设计模式之观察者模式（java）

观察者模式 = 主题 + 观察者（也叫订阅者）

观察者订阅一个主题，当主题内容有更新的时候，观察者可以接受到更新通知，在这里还分为俩种获取更新的方式

推送 ： 一旦主题有更新，就主动推送更新通知到各个观察者

拉取 : 主题有更新的时候，观察者需要主动去获取更新

下面我们来看一下基于推送的天气预报的例子，气象站是收集天气信息，WeatherData对象来保存气象数据，并且当WeatherData数据变化时，显示装置（CurrentConditionsDisplay）要动态显示数据的改变。

首先先定义Subject主题接口

public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void notifyObservers();
}

定义观察者接口Observer

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

定义显示装置的展示功能接口

public interface DisplayElement {
public void display();
}

定义WeatherData类实现主题接口：

public class WeatherData implements Subject {
private ArrayList observers;
private float temperature;
private float humidity;
private float pressure;

public WeatherData() {
observers = new ArrayList();
}

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

@Override
public void removeObserver(Observer o) {
int i = observers.indexOf(o);
if(i > 0) {
observers.remove(i);
}
}

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

public void measurementsChanged() {
notifyObservers();
}

public void setMeasurements(float temperature, float humidity, float pressure) {
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}

public void setTemperature(float temperature) {
this.temperature = temperature;
}

public void setHumidity(float humidity) {
this.humidity = humidity;
}

public void setPressure(float pressure) {
this.pressure = pressure;
}

public float getTemperature() {
return temperature;
}

public float getHumidity() {
return humidity;
}

public float getPressure() {
return pressure;
}
}

定义观察者实现类并实现显示装置的显示功能接口：

public class CurrentConditionsDisplay implements Observer, DisplayElement {
private float temperature;
private float humidity;
private float pressure;
private Subject weatherData; //这个引用在下文没有用到，取消注册就会用到

public CurrentConditionsDisplay(Subject weatherData) {
this.weatherData = weatherData;
weatherData.registerObserver(this); //注册当前的观察者
}

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

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

主要的执行类（气象站）：

public class WeatherStation {
public static void main(String[] args) {
WeatherData weatherData = new WeatherData();

CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);

weatherData.setMeasurements(80, 65, 30);
weatherData.setMeasurements(82, 70, 29);
weatherData.setMeasurements(83, 90, 28);
}
}

运行结果截图：


上面时我们完全自己定义接口，下面我们看一下jdk原生内置的观察者模式，java.util.Obervable。（改一下上述的代码就可以，也能相互比较一下）

因为jdk帮我们实现了接口，我们就不需要定义Subject和Observer接口了，首先改变以下WeatherData：

public class WeatherData extends Observable {
private float temperature;
private float humidity;
private float pressure;

public WeatherData() {}

public void measurementsChanged() {
setChanged();
notifyObservers();
}

public void setMeasurements(float temperature, float humidity, float pressure) {
this.temperature = temperature;
this.humidity = humidity;
this.pressure = pressure;
measurementsChanged();
}

public void setTemperature(float temperature) {
this.temperature = temperature;
}

public void setHumidity(float humidity) {
this.humidity = humidity;
}

public void setPressure(float pressure) {
this.pressure = pressure;
}

public float getTemperature() {
return temperature;
}

public float getHumidity() {
return humidity;
}

public float getPressure() {
return pressure;
}
}

再修改一下CurrentConditionsDisplay：

public class CurrentConditionsDisplay implements Observer, DisplayElement {
private float temperature;
private float humidity;
private float pressure;
private Observable observable;

public CurrentConditionsDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}

public void update(Observable obs, Object arg) {
if(obs instanceof WeatherData) {
WeatherData weatherData = (WeatherData) obs;
this.temperature = weatherData.getTemperature();
this.humidity = weatherData.getHumidity();
this.pressure = weatherData.getPressure();
display();
}
}

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

运行结果：



原生的不好的地方大家应该都看到了，就是直接使用继承Observable类来实现观察者模式的，这样会让代码耦合比较紧密，好的设计原则应该是“多用组合，少用继承”。