How to deal with KeyPressFcn recording multiple key presses within a loop?

Hello there,

I'm trying to make a simple game in MATLAB, and I am using KeyPressFcn and KeyReleaseFcn callbacks to get player input from within a loop. It looks something like this:

boxfig=figure;
drawnow
akey='a';
set(boxfig,'KeyPressFcn',{@Key_Down,akey},'KeyReleaseFcn',{@Key_Up,akey});
lrshift=0;
kk=1;
while kk<2
        drawnow   
        fprintf([num2str(lrshift) '\n']);
end %kk

function Key_Down(~,event,leftkey)
if strcmp(event.Key,leftkey),  assignin('base','lrshift',1);  end    
end

function Key_Up(~,event,leftkey)
if strcmp(event.Key,leftkey),  assignin('base','lrshift',0);  end    
end

The code sets lrshift to 1 when the a-key is pressed, and to 0 when it's not pressed, and then prints the value of lrshift. However, the loop obviously runs faster than one can press and release a key, so it always records multiple subsequent key-presses. I would like the program to set lrshift to 1 once only when the key is first pressed, and then immediately back to 0 until the key has been released and is pressed again. At the moment, the output looks something like this:

0
0
0
0
1 %<- a-key pressed
1
1
1
1 
0 %<- a-key released
0
0
1 %<- a-key pressed
1
1
1
1 
0 %<- a-key released
0
0

I would like it to look like:

0
0
0
0
1 %<- a-key pressed
0
0
0
0
0 %<- a-key released
0
0
0
1 %<- a-key pressed
0
0
0
0
0 %<- a-key released
0
0

I have tried to disable the callback to KeyPressFcn after the key is first pressed within the loop, but the result is still the same:

while kk<2       
        if lrshift==1                        
            set(boxfig,'KeyPressFcn',[],'KeyReleaseFcn',{@Key_Up,akey});  
        else
            set(boxfig,'KeyPressFcn',{@Key_Down,akey},'KeyReleaseFcn',{@Key_Up,akey});
        end      
        drawnow   
        fprintf([num2str(lrshift) '\n']);
end %kk

Pausing for a short time is not an option since the game is still doing other things in the background.

I would be grateful if anyone could help me with this! Is KeyPressFcn even the right way to go about it?

Thank you :)

Answer 1

[Anmol Dhiman]

Hi Lp,

Update your Key_Down function as below for a workaround

function Key_Down(~,event,leftkey)
    if strcmp(event.Key,leftkey)
        lrshift = 1;
        fprintf([num2str(lrshift) '\n']);
        assignin('base','lrshift',0); 
    end    
end

Regards,

Anmol Dhiman

[lp18692]

Thank you Anmol!

Your workaround does work in the example I provided, but in the real code I needed the main loop to do other things besides printing the value, and letting it all happen within the function seemed messy. I have since gotten it to work this way:

boxfig=figure;
drawnow
akey='a';
set(boxfig,'KeyPressFcn',{@Key_Down,akey},'KeyReleaseFcn',{@Key_Up,akey});
lrshift=0;
kk=1;
down1=0; up1=1;
while kk<2
    drawnow
    if downl==1 && upl==1
        lrshift=1;
        downl=0;
    end    
    fprintf([num2str(lrshift) '\n']);
    lrshift=0;
end %kk

function Key_Down(~,event,leftkey)
if strcmp(event.Key,leftkey),  assignin('base','downl',1);  end
end

function Key_Up(~,event,leftkey)
if strcmp(event.Key,leftkey),  assignin('base','upl',1);  end
end

This way lrshift is immediately set back to 0 and the code waits for the button to be released before it can be set to 1 again.

Thanks and kind regards!