listeners-and-event-handlers.md

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Listeners and Event Handlers

How are listeners/event handlers implemented?

PHP

There is no specific internal event handling mechanism, but one can be approximated by implementing one yourself. For example, a basic implementation of an observer model event handling system:

<?php
    abstract class SubjectBase {
        abstract public function handleEvent();
    }

    class Observer {
        private $subjects = array();
        public function registerSubject($subject) {
            array_push($this->subjects, $subject);
        }
        public function notifySubjects() {
            foreach($this->subjects as $s) {
                $s->handleEvent();
            }
        }
    }
    class Subject extends SubjectBase {
        private $name;
        public function __construct($name) {
            $this->name = $name;
        }
        public function handleEvent($event) { // In this example, events are just Strings. However more a more sophisticated Event object could be constructed and utilized to increase usefulness
            print "Subject name: ".$this->name." is handling this event: ".$event;
        }
    }

    $a = new Subject("foo");
    $b = new Subject("bar");
    $c = new Subject("baz");

    $observer = new Observer();
    $observer->registerSubject($a);
    $observer->registerSubject($b);
    $observer->registerSubject($c);

    $observer->notifySubjects("myEvent");
?>

Will output:

Subject name: foo is handling this event: myEvent
Subject name: bar is handling this event: myEvent
Subject name: baz is handling this event: myEvent

Python

Much like PHP, there exists no native library functions specifically for event handling. Yet, just like PHP, event-based architectures can be created. A basic observer model pattern can be approximated in a way similar to which follows:

class Subject:
    def __init__(self, name):
        self.name = name
    def handleEvent(self, event):
        print("Subject name: %s is handling this event: %s"%(self.name, event))

class Observer:
    subjects = []
    def registerSubject(self, subject):
        self.subjects.append(subject)
    def notifySubjects(self, event):
        for s in self.subjects:
            s.handleEvent(event)

a = Subject("foo")
b = Subject("bar")
c = Subject("baz")

observer = Observer()
observer.registerSubject(a)
observer.registerSubject(b)
observer.registerSubject(c)

observer.notifySubjects("myEvent")

Will output:

Subject name: foo is handling this event: myEvent
Subject name: bar is handling this event: myEvent
Subject name: baz is handling this event: myEvent