Visual Basic .NET BASICS Lesson 16 Case Study — Snake Game

Objectives Run the Snake Game. yo 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.

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.

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

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.

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.

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.

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.

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.

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

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.

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.

The Start Game Button’s 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.

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.

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.

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

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.

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.

Summary (continued) Once in play, the Snake Game continues until the user runs the snake into a wall or the snake’s 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.

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.