Stop Matlab timer from within its callback (after outside input/event) - matlab

Question

I am running a long data preload within a timer callback, and I would like to be able to stop the callback halfway through with an outside input (for instance, the click of a GUI button).
The stop() function will stop the timer calls, but not the callback function itself.
Here is a simple example:
timerh = timer('TimerFcn' , #TimerCallback,'StartDelay' , 1, 'ExecutionMode' , 'singleShot');
NeedStopping = false;
start(timerh)
disp('Running')
pause(1)
disp('Trying to stop')
NeedStopping = true;
function TimerCallback(obj, event)
% Background data loading in here code in here; in this example,
% the callback simply displays numbers from 1 to 100
for k = 1 : 100
drawnow(); % Should allow Matlab to do other stuff
NeedStopping = evalin('base' , 'NeedStopping');
if NeedStopping
disp('Should stop now')
return
end
disp(k)
pause(0.1)
end
end
I expect this script to display numbers between 1 and (approximately) ten, but the timer callback does not stop until 100.
Strangely, the code reaches the line just before before the pause(1) and correctly prints 'Running', but then it stops there and waits for the timer to finish.
Even more perplexing, if I change the 1 second pause to 0.9 seconds, the timer stops immediately with this output:
Running
Trying to stop
Should stop now
I am aware that Matlab is mostly single-threaded, but I thought the drawnow() function should allow it to process other stuff.
Edit: Specific use behind my question:
I have a GUI with a "next" button that loads several images and shows them all side by side. The images are large so loading takes time; therefore, while the user looks at the pictures, I want to preload the next set. This can be done in the background with a timer, and it works.
However, if the user clicks "next" before the preloading has finished, I need to stop it, show the current images, and launch the preloading for the next step. Hence, the timer needs to stop during callback execution.

Answer 1

This is a demo of how to set up an interruptible callback. The way your example is set up I didn't see the need for an actual timer so I made it as a standard button callback.
note: If you are dead set on using it for a timer you can use exactly the same solution, just assign the startProcess callback to the timer instead of the gui button.
function h = interuptible_callback_demo
% generate basic gui with 2 buttons
h = create_gui ;
guidata( h.fig , h )
% create application data which will be used to interrupt the process
setappdata( h.fig , 'keepRunning' , true )
end
function startProcess(hobj,~)
h = guidata( hobj ) ;
% set the 'keepRunning' flag
setappdata( h.fig , 'keepRunning' , true )
% toggle the button states
h.btnStart.Enable = 'off' ;
h.btnStop.Enable = 'on' ;
nGrainOfSand = 1e6 ;
for k=1:nGrainOfSand
% first check if we have to keep running
keepRunning = getappdata( h.fig , 'keepRunning' ) ;
if keepRunning
% This is where you do your lenghty stuff
% we'll count grains of sand for this demo ...
h.lbl.String = sprintf('Counting grains of sands: %d/%d',k,nGrainOfSand) ;
pause(0.1) ;
else
% tidy up then bail out (=stop the callback)
h.lbl.String = sprintf('Counting interrupted at: %d/%d',k,nGrainOfSand) ;
% toggle the button states
h.btnStart.Enable = 'on' ;
h.btnStop.Enable = 'off' ;
return
end
end
end
function stopProcess(hobj,~)
h = guidata( hobj ) ;
% modify the 'keepRunning' flag
setappdata( h.fig , 'keepRunning' , false )
end
function h = create_gui
h.fig = figure('units','pixel','Position',[200 200 350 200]) ;
h.btnStart = uicontrol('style','pushbutton','string','Start process',...
'units','pixel','Position',[50 100 100 50],...
'Callback',#startProcess) ;
h.btnStop = uicontrol('style','pushbutton','string','Stop process',...
'units','pixel','Position',[200 100 100 50],...
'Callback',#stopProcess,'enable','off') ;
h.lbl = uicontrol('style','text','string','','units','pixel','Position',[50 20 200 50]) ;
end
To see it in action: