关于观察者模式总结

观察者模式(发布订阅模式):在对象之间定义一对多的依赖,这样一来,当一个对象改变状态,依赖他的对象都会收到通知,并自动更新。java

使用场景:当一个对象状态改变同时须要改变其余对象,且不知道多少观察者受影响。ide

观察者模式包含如下几个主要部分:this

  1. 抽象主题接口:该主题接口主要包含的功能有注册/删除观察者、通知全部观察者。
  2. 具体主题类:实现抽象的主题接口,将全部属于它的观察者保存在一个集合中,每一个主题能够拥有任意数量的观察者,当具体主题类内部状态发生改变时,将通知全部登记过的观察者。
  3. 抽象观察者接口:为全部观察者定义一个通用的接口,当收到主题的通知时将更新本身。
  4. 具体观察者类:存储与主题状态自恰的状态,实现抽象观察者接口所需的更新接口,以便自身与主题类的状态相协调,同时能够定义一个关于主题接口,指向本身订阅具体主题对象的引用,方便本身注销该主题。

观察者模式中,为分为拉模型和推模型。code

  • 推模型:主题对象向观察者推送主题的信息,无论观察者是否须要,推送的信息包含:部分或所有信息
public interface DisplayElement {
	public void display();
}
public interface Subject {
	public void registerObserver(Observer o);
	public void removeObserver(Observer o);
	public void notifyObservers();
}
public interface Observer {
	public void update(float temperate,float humidity,float pressure);
}
public class WeatherData implements Subject {
	private ArrayList<Observer> observers;
	private float temperate;
	private float humidity;
	private float pressure;
	public WeatherData(){
		observers = new ArrayList<Observer>();
	}
	@Override
	public void registerObserver(Observer o) {
		// TODO Auto-generated method stub
		observers.add(o);
	}

	@Override
	public void removeObserver(Observer o) {
		// TODO Auto-generated method stub
		int i = observers.indexOf(o);
		if(i >= 0)
		{
			observers.remove(i);
		}
	}

	@Override
	public void notifyObservers() {
		// TODO Auto-generated method stub
		for (int i = 0; i < observers.size(); i++) {
			Observer observer =observers.get(i);
			observer.update(temperate, humidity, pressure);
		}
	}
	
	public void measurementsChanged(){
		notifyObservers();
	}
	
	public void setMeasurements(float temperate,float  humidity,float  pressure){
		this.temperate = temperate;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}
}
public class CurrentConditionDisplay2 implements Observer,DisplayElement{
	private float temperate;
	private float humidity;
	private Observable observable;
	
	public CurrentConditionDisplay2(Observable weatherData) {
		this.observable = weatherData;
		this.observable.addObserver(this);
	}

	@Override
	public void display() {
		// TODO Auto-generated method stub
		System.out.println("Current conditions:"+temperate
				+" F degrees and "+humidity+" % humidity");
	}

	@Override
	public void update(Observable o, Object arg) {
		// TODO Auto-generated method stub
		 if (o instanceof WeatherData2) {
			 WeatherData2 weatherData2 = (WeatherData2) o;
			 this.humidity = weatherData2.getHumidity();
			 this.temperate = weatherData2.getTemperate();
			 display();
		}
	}



}
  • 拉模型:主题对象在通知观察者时,只传递少许信息。由观察者主动获取信息,把主题对象经过update()传递给观察者,观察者根据本身的需求,经过该引用获取数据。
public class WeatherData2 extends Observable {
	
	private float temperate;
	private float humidity;
	private float pressure;
	public WeatherData2(){
		
	}

	public void measurementsChanged(){
		setChanged();
		notifyObservers();
	}
	
	public void setMeasurements(float temperate,float  humidity,float  pressure){
		this.temperate = temperate;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}
	public float getTemperate() {
		return temperate;
	}
	public void setTemperate(float temperate) {
		this.temperate = temperate;
	}
	public float getHumidity() {
		return humidity;
	}
	public void setHumidity(float humidity) {
		this.humidity = humidity;
	}
	public float getPressure() {
		return pressure;
	}
	public void setPressure(float pressure) {
		this.pressure = pressure;
	}
	
}
public class CurrentConditionDisplay2 implements Observer,DisplayElement{
	private float temperate;
	private float humidity;
	private Observable observable;
	
	public CurrentConditionDisplay2(Observable weatherData) {
		this.observable = weatherData;
		this.observable.addObserver(this);
	}

	@Override
	public void display() {
		// TODO Auto-generated method stub
		System.out.println("Current conditions:"+temperate
				+" F degrees and "+humidity+" % humidity");
	}

	@Override
	public void update(Observable o, Object arg) {
		// TODO Auto-generated method stub
		 if (o instanceof WeatherData2) {
			 WeatherData2 weatherData2 = (WeatherData2) o;
			 this.humidity = weatherData2.getHumidity();
			 this.temperate = weatherData2.getTemperate();
			 display();
		}
	}



}

        以上两个例子中,推模式是用自定义接口,拉模式使用java内置观察者模式,拉/推模式均可以用自定义接口或java内置实现的。server

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