[Feature] Make Back button on EV3 brick usable in user program

[XilefTech]

Describe the solution you'd like
I think it would be useful to be able to use the back-button on the ev3 brick in your programs too. I thought of a method that would stop it from exiting the program and give it it's own entry in brick.buttons.pressed()

Describe alternatives you've considered
Just an idea for a possible feature.

Additional context
I would love to use a discrete back-button for some menus and stuff, I'm trying to do on the display

[dlech]

This is currently not possible for technical reasons. The way ev3dev works would need to be changed. Some related discussion: ev3dev/ev3dev#794.

[XilefTech]

Okay, so you wrote there:

It would be simple enough to add a ev3.interrupt_on_back_button() function to ev3dev-lang-python that users could add to the beginning of their program that handles this. It could send SIGINT just like a keyboard interrupt. Then the basic program template would look like...
together with a code snippet.
If my interpretation is correct, you could abuse that to just call a funktion and go back to your main program like

from ev3dev import ev3

if __name__ == "__main__":
    try:
        ev3.interrupt_on_back_button()
        # init other stuff
        
        while True:
            # do stuff
    except:
        # thing to do that sould happen, when the button was pressed
        # just make something to go back into your program

would that be correct? or is it just something that I overread that would prevent this?
to be clear: I don't plan on implementing it that way in near future but it would be a possible workaround for me if I finished the other stuff on my program xD

Also I'd love to see these changes done in ev3dev so that this would be possible.

[devinbreise]

So my FLL kids have this large program that does all of their "runs" and controls the whole thing via a master control program so they don't need to "exit" to Brick Man during their competition.

Occasionally, things go wrong and they want to "stop a run", without exiting their "master control" program (because relaunching takes a long time while the clock is ticking). I'm looking for a solution I can point them at that doesn't involve littering their code with checks against some global state. If the "back" button on the EV3 were to raise a catchable exception, that would be a very elegant solution for them. Their "master control" program could simply try each of the "runs" and catch the exception if it were thrown. But alas, apparently that is not yet an option.

Any other suggestions?

[dlech]

There is an experimental run_parallel() method in v2. https://github.com/pybricks/pybricks-micropython/blob/v2/bricks/ev3dev/modules/pybricks/experimental.py

So maybe you could do something like this (untested)?

from pybricks.experimental import run_parallel

# if this is true, it means we are running a mission
running = False

...

def cancel_button_monitor():
    while not any(ev3.buttons.pressed()):
        wait(100)
        if not running:
            return
    throw RuntimeError("canceled")

def run_1():
    # mission run 1
    try:
        ...
    finally:
        global running
        running = False

def run_2():
    # mission run 2
    try:
        ...
    finally:
        global running
        running = False

def master_control():
    while True:
        # get input
        ...
        try:
            global running
            if selected == 1:
                running = True
                run_parallel(run_1, cancel_button_monitor)
            elif selected == 2:
                running = True
                run_parallel(run_2, cancel_button_monitor)
        catch Exception as ex:
            print("error during run:")
            print(ex)
        finally:
            # stop all motors here
            ...

If a button is pressed during a run, the cancel_button_monitor() function will end the while loop and raise an exception. When this happens, run_paralle()l will raise a SystemExit exception in the run_x() function to stop it.

[devinbreise]

Pybricks never disappoints! We'll give it a try and report back. Only concern is that they have some pretty tight loops in a few places (scanning for lines at high speed, line follows, etc.). Presumably running something like cancel_button_monitor() every 100ms wouldn't impact any of that...

[dlech]

I forgot, there is a file /etc/brickrun.conf in ev3dev. If you add a line delay=1, this will delay the back button from killing the program by one second. So with this change, the back button should be usable as Button.LEFT_UP.