Event-driven programming

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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.


[edit] Contrast with batch programming

In contrast, in batch programming, the flow is determined by the programmer. Although batch programming is the style taught in beginning programming classes, the more complex event-driven programming is the standard architecture of modern interactive programs.

Here are two pseudocode versions of a trivial program to add two numbers:

[edit] Batch version

read a number (from the keyboard) and store it in variable A[0]
read a number (from the keyboard) and store it in variable A[1]
print A[0]+A[1]

[edit] Event-driven version

set counter K to 0
   if a number has been entered (from the keyboard)
     store in A[K] and increment K
     if K equals 2 print A[0]+A[1] and reset K to 0

At first sight, the event-driven program seems more cumbersome and for such a trivial task is indeed so. However, the second program can be generalized far more easily than the first. Instead of checking just for a number entry we may add code to check whether any of several events has occurred. Then for each event we can execute a particular piece of code that is commonly referred to as an event handler.

A slight variation in the above further illustrates the point:

set counter K to 0
   whenever a number has been entered (from the keyboard) // keyboard-number event
     store in A[K] and increment K // keyboard-number handler
   if K equals 2 // ready-to-sum event
     print A[0]+A[1] and reset K to 0 // ready-to-sum handler

[edit] Example: reading from a socket

This example uses pseudocode to illustrate how data is read from a socket using an event-driven approach:

function read_next_data(fd)
   data = read_async( fd )
   if len(data) == 0
       => Nothing to read, register to be called back when something is ready
       event_polling_register( fd, read_next_data )
       => Go back to doing something else
       => Data was available and len(data) was received
       add_data_to_buffer( buffer, data )

This example uses Tcl code to illustrate how data is read from a socket using an event-driven approach:

 # open channel
 set chan [socket $host $port]
 set buffer ""
 fconfigure $chan -blocking none
 # register event handler
 fileevent $chan readable [list read_next_data $chan buffer]  
 # process event until end of file
 proc read_next_data {chan bufferVar} {
    upvar #0 $bufferVar buffer
    append buffer [read $chan]
    if {[eof $chan]} {close $chan}

[edit] Event handlers

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 event handler called OnKeyEnter() that includes an argument with a string of characters, corresponding to what the user typed before hitting the ENTER key. If we want to add two numbers we need to use storage outside the event handler, so the implementation might look like this

[edit] A trivial event handler

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.

[edit] 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 that the main program will respond to. For example, in a GUI program, we might be interested in a single (as opposed to a double) left-button mouse-click on a command button. So a routine would be written to respond to such an event. The routine might 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 need only supply the event code.

[edit] Binding event handlers

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.

[edit] Main loop

The third step in developing an event-driven program is to write the main loop: a function that checks for events, and then calls the matching event handler. Most event-driven programming environments already provide this main loop, so it need not be rewritten.

[edit] See also

[edit] References

[edit] External links

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