processing4/java/examples/Basics/Structure/Redraw/Redraw.pde

/**
 * Redraw. 
 * 
 * The redraw() function makes draw() execute once.  
 * In this example, draw() is executed once every time 
 * the mouse is clicked. 
 */
 
// The statements in the setup() function 
// execute once when the program begins
void setup() 
{
  size(200, 200);  // Size should be the first statement
  stroke(255);     // Set line drawing color to white
  noLoop();
}

float y = 100;

// The statements in draw() are executed until the 
// program is stopped. Each statement is executed in 
// sequence and after the last line is read, the first 
// line is executed again.
void draw() 
{ 
  background(0);   // Set the background to black
  y = y - 1; 
  if (y < 0) { y = height; } 
  line(0, y, width, y);  
} 

void mousePressed() 
{
  redraw();
}