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Observer pattern refers to a class called “subject” that has a list of dependents, called observers, and notifies them automatically each time an action is taking place.
A small example of why is used:
– let’s say we have a class with does someting:
1class Actiune { 2 private $val; 3 function __construrct() { 4 // someting in the constructor 5 } 6 7 function change($val) { 8 $this->val = $val; 9 } 10}Each time $val changes we want to call a method of an “observer” object:
1class Actiune { 2 private $val; 3 function __construrct() { 4 // someting in the constructor 5 } 6 7 function change($val, $observator) { 8 $this->val = $val; 9 $observator->update($this); 10 } 11}Theoretically is not bad, but the more methods there are so does the dependence grows bigger and each time we add a new observer object we must modify the class, with will probably result in chaos, which will be almost impossible to port.
Now, the observator pattern looks something like this:
SPL (Standard PHP Library), which is well known for it’s defined iterators, comes with the interfaces SplSubject and SplObserver, for the subject and respectively the observer.
An implementation looks someting like this:
1/** 2 * the class which must be monitored 3 */ 4class Actiune implements SplSubject { 5 private $observatori = array(); 6 private $val; 7 8 /** 9 * method to attach an observer 10 * 11 * @param SplObserver $observator 12 */ 13 function attach(SplObserver $observator) { 14 $this->observatori[] = $observator; 15 } 16 17 /** 18 * method to detach an observer 19 * 20 * @param SplObserver $observator 21 */ 22 function detach(SplObserver $observator) { 23 $observatori = array(); 24 foreach($this->observatori as $observatorul) { 25 if($observatorul != $observator) $observatori[] = $observatorul; 26 } 27 $this->observatori = $observatori; 28 } 29 30 /** 31 * method that notifies the observer objects 32 */ 33 function notify() { 34 foreach($this->observatori as $observator) { 35 $observator->update($this); 36 } 37 } 38 39 /** 40 * method for makeing changes in the class 41 * 42 * @param int $val 43 */ 44 function update($val) { 45 echo 'updateing...'; 46 $this->val = $val; 47 $this->notify(); 48 } 49 50 /** 51 * public method with the subject's status 52 * 53 * @return int 54 */ 55 function getStatus() { 56 return $this->val; 57 } 58} 59 60/** 61 * and observer class 62 */ 63class Observator implements SplObserver { 64 function update(SplSubject $subiect) { 65 echo $subiect->getStatus(); 66 } 67} 68 69// an observer instance 70$observator = new Observator(); 71 72// an subject instance 73$subiect = new Actiune(); 74 75// attaching an observer to the subject 76$subiect->attach($observator); 77 78// update subject 79$subiect->update(5);What seems strange is that there isn’t any documentation on this SPL interfaces. Even on the Zend website there is an article PHP Patterns: The Observer Pattern which does not use SPL, but for something like namespaces there was documentation even before PHP 5.3 was out.