I'm trying to do event-driven programming, typically playing a musical sound, then doing some graphics. I always have probs with delay.
For example:
int i=10;
void setup(){
size(300,300);
}
void draw(){
 background(255);
 fill(0);
rect(50+i,50,40,20);
 delay(400);
 
  rect(50+i,250,40,20);
 i=i+10;
 delay(400);
// background(255);  // if you  call it here, its a blank screen
}

won't run as it should; and does odd things at times.
Cheers.