Event-driven programming model

Event-driven programming is a programming model where flow control is determined by events. Examples of events are mouse clicks, key presses, gestures, sensor data, messages from other programs, and so on. An event-driven application has the mechanism to detect events on a near real-time basis, and respond or react to them by invoking the appropriate event handling procedure. Since the bulk of the earlier event processing programs were written using C/C++, they resorted to low-level techniques such as callbacks (using function pointers) to write those event handlers. Later systems such as Visual Basic, Delphi, and other rapid application development tools did add native support for event-driven programming. To make matters more clear, we will take a tour of the event handling mechanism of the various platforms. This will help readers appreciate the issues that reactive programming models are solving (from a GUI programming context).

Reactive programming treats data as streams and events in windowing systems can be treated as streams to be processed in a uniform manner. The Reactive programming model provides support for gathering events from different sources as streams, filtering streams, the transformation of streams, performing actions on streams, and so on. The programming model handles asynchrony, scheduling details as part of the framework. This chapter is mostly based on the key data structures of the Reactive programming model and how we can implement basic Reactive programs. In an industrial-strength reactive program, the code written will be asynchronous and the examples from this chapter are synchronous. We give the necessary background information and language constructs in the following chapters before out of order execution and schedules are discussed. These implementations are here for elucidation and can be treated as learning examples.