paint event handler can be used to redraw
  objects that are erased when a form is covered or
  minimized.
• The Snake Game keeps track of a point on the form
  where the next pixel will appear as the snake
  grows.
• The snake is controlled by two factors. One
  factor specifies the change in the X coordi-nate
  and the other factor specifies the change in the
  Y coordinate.

19
Summary (continued)

• Once in play, the Snake Game continues until the
  user runs the snake into a wall or the snakes
  tail. The Stop Game and Quit buttons also end the
  game.
• The direction buttons change the factors that
  control the direction of the snake. The variables
  that store the factors are form-level so that
  multiple buttons can change the value of the
  factors.

20
Summary (continued)

• The color of the wall and snake are con-trolled
  by changing the value of clrSnakeColor.
• The speed of the snake is controlled by adjusting
  the value in the Sleep method call.