Callbacks - GNU Octave
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#### 15.4.4 Callbacks

Callback functions can be associated with graphics objects and triggered after certain events occur. The basic structure of all callback function is

         function mycallback (src, data)
         ...
         endfunction

where `src` gives a handle to the source of the callback, and `code` gives some event specific data. This can then be associated with an object either at the objects creation or later with the `set` function. For example,

         plot (x, "DeleteFcn", @(s, e) disp("Window Deleted"))

where at the moment that the plot is deleted, the message "Window Deleted" will be displayed.

Additional user arguments can be passed to callback functions, and will be passed after the 2 default arguments. For example:

         plot (x, "DeleteFcn", {@mycallback, "1"})
         ...
         function mycallback (src, data, a1)
           fprintf ("Closing plot %d\n", a1);
         endfunction

The basic callback functions that are available for all graphics objects are

- CreateFcn This is the callback that is called at the moment of the objects creation. It is not called if the object is altered in any way, and so it only makes sense to define this callback in the function call that defines the object. Callbacks that are added to `CreateFcn` later with the `set` function will never be executed.
- DeleteFcn This is the callback that is called at the moment an object is deleted.
- ButtonDownFcn This is the callback that is called if a mouse button is pressed while the pointer is over this object. Note, that the gnuplot interface does not respect this callback.

The object and figure that the event occurred in that resulted in the callback being called can be found with the `gcbo` and `gcbf` functions.

— Function File: h = gcbo ()
— Function File: [h, fig] = gcbo ()

> Return a handle to the object whose callback is currently executing. If no callback is executing, this function returns the empty matrix. This handle is obtained from the root object property "CallbackObject".

> Additionally return the handle of the figure containing the object whose callback is currently executing. If no callback is executing, the second output is also set to the empty matrix.

> See also: gcf, gca, gcbf.

— Function File: fig = gcbf ()

> Return a handle to the figure containing the object whose callback is currently executing. If no callback is executing, this function returns the empty matrix. The handle returned by this function is the same as the second output argument of gcbo.

> See also: gcf, gca, gcbo.

Callbacks can equally be added to properties with the `addlistener` function described below.