Can't get variable out of Callback function. Already defined as global. Matlab - matlab

Question

I have a serialport bytesavailable callback function.
function readFrame(src,~)
global data
global payloadBytes
message = read(src,payloadBytes,"uint8");
src.UserData = message;
data = message;
return
I read the buffer into the variable message and then save it as global variable data.
i get the variable in my workspace. Then something happens that I can't explain. I try to save this global variable in a local variable in my Mainscript. Notice: global data is defined in mainscript.
global data;
global payloadBytes;
msg=[];
frameCount = 1;
numFrames = 10;
messageByte = zeros(numFrames,payloadBytes)
while(app)
%wait till callback triggered and serial data read
while isempty(msg)
%save global variable data in local variable msg
msg = data;
disp("wait for data")
end
%save msg in an expanding 2D Variable messagebyte
messageByte(frameCount,:) = msg;
%empty msg variable
msg =[]
frameCount = frameCount +1
%stop when 10 frames are caught
if frameCount == 11
app = 0;
end
end
Problem is processedData is always empty. So when i want to make a 2D Matrice to save different data like this:
processedData(frameCount,:) = data;
I get an exception: indices doesn't match. Thats no wonder.
Can somebody tell me what I am doing wrong.
Thank you.

Answer 1

MATLAB is single-threaded. Stuff doesn’t run in parallel unless you explicitly use the Parallel Processing Toolbox. So for your callback to write to data, your main loop must give the callback a chance to execute. Add a pause to your loop, this will allow callbacks to execute, as well as timer objects.
while isempty(msg)
pause(0.1) % find a suitable pause length here
%save global variable data in local variable msg
msg = data;
disp("wait for data")
end
The pause doesn’t need to be long, but make it as long as you can tolerate, since that will make your wait less processor intensive.