Event functions are used to receive and process user actions at the GUI interface. Every GUI object listens for user actions (e.g., mouse click, mouse drag, typing a key, etc), and allows you to specify a function to be called if and when a user action occurs.
You do not have to specify a function for every user action on every object on your GUI, only for the ones that you want. For example, you could make a program that draws a circle when the mouse is clicked on a particular location, and clean up all circles when the mouse exits the window.
The following event functions are available for all GUI library objects (i.e., Display, Label, Button, Checkbox, DropDownList, Slider, TextBox, Line, Circle, Rectangle, and Icon) except menus.
Note: In the case of overlapping objects (e.g., a label and display), user events are handled by the object on top (front-most).