Head First--设计模式之观察者模式分析

Head First–设计模式之观察者模式分析

基本简介
观察者模式（Observer）完美的将观察者和被观察的对象分离开。举个例子，用户界面可以作为一个观察者，业务数据是被观察者，用户界面观察业务数据的变化，发现数据变化后，就显示在界面上。
观察者设计模式定义了对象间的一种一对多的组合关系，以便一个对象的状态发生变化时，所有依赖于它的对象都得到通知并自动刷新。[注：摘自百度百科 ]

在书中阐述的是气象站的实现，即当气象观测值改变时，主题会把这些状态值当作方法的参数，传给观察者。

气象站的UML类图

Subject.java

public interface Subject {

    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

Observer.java

public interface Observer {
    public void update(float temp, float humidity, float pressure);
}

DisplayElement.java

public interface DisplayElement {
    public void display();
}

WeatherData.java

public class WeatherData  implements Subject{

    private ArrayList<Observer> observers;
    private float temperature;
    private float humidity;
    private float pressure;

    public WeatherData(){
        this.observers = new ArrayList<Observer>();
    }

    @Override
    public void registerObserver(Observer o) {
        observers.add(o);

    }

    @Override
    public void removeObserver(Observer o) {
        if(observers.indexOf(o)>-1){
            observers.remove(o);
        }

    }

    @Override
    public void notifyObservers() {
        for(int i = 0; i< observers.size();i++){
            Observer o = observers.get(i);
            o.update(temperature, humidity, pressure);
        }
    }

    public void mesaurementsChanged(){
        notifyObservers();
    }

    public void setMeaseurements(float temperature, float humidity, float pressure){
        this.temperature = temperature;
        this.humidity = humidity;
        this.pressure = pressure;
        mesaurementsChanged();
    }
}

CurrentConditionsDisplay.java

public class CurrentConditionsDisplay implements DisplayElement, Observer{

    private float temperature;
    private float humidity;
    private Subject weatherData;

    public CurrentConditionsDisplay(Subject s){
        this.weatherData = s;
        weatherData.registerObserver(this);
    }

    @Override
    public void update(float temp, float humidity, float pressure) {
        this.temperature = temp;
        this.humidity = humidity;
        display();
    }


    @Override
    public void display() {
        System.out.println("Current conditions:" + temperature + "F degrees and " + humidity + "% humidity");
    }

}

StatisticsDisplay.java

public class StatisticsDisplay implements DisplayElement, Observer{
    private ArrayList temperature;
    private Subject weatherData;
    public StatisticsDisplay(Subject s){
        this.weatherData = s;
        weatherData.registerObserver(this);
        if(temperature == null){
            temperature = new ArrayList();
        }
    }
    @Override
    public void update(float temp, float humidity, float pressure) {
        this.temperature.add(temp);
        display();
    }

    @Override
    public void display() {
        float total = 0, max = (float) temperature.get(0), min = (float) temperature.get(0);
        for(int i = 0;i<temperature.size();i++){
            max = ((float) temperature.get(i))>max? (float) temperature.get(i):max;
            min = ((float) temperature.get(i))<min? (float) temperature.get(i):min;
            total = total + (float)temperature.get(i);

        }
        float avg = total/(temperature.size());

    System.out.println("Avg/Max/Min temperature = " + avg + "/" + max + "/" + min );
    }

}

测试代码 WeatherStation.java

public class WeatherStation {

    public static void main(String[] args) {
        WeatherData wd = new WeatherData();
        CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(wd);
        StatisticsDisplay currentDisplay1 = new StatisticsDisplay(wd);
        wd.setMeaseurements(80, 65, 30.4f);
        wd.setMeaseurements(82, 70, 29.4f);
    }

}

控制台输出结果：

Current conditions:80.0F degrees and 65.0% humidity
Avg/Max/Min temperature = 80.0/80.0/80.0
Current conditions:82.0F degrees and 70.0% humidity
Avg/Max/Min temperature = 81.0/82.0/80.0

总结一下，需要观察的对象主动通知观察者，需要将观察者创建观察者列表，然后对其进行增删操作，在notify方法中，遍历了列表中所有的对象，调用观察中的update方法。

java.util包（package）内最基本的包含 Observer接口与Observerable类，这和我们的Subject接口和Observer接口很相似。但是Observer接口与Observerable类 使用上更方便，可以用于push 或 pull 传送数据。