Handle of object whose callback is executing
h = gcbo
[h,figure] = gcbo
The MATLAB® software stores the handle of the object whose callback is executing in the root CallbackObject property. If a callback interrupts another callback, MATLAB replaces the CallbackObject value with the handle of the object whose callback is interrupting. When that callback completes, MATLAB restores the handle of the object whose callback was interrupted.
The root CallbackObject property is read only, so its value is always valid at any time during callback execution. The root CurrentFigure property, and the figure CurrentAxes and CurrentObject properties (returned by gcf, gca, and gco, respectively) are user settable, so they can change during the execution of a callback, especially if that callback is interrupted by another callback. Therefore, those functions are not reliable indicators of which object's callback is executing.
When you write callback routines for the CreateFcn and DeleteFcn of any object and the figure ResizeFcn, you must use gcbo since those callbacks do not update the root's CurrentFigure property, or the figure's CurrentObject or CurrentAxes properties; they only update the root's CallbackObject property.
When no callbacks are executing, gcbo returns  (an empty matrix).