MATLAB: How do i make function handles work

callbackfunction handles

Hello, I have a really annoying error that i can't find a way to make it work, my code looks like this:

function [h] = test_listeners()
f = figure('Units','Normalized');
h.Red = uicontrol('Parent',f,...
                  'Units','Normalized',...
                  'Style','slider',...
                  'position',[0.1, 0.6, 0.3, 0.1],...
                  'backgroundColor',[1,0.85,0.85],...
                  'FontSize',15);
h.Blue = uicontrol('Parent',f,...
                   'Units','Normalized',...
                   'Style','text',...
                   'position',[0.1, 0.3, 0.3, 0.1],...
                   'backgroundColor',[0.85,0.85,1],...
                   'FontSize',15);
addlistener(h.Red,'Action',@()callbackBlue(h.Blue));
end
function [] = callbackBlue(varargin)
disp(varargin)
end

in one sentence it should change the value of BLUE according to the value of RED, but even though in input of the callback is varargin when running the code i get this error message:

Warning: Error occurred while executing the
listener callback for event Action defined
for class matlab.ui.control.UIControl:
Error using
test_listeners>@()callbackBlue(h.Blue)
Too many input arguments.

I am currently stuck and will love some help.

Best Answer

addlistener(h.Red, 'Action', @(varargin)callbackBlue(varargin{:}, h.Blue));

Remember, listeners have two arguments automatically added, just like all other callbacks (except for callbacks that are acting as filters, such as zoom constraint functions.)

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MATLAB: How to update guidata in the addlistener callback function

Note:

for i = 1:10
   checkbox(i-1) = ...

will error on the first iteration, since checkbox(1-1=0) is not a valid index.

Note 2:

@(hObject, eventdata, handles) callback(hObject, eventdata, handles)

Since this anonymous function only job is to pass all its inputs to another function unchanged, it could just be skipped. That is

@callback

would do the same.

Note 3:

function handle = callback(...)

A proper callback never returns anything since there is no mechanism for getting that return value.

Now for your question, the immediate problem comes from the anonymous function you've defined. An listener callback must have two inputs arguments and only two. These are, the object generating the event, and some event data. Therefore, the inputs to your anonymous function can only be:

@(hobject, eventdata) ...

handles cannot be an input since it won't be sent by the event. Now, if you wanted handles to be passed to your callback function, you could define your anonymous function as:

@(hobject, eventdata) callback(hobject, eventdata, handle)

This would work with you example, but I wouldn't recommend that. The anonymous function captures the content of the handle structure at the point it is defined, so any change to that handle structure after that line won't be seen by your callback, e.g.:

lh = addlistener(handles.checkboxes, 'Value', 'PostSet', @(hObject, eventdata) callback(hObject, eventdata, handles));
handles.text = uicontrol('Style', 'text', 'String', 'some new control');  %add a new control to handle

The handle received by your callback will not contain the text field as it was created after handle was captured.

A safer way is simply to get the handle structure with guidata inside your callback, so:

lh = addlistener(handles.checkboxes, 'Value', 'PostSet', @callback);  %not passing handle to callback
function callback(hObject, eventdata) %not receiving handle
   handle = guidata(hobject);   %get handle inside the callback. 
   %...

MATLAB: Addlistener question…

The posted example does not work on my computer under Matlab 2011b:

addlistener(handles.axes1,'XLim', 'PostSet', @myfunction);
function myfunction   % ERROR

Calling the callback function fails, because it needs at least 2 inputs:

function myfunction(ObjH, EventData)  % OK

If you need a third input:

addlistener(handles.axes1,'XLim', 'PostSet', ...
   @(ObjH, EventData) myfunction(ObjH, EventData, handles));
function myfunction(ObjH, EventData, handles)

Best Answer

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MATLAB: How to update guidata in the addlistener callback function

MATLAB: Addlistener question…

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