Some code does not execute in a predetermined sequence. Instead, it executes when the ActionScript interpreter notices that an event has occurred. Many events involve a user action, such as clicking the mouse, resizing the movie, or pressing a key. Other events happen without user intervention, such as a sound completing or an XML document loading. Just as the playhead entering a new frame executes synchronous code attached to the frame, events can cause event-based code to execute. Event-based code (code that executes in response to an event) is said to be executed asynchronously because the triggering of events can occur at arbitrary times.
Synchronous programming requires us to dictate, in advance, the timing of our code's execution. Asynchronous programming, on the other hand, gives us the ability to react dynamically to events as they occur. Asynchronous code execution is critical to ActionScript and interactivity.
This chapter explores asynchronous (event-based) programming in Flash.
