All Visio SmartShape symbols are objects. Those of you who are programmers already know an object has three things that distinguish it as such: properties, methods, and events. The cells of a SmartShape symbol's ShapeSheet equate to the properties. The user directs many of the methods, and the Events section of a ShapeSheet controls the most basic of the available events.
Drag out a rectangle SmartShape symbol onto a page and open its ShapeSheet. If you must, ensure that all sections are visible by selecting View, Sections. Click All and then OK. Now scroll down in the ShapeSheet until you find the Events section of the ShapeSheet. Notice that there are five supported events. They are