Event-driven programming

Event-driven programming

In computer programming, event-driven programming or event-based programming is a programming paradigm in which the flow of the program is determined by events—i.e., sensor outputs or user actions (mouse clicks, key presses) or messages from other programs or threads.

Event-driven programming can also be defined as an application architecture technique in which the application has a main loop which is clearly divided down to two sections: the first is event selection (or event detection), and the second is event handling. In embedded systems the same may be achieved using interrupts instead of a constantly running main loop; in that case the former portion of the architecture resides completely in hardware.

Event-driven programs can be written in any language, although the task is easier in languages that provide high-level abstractions, such as closures. Some integrated development environments provide code generation assistants that automate the most repetitive tasks required for event handling.


Event handlers

A trivial event handler

Because the code for checking for events and the main loop does not depend on the application, many programming frameworks take care of their implementation and expect the user to provide only the code for the event handlers. In this simple example there may be a call to an event handler called OnKeyEnter() that includes an argument with a string of characters, corresponding to what the user typed before hitting the ENTER key. To add two numbers, storage outside the event handler must be used so the implementation might look like below.

globally declare the counter K and the integer T.
OnKeyEnter(character C)
   convert C to a number N
   if K is zero store N in T and increment K
   otherwise add N to T, print the result and reset K to zero

While keeping track of history is straightforward in a batch program, it requires special attention and planning in an event-driven program.

Exception handlers

In some programming languages (e.g. PL/1), even though a program itself may not be predominantly event driven, certain abnormal events such as a hardware error, overflow or "program checks" may occur that possibly prevents further processing. Exception handlers may be provided by "ON statements" in (unseen) callers to provide housekeeping routines to clean up afterwards before termination.

Creating event handlers

The first step in developing an event-driven program is to write a series of subroutines, or methods, called 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 GUI program may trigger a routine that will open another window, save data to a database or exit the application. Many modern day programming environments provide the programmer with event templates so that the programmer only needs to supply the event code.

The second step is to bind event handlers to events so that the correct function is called when the event takes place. Graphical editors combine the first 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.

The third step in developing an event-driven program is to write the main loop. This is a function that checks for the occurrence of events, and then calls the matching event handler to process it. Most event-driven programming environments already provide this main loop, so it need not be specifically provided by the application programmer. RPG, an early programming language from IBM, whose 1960s design concept was similar to event driven programming discussed above, provided a built-in main I/O loop (known as the "program cycle") where the calculations responded in accordance to 'indicators' (flags) that were set earlier in the cycle.

Criticism and best practice

Event-driven programming is widely used in graphical user interfaces because it has been adopted by most commercial widget toolkits as the model for interaction. The design of those toolkits has been criticized for promoting an over-simplified model of event-action, leading programmers to create error prone, difficult to extend and excessively complex application code:[1]

Such an approach is fertile ground for bugs for at least three reasons:

  1. It is not thread-safe.
  2. It can lead to writing code within the event handler for each possible value of various values in the program, making the source code hard to understand.
  3. In places where the event code changes variables shared throughout the program, it can require the programmer to write convoluted algorithms to avoid unwanted ripple effects.

Stackless threading

An event driven approach is used in hardware description languages. A thread context only needs a cpu stack while actively processing an event, once done the cpu can move on to process other event-driven threads, that allows an extremely large number of threads to be handled. This is essentially an Finite-state machine approach.

See also


  1. ^ Samek, Miro (April 2003). "Who Moved My State?". C/C++ Users Journal, The Embedded Angle column. http://www.ddj.com/cpp/184401643. 

External links

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