MATLAB: Addlistener question…

addlistener listener function input arguments handle

I have created a listener which calls 'myfunction' whenever the limits of a certain set of axes changes:

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

This works fine. Now I want to pass the handles structure to myfunction:

      addlistener(handles.axes1,'XLim', 'PostSet',{@myfunction,handles});
      function myfunction(handles)
      ...

However I recieve the error:

      callbacks need to be of type function handle

I cannot pass handles, or any additional arguments to myfunction in this case. Why is this so? How would I set up a listener object properly?

Best Answer

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)

Related Solutions

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 Behavior/Syntax

There is no way to merge the two events into one event. Even when you call set, the properties are still set one at a time, in order.

In other words:

set(handles.axes1,'XLim',newXLim,'YLim',newYLim);

is effectively equivalent to:

handles.axes1.XLim = newXLim;
handles.axes1.YLim = newYLim;

In both versions, the XLim property is set first, followed by the YLim property. The PostSet listener for the XLim property has no way to know that you are about to set the YLim property, so it cannot suppress the first event while waiting for the YLim to be set.

Best Answer

Related Solutions

MATLAB: How to update guidata in the addlistener callback function

MATLAB: Addlistener Behavior/Syntax

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