App Designer initial value for UserData for Slider

I want to return from a slider a different value than its Value property (that is, a value computed based on its Value).

In GUIDE, I would do this using the UserData propery, however, the App Designer GUI provides only the "Tag" property from the Identifiers section.

If I inspect the object in code I can see that there is a UserDate property available for the slider:

               Position: [50 522 693 3]
                    Tag: ''
                Tooltip: '30.5px'
                   Type: 'uislider'
               UserData: []
                  Value: 0.3000
        ValueChangedFcn: ''
       ValueChangingFcn: @(source,event)executeCallback(ams,app,callback,requiresEventData,event)
                Visible: on

I was thinking that I can initialize the UserData in the CreateFcn of the slider - but the slider comes only with ValueChangedFcn and ValueChangingFcn.

So how can I set an initial value to the UserData?

I'm using Matlab R2023a.

[Stephen23]

Both UISLIDER (for UIFIGURE) and UICONTROL (for FIGURE) provide object creation functions:

https://www.mathworks.com/help/matlab/ref/matlab.ui.control.slider.html#buh_e24-86_sep_shared-CreateFcn

https://www.mathworks.com/help/matlab/ref/matlab.ui.control.uicontrol-properties.html#bt6ck7c-1_sep_shared-CreateFcn

[Andrei]

@Stephen23 How do you insert the CreateFcn from AppDesigner?

The documentation also has UserData for the slider, but the App Designer does not provide a way to initialize it. The same for a way to directly add a CreateFcn for a slider object.

It is interesting the documentation for Adding App Designer Callbacks uses the Slider object as an example: Callbacks in App Designer

Only ValueChanged/ing Fcns seem to be available for sliders in AppDesigner.

(Last edit for minor typos)

[Andrei]

For the moment, I have a workaround: I moved the code that computes the needed value based on the slider's position into a separate function and I call that function each time that I need to use the output of the slider (what would have been the UserData).

In this way I can update the code for the slider without needing to make further changes in the app code. However, some transparency is lost with this - it would have been much easier to just set (easily) the initial value for the UserData.

As a note, I cannot prevent the user using the default slider position so the UserData is the empty array [ ] when the core function for the app is called.

Answer 1

[Voss]

You can initialize the slider's UserData in the app's StartupFcn.

[Andrei]

I have the same question as for Stephen23 - How do you insert in App Designer the app's StartUpFcn?

[Voss]

In App Designer Component Browser, select the app then select Callbacks below that; you'll see StartupFcn; type a name to define a new StartupFcn.

[Andrei]

Thank you, it worked. I was trying to add the function to the UIFigure, not to the app....

I will accept this answer, but I believe this needs to be posed as a feature request as the App Designer' GUI doesn't give access to all documented object properties / callbacks so the user is left with trying to figure out how to set them programmatically and this does not lead to self explanatory / concise code (just try to figure 2 years in the future where and why in the code the initial value of the property was set...)

[Voss]

You're welcome!

You can make an enhancement request as follows: https://www.mathworks.com/matlabcentral/answers/460998-how-can-i-make-a-features-request#answer_374172