Editing Event-driven programming (section)

===Event handlers===
The actual logic is contained in event-handler routines. These routines handle the events to which the main program will respond. For example, a single left-button mouse-click on a command button in a [[graphical user interface|GUI]] program may trigger a routine that will open another window, save data to a [[database]] or exit the application. Many [[Integrated development environment|IDEs]] provide the programmer with GUI event templates, allowing the programmer to focus on writing the event code.

Keeping track of history is normally trivial in a sequential program.  Because event handlers execute in response to external events, correctly structuring the handlers to work when called in any order can require special attention and planning in an event-driven program.

In addition to writing the event handlers, event handlers also need to be bound to events so that the correct function is called when the event takes place. For UI events, many IDEs combine the two steps: double-click on a button, and the editor creates an (empty) event handler associated with the user clicking the button and opens a text window so you can edit the event handler.