How to pause/play a sketch in processing with the same button? - processing

Question

I'm trying to implement a pause/play for a sketch with the same key, for example if I press p the sketch should stop and if I press p again, the sketch should start again. So far I used the noLoop()/loop() to do this but with two different keys (p for pause, r for start). It does work if I use keyPressed() and keyReleased() but this means to hold down the key but this doesn't answer my question. Also in the pause mode I used redraw() for a single step while noLoop() and works good.
Here is some code I tried so far with two different keys:
public void draw(){
background(random(255));
}
public void keyPressed(){
if ( key == 'p' )
noLoop();
if ( key == 'r' )
loop();
if ( key == 's' )
redraw();
}
And this is the code with the same key:
public void draw(){
background(random(255));
}
public void keyPressed(){
if ( key == 'p' )
noLoop();
if ( key == 'p' )
loop();
if ( key == 's' )
redraw();
}
In this case when I press key it doesn't have any effect.
And the last one I tried is this:
public void draw(){
background(random(255));
}
public void keyPressed(){
if ( key == 'p' )
noLoop();
else
loop();
if ( key == 's' )
redraw();
}
In this case when I press 'p' it stops the sketch but is doesn't play again. Because of the 'else' it plays again when I press any key including 's' which suppose to be just for a single step.
Any help is more than welcome.
Thanks!

Answer 1

Please try to post a MCVE that we can copy and paste to run ourselves. The code you've posted is almost a MCVE, but please include a draw() function so we can see exactly the same thing as you.
Here's a simple MCVE that demonstrates the problem you're having:
void draw() {
background(random(255));
}
public void keyPressed() {
if ( key == 'p' ) {
noLoop();
}
if ( key == 'p' ) {
loop();
}
}
This code will show a randomly flashing background. If you press the P key, you're expecting it to pause, but it just keeps flashing.
You need to get into the habit of stepping through your code to understand exactly what it's doing. Read through the keyPressed() function line by line, and imagine what the code will do when you press the P key.
The first if statement checks whether the key is P, and since it is, it calls the noLoop() function. Then the second if statement checks whether they key is P, and since it is, it calls the loop() function. This reverses the call to noLoop() we just made, which makes it appear as though nothing happens.
You need to track the state of your sketch. In your case, you need to track whether the sketch is currently paused. You could use a single boolean variable for this. Then use that variable to make sure you do the right thing when the P key is pressed. Something like this:
boolean paused = false;
void draw() {
background(random(255));
}
public void keyPressed() {
if ( key == 'p' ) {
paused = !paused;
if (paused) {
noLoop();
} else {
loop();
}
}
}