You should be passing into the callback function the handle to your edit box. For example
... ,'Callback',{@pushbutton_callback,myvar,ui})
Then get the value from the edit box inside the callback. The way you are doing it, MATLAB is evaluating the value of the string and saving the literal as part of the callback function call. It will not evaluate it again.
With the revisions, your code becomes:
function testui()
f=figure();
f.Color='black';
f.ToolBar='none';
ui=uicontrol('Style','edit');
set(ui,'HorizontalAlignment','right') % set works in R2014a and older, too
set(ui,'String','0')
myvar=2;
uicontrol('Style','pushbutton','Position',[250 20 60 20],'String', ...
'Evaluate', 'Callback',{@pushbutton_callback,myvar,ui});
end
function pushbutton_callback(~,~,x,Y) % use ~ to ignore inputs we don't use
F=str2double(get(Y,'String'));
display(F)
display(x)
end
