CSE120 Sp17 Assignment - Events

Goals

• Practice controlling action on the screen and following the logic of the computation.
• Learn about events, which interrupt Processing when they occur (specifically keyPressed() and mousePressed()).

Events

Step 1: Write the Starting Program

Write a Processing program that has an 800x100 drawing canvas and moves a red ball from left to right across the screen.

• The ball is 80 pixels in diameter and is centered vertically (i.e. same amount of space above and below the ball).
• The ball's horizontal position is an integer variable named x_position.
• The ball's initial position is 100 pixels from the left.
• The ball moves one pixel each time the screen is redrawn.

Step 2: Update Your Program with Variables

Declare four more integer variables at the top of your program:

Variable Name	Initial Value	Description
move	1	The number of pixels the ball will move on each repetition of draw() (i.e. ball speed)
rVal	255	The red ('R') value for the ball's fill
gVal	0	The green ('G') value for the ball's fill
bVal	0	The blue ('B') value for the ball's fill

Now make the changes that are detailed below. Make sure to run your code after each change to ensure that it still works correctly!

• Change your program so the amount the ball moves each time draw() repeats is move instead of the value 1.
• Change the RGB values in your fill() command, replacing them with the three variables rVal, gVal, and bVal.
• Add a keyPressed() function at the bottom of your code to reverse the direction of the ball as shown below:

  void keyPressed() {
     move = -1 * move;
  }
  

• Add a mousePressed() function to the bottom of your code to change the ball's color everytime the mouse is pressed as shown below:

  void mousePressed() {
      int temp;
      temp = bVal;  // save bVal (old) in a temporary variable
      bVal = gVal;  // make new bVal the old gVal
      gVal = rVal;  // make new gVal the old rVal
      rVal = temp;  // make new rVal the old bVal
  }
  

Step 3: Understanding

Open a Google Sheet and set it up to have the form shown below. Fill in the numbers for the first 10 clicks as they exist after the last assignment statement (rVal = temp;). Then explain in your own words (a short paragraph, in your spreadsheet) what is happening with these assignment statements in the mousePressed() function.