Please provide a little more context. From what you describe it sounds like your Simulink model has an interpreted MATLAB function block ("I have a value within a MATLAB function which is updated each interval of my Simulink model") is that correct? Otherwise exactly what kind of value in your Simulink model do you want to get?
If you just want a block parameter value that may change, you can use get_param to get the value. If you want data from an InputPort or OutputPort you will need to access the block's Run-TimeObject https://www.mathworks.com/help/simulink/ug/accessing-block-data-during-simulation.html
In any case, what I have done regarding communicating between Simulink and my App Designer Gui for soft real time application is (omitting many details) the following:
- To provide some decoupling between my app and the simulink model I make an interface object that can get data in an out of the simulink model. In this interface object I have an event that the gui can listen for that lets it know when new data is available.
- I use a MATLAB timer, and corresponding timer callback that periodically gets the desired value from the Simulink model (e.g. using the Run-Time object). This timer callback function saves the data as a property of my interface object and notifies event listeners (in particular the gui app) that a value has been changed.
- My app listens for this event, and when it occurs it gets the data from the interface object (it is just a property of the interface object) and it writes the data to the appropriate data fields (edit fields, tables etc) in my gui.
