Head First-觀察者模式

什麼是觀察者模式?觀察者模式定義了對象之間一對多的關係。php

觀察者模式中有主題(便可觀察者)和觀察者。主題用一個共同的接口來通知觀察者,主題不知道觀察者的細節,只知道觀察者實現了主題的接口。編程

廣泛的觀察者模式中的推的方式更適合點,下面咱們就寫一個推的例子,天氣站提供一個接口,當天氣變化時,會將數據通知給各個看板顯示。this

<?php
//使用接口,類必須實現幾個功能註冊,刪除,通知這幾個動做
interface Subject{
	public function registerObserver(Observer $o);
	public function removeObserver(Observer $o);
	public function notifyObservers();
}
interface Observer{
	public function update($a,$b,$c);
}
//各個面板不一樣將改行爲以接口實現
interface DisplayElement{
	public function display();
}

class Weather implements Subject{
	public $observers;
	public $changed=false;
	public $a;
	public $b;
	public $c;

	public function __construct(){
		$this->observers = array();
	}
	public function registerObserver(Observer $o){
		$this->observers[] = $o;
	}
	public function removeObserver(Observer $o){
		$key = array_search($o,$this->observers);
		if($key!==false){
			unset($this->observers[$key]);
		}
	}
	public function notifyObserver(){
		if($this->changed){
			foreach($this->observer as $ob){
				$ob->update($this->a,$this->b,$this->c);
			}
		}
	}
	public function setChanged(){
		$this->changed = true;
	}
	//當數值改變時通知各個觀察者
	public function measurementsChanged(){
		$this->setChanged();
		$this->notifyObserver();
	}

	public function setMeasurements($a,$b,$c){
		$this->a = $a;
		$this->b = $b;
		$this->c = $c;
		$this->measurementsChanged();		
	}
}

class CurrentConditionsDisplay implements Observer, DisplayElement{
	public $a;
	public $b;
	public $c;
	public $subject;

	public function __construct(Subject $weather){
		$this->subject = $weather;
		$this->subject->registerObserver($this);
	}

	public function update($a,$b,$c){
		$this->a = $a;
		$this->b = $b;
		$this->c = $c;
		$this->display();
	}

	public function display(){
		echo $this->a.$this->b.$this->c;
	}
}
?>

咱們在這些對象之間用鬆耦合的方式進行溝通,這樣咱們在後期維護的時候,能夠大大的提升效率。設計

設計原則:找出程序中會變化的方面,而後將其進行分離;針對接口編程,不針對實現編程;多用組合,少用繼承server

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