Problem with fast and slow function running after each other

Dear MathWorks Community,

I am trying my hands at the app designer currently and defined some functions that run after each other for plotting a physics problem. For starters I want to mention that I just use Matlab as a better calculator with little knowledge about code optimization and how it really works.

This is what I currently try to do:

On change of a drop down element do the following:

1. Lock all editable elements in a graphic container (Tab in my case)
2. Calculate and Plot a function
3. Unlock all editable elements in the graphic container

For 1 and 3 I wrote myself a private function, since I use this regulary for "Locking"

 function LockTab(app,input)
            elements = findobj(input,"Enable","on");
            for ii=1:numel(elements)
                elements(ii).Enable="off";
            end
 end

and "Unlocking" a Tab, while calculations are running

function UnlockTab(app,input)
            elements = findobj(input,"Enable","off");
            for ii=1:numel(elements)
                elements(ii).Enable="on";
            end
end

now I want to call those functions and the slow plotting function

        function MyDropDownValueChanged(app, event)
            LockTab(app,app.MyTab)
            Plottingfunction(app)
            UnlockTab(app,app.MyTab) 
        end

My expectations now are, that MyTab is "locked", then the PlottingFunction runs, updating my plot, afterwards MyTab is "unlocked".

I measured the computation time of my functions and they take around

0.003seconds

4-6.5 seconds (depending on the DropDown)

0.003 seconds

What happens however is, that MyTab is not locked and just the plotting is done. My solve right now is to add a pause(0.1) line between LockTab and Plottingfunction, but that seems like a bandaid at best and I am puzzled as to why the LockTab function isn't doing anything unless I pause, as the functions should run sequentally and not influence each other to my understanding. It feels as though the PlottingFunction is overwriting the LockTab funciton though.

Thank you in advance for your help.

Answer 1

[dpb]

Instead of pause, try drawnow

[PrinzM]

My issue wasn't really with needing to use pause specifically.

It is more, why the update doesn't happen automatically, because to me, that is what should happen and I feel like I did something wrong conceptually.

[dpb]

As the doc for drawnow explains, MATLAB may choose to not immediately process graphics or other callback actions that don't affect screen visibility or other events that don't matter from the standpoint of actual end results immediately when the event(s) are posted/queued. If it is desired to force such action as here, drawnow is the documented way in which to do that.

As you've noted, pause may result in the same thing happening as there is then time for the callbacks to be processed, but that is not the reliable way.

The callbacks will get processed, regardless, just not necessarily in the precise order of the instructions as they are queued. That's the thing about interrupt-driven code; it no longer is strictly sequential in execution; queueing delays for short-lived events such as this may end up with the event being over before the queue is cleared unless the system is forced to process any pending events.

[PrinzM]

Thank you very much!