UI Control callback function problem

I have done a small program based on show in a static text the values of the slider everytime I touch the slider. I'm working in dot notation ( e.g. no using get and set )and without using GUIDE. I don't know how to use the callback function for make it works, even I know the command that works.

Here is the program:

%%Figure Creation

fig = figure

fig.Color = 'b'

fig.Tag= 'fig1'

fig.Name= 'SliderGUI'

%%Slider Creation

slider = uicontrol('Parent',fig,'Style','Slider','Min',0,'Max',1, ...
    'SliderStep',[0.01 0.10], 'Position', [100,100,300,25]);

slider.BackgroundColor = 'k'

slider.Tag = 'slider1'

slider.Callback=@sliderCallback

%%Create of UIControl Static text

static = uicontrol ('Parent',fig,'Style','text','Position', ...
         [100,150,300,250], 'Value', 0 ,'String', 'Slider Value');

static.FontName = 'Arial';

static.FontSize =100;

static.BackgroundColor = 'c'

static.Tag = 'static1'

static.Callback=@staticCallback

%%Slider Callback function

% Here is where I have problems 

function sliderCallback(slider,eventdata)

static.String=slider.Value

end

The command

static.String=slider.Value

Works good, but I don't know how to define the callback function. I try a lot of things but anything have worked.

Thank's for the help!

Answer 1

[Walter Roberson]

Change

function sliderCallback(slider,eventdata)

static.String=slider.Value

end

to

function sliderCallback(slider,eventdata)
  static = findobj(ancestor(slider, 'figure'), 'Tag', 'static1');
  static.String = slider.Value
end

That is, your problem is that "static" is not in scope in the sliderCallback so you have to locate it.

[Walter Roberson]

You need R2016b or later in order to be allowed to have a function definition inside a .m file that does not start with either function or classdef

You can do one of the following:

• move the function to its own sliderCallback.m file; or
• upgrade to R2016b; or
• put function followed by your .m file name at the beginning of your code, before that '%% Figure Creation'; or
• remove the function entirely and change

static.Callback=@staticCallback

to

static.Callback = @(src, event) set(static, 'String', get(slider, 'Value') );

[Pau Papu]

Thank's Walter,

I have realized also that my problem is that the callback function is a nested function and for work well it needs to have a general function. Then, even if I just do:

static.String=slider.Value

The slider callback works well. But I understand that I have always to put my variable in scope

I have a question concerning this topic: Can I add the handles of the different objects I will need just adding arguments into the callback cell array? I mean:

slider.Callback={@sliderCallback,static}

Is that another way to have the variables in scope?

The function works well doing that but I want to verify if I'm understanding well the concept.

Thank you very much!

Thank you

[Walter Roberson]

You have to be a bit careful with something like

slider.Callback={@sliderCallback,static}

It is a quite important technique, but you need to remember that when you do that, what will be passed to each callback invocation will not be the whatever the variable static has been assigned at the time of the invocation. Instead, what will be passed to the callback invocation is a copy of whatever static was at the time that the cell array was constructed. If static is changed after that, then the callback will not get the updated version.

If static is a handle (as it is in your code) then provided the handle is not destroyed, then that works out fine because the handle does not itself hold the properties and instead just says where the properties are to be found, and the location to be found at does not change even when the properties change.

But if static is instead something like a structure, such as the handles structure, then any new fields added to the structure after the callback is created will not exist in the invocation of the callback, and any non-handle fields would retain their old values. For example,

handles = guidata(hObject);
handles.filename = '';
handles.folder = '';
handles.slider = uicontrol(....);
handles.slider.Callback = {@sliderCallback, handles};
[handles.filename, handles.folder] = uigetfile('*.jpg', 'Select an image');
guidata(hObject, handles);

then the version of handles received by callback invocations would not have the updated filename and folder fields.

[Pau Papu]

Thanks for the answer Mr.Robertson,

I understand your explanation but I tried to run this last small code but doesn't work. I will appreciate to understand more the context. For what I have read, I should do that:

fig1=figure

handles=guihandles(fig1)

And then, fill the fields:

handles.filename=''; etc

And finally store this data using guidata:

guidata(fig1,handles)

I'm understanding that well? I have some doubts concerning what is a handle...

I have some other questions:

1-You say that in my code, static is a handle because everytime I create a uicontrol ( or a figure ) it's by definition a handle object from the handle class?

2- In your code, you first create a structure of handles but you can add fields that are not handles ( as character vectors, and other structures...) using guihandles/guidata. It's better to proceed like that? Because I am not habituated to use guihandles/guidata.

3-There is any reference for understand all that?

Thank you very much!

[Walter Roberson]

graphics routines were defined as returning handles long before object oriented classes existed in MATLAB.

People tend to over-use adding non-handle fields to the handles structure. It is a useful structure, but it can lead to performance problems. GUIDE uses the handles structure for every graphics callback, so for every GUIDE graphics callback, the entire contents of the handles structure is copied. If a lot of non-handle data has been stored in handles then that can take a lot of time and memory. Most callbacks only need access to a modest number of entries from the handles structure. My suggestion is that you never save large arrays to the handles structure: instead store them using setappdata(), as then you can request just that data with getappdata() when you need it, without it being copied when it is not needed.

http://blogs.mathworks.com/pick/2008/04/17/advanced-matlab-handles-and-other-inputs-to-guide-callbacks/

https://www.mathworks.com/matlabcentral/answers/153052-how-to-understand-gui-handles

https://www.mathworks.com/help/matlab/matlab_oop/handle-objects.html

https://www.mathworks.com/help/matlab/learn_matlab/understanding-handle-graphics-objects.html