Continuous slider callback, how to get Value from addlistener?

Hi I use GUIDE to create slider I tried to get value from continuous slider call back using addlistener I follow instruction from http://www.mathworks.com/matlabcentral/answers/51668-getting-gui-slider-updates-while-dragging

which utilize

h = uicontrol('style','slider','callback',@(src,evt)disp(get(src,'value')));
addlistener(h,'Value','PreSet',@(~,~)disp('hi'));

and I adjust the code accordingly, and it work as it should.

But it seem I cant get to contain the displayed value into variable, How to put the display value into variable?

here is my code.

% --- Executes on slider movement.
function kvslider_Callback(hObject, eventdata, handles)
% hObject    handle to kvslider (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hints: get(hObject,'Value') returns position of slider
%        get(hObject,'Min') and get(hObject,'Max') to determine range of slider

kV= get(hObject,'Value');
kv_value =(((round(kV/5))*5))
assignin('base','kV',kV);
assignin('base','kVp',kv_value);

set(handles.kvvalue,'String',strcat((num2str(kv_value)),' kVp'));
set(handles.kvslider,'Value',(kv_value));
setappdata(handles.kvslider, 'Value', num2str(kv_value));
%slidervalue = (round(slidervalue/3))*3

%experimental
kv_update = handles.kvslider;
kv_up =addlistener(kv_update,'Value','PreSet',@(~,~)disp(((round((get(kv_update , 'Value'))/5))*5)));
assignin('base','kv_up',kv_up);
%addlistener(kv_update,'Value','PreSet',@Apply_Callback);

Answer 1

[Stephen23]

A typical usage would be like this:

addlistener(Handle, 'Value', 'PostSet',@MyCallBack);

And note that while MyCallBack is a function, it does not have any output arguments. However when that function is called you can access the slider value very easily within that function.

Here is a minimal example that simply displays the new value:

MyCallBack = @(a,b) disp(get(b,'NewValue'));
F = figure();
H = uicontrol(F,'Style','slider');
addlistener(H, 'Value', 'PostSet',MyCallBack);

Again note that the new value is available inside the MyCallBack function workspace, so it is within this callback function that you need to use the new value as a variable. The easiest way is to write a new function just for the callback.

Tip

It would improve your code if you used guidata instead of using assignin:

https://www.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html

https://www.mathworks.com/help/matlab/matlab_prog/share-data-between-workspaces.html

[Mohamad Roslan Mohd Roshdi]

At @CallBack

How should I write to have the display value into a variable?

@(~,~)disp(((round((get(kv_update , 'Value'))/5))*5))

The code seem to only display value in Command Window, but not assigning any variable to it value.

I tried to create a new function for @CallBack, but dont know how.

[Stephen23]

I do not use GUIDE, as I prefer to write my own code. This is an example callback function for a slider, showing how to get the slider value into a variable:

function temp
F = figure();
H = uicontrol(F,'Style','slider');
addlistener(H, 'Value', 'PostSet',@myCallBack);
end
function myCallBack(hObj,event)
val = get(event,'newValue')
end

Where val is a variable with the updated value of the slider. This basically the same example as is given in the documentation, which you should have a read of:

http://www.mathworks.com/help/matlab/creating_guis/write-callbacks-using-the-programmatic-workflow.html

Note that you can use guidata or setappdata to pass values between callbacks. This is the recommended way to pass values: do not use assignin or similar methods.

[Hann-Shin]

Thank you for the detailed response. When I run your code on 2015a, I get

Warning: Error occurred while executing callback: Error using get:

Cannot find 'get' method for event.PropertyEvent class.

Error in slidertestCOnt>myCallBack (line 7) val = get(event,'newValue')

[zhikai]

I think Matlab updated this function recently. For this part:

get(event,'newValue')

Now, maybe you should try the following:

get(event.AffectedObject,'Value')

It worked on my R2016b version.

[Michiel Hermes]

Zhikai is correct in the 2018 version you seem to need:

CallBack = @(~,b) disp(b.AffectedObject.Value);
F = figure();
H = uicontrol(F,'Style','slider');
addlistener(H, 'Value', 'PostSet',CallBack);

Answer 2

[AG]

The following worked for me. I get a 'live' scroll-bar update by calling this within another function:

slider_value = get(gcf.Children(j), 'Value');

...where j is the value of the UIcontrol corresponding to the scroll. You can find out which UIControl it is in the figure by putting a break in the code and using:

gcf.Children

This could also be ascertained in run-time using:

for j = 1:length(gcf.Children)
    get(gcf.Children(j), 'Tag')
end

which will return a char array with the Tag of each of the children which could then be compared using strcmp(), for example. I've done this using a GUI created with GUIDE.

[Stephen23]

To get continuous updates with the slider movement (which the original question requested) requires adding a listener to that slider object. Your answer does not use a listener, and does not provide continuous updates based on the slider movement. All you are doing is polling the slider value by "another function".

Note that obtaining and using explicit handles for all graphics objects is ultimately simpler and much more reliable that than using gcf and the very indirect approach that you are using.