观察者模式

观察者模式定义了对象之间的一对多依赖，这样一来，当一个对象改变状态时，它的所有依赖都会收到通知并自动更新。

示例—气象观测站的监测系统

已有一个WeatherData对象，负责追踪目前的天气状况（温度、湿度、欺压）。建立一个应用， 有三种布告板，分别显示目前的状况、气象统计及简单的预报。当WeatherData对象获得最新的测量数据时，三种布告板必须实时更新。而且可以扩展，公布一组api，可让其他开发人员写出自己的气象布告板，并插入此应用。

有两种方式实现：自定义观察者和内置观察类

自定义观察者

• UML图表示

气象站三种布告板的设计-自定义观察者

• 代码演示

import java.util.ArrayList;
import java.util.List;

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

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();
    }
}

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

interface DisplayElement{
    void display();
}

class CurrentConditionsDisplay implements Observer,DisplayElement{

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

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

class StatisticsDisplay implements Observer, DisplayElement{

    private List<Float> temperatureList;
    private List<Float> humidityList;
    private List<Float> pressureList;

    private Subject weatherData;

    public StatisticsDisplay(Subject weatherData){
        this.weatherData = weatherData;
        temperatureList = new ArrayList<Float>();
        humidityList = new ArrayList<Float>();
        pressureList = new ArrayList<Float>();
        weatherData.registerObserver(this);
    }

    @Override
    public void update(float temp, float humidity, float pressure) {
        temperatureList.add(temp);
        humidityList.add(humidity);
        pressureList.add(pressure);
        display();
    }

    @Override
    public void display() {
        System.out.println("Statistics Average Data: " + calAverage(temperatureList)
                + "F degrees and " + calAverage(humidityList) + "% humidity and average pressure is "
                + calAverage(pressureList));
    }

    private Float calAverage(List<Float> list){
        Float sum = 0.0f;
        for (Float item : list){
            sum += item;
        }
        return sum / (float) list.size();
    }
}

class ForecastDisplay implements Observer, DisplayElement{

    private float temperature;
    private float humidity;
    private float pressure;

    private Subject weatherData;

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


    @Override
    public void update(float temp, float humidity, float pressure) {
        this.temperature = temp + (float) 1;
        this.humidity = humidity - (float) 1;
        this.pressure = pressure +  0.5f;
        display();
    }

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

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

        CurrentConditionsDisplay currentConditionsDisplay = new CurrentConditionsDisplay(weatherData);
        StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
        ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);


        weatherData.setMeasurements(80,65,30.4f);
        weatherData.setMeasurements(82,70,29.2f);
        weatherData.setMeasurements(78,90,29.2f);

    }
}

• 测试结果

Current conditions: 80.0F degrees and 65.0% humidity
Statistics Average Data: 80.0F degrees and 65.0% humidity and average pressure is 30.4
Forecast conditions: 81.0F degrees and 64.0% humidity and pressure is 30.9
Current conditions: 82.0F degrees and 70.0% humidity
Statistics Average Data: 81.0F degrees and 67.5% humidity and average pressure is 29.8
Forecast conditions: 83.0F degrees and 69.0% humidity and pressure is 29.7
Current conditions: 78.0F degrees and 90.0% humidity
Statistics Average Data: 80.0F degrees and 75.0% humidity and average pressure is 29.6
Forecast conditions: 79.0F degrees and 89.0% humidity and pressure is 29.7

内置观察类

• UML图表示

气象站三种布告板的设计-内置观察类

• 代码演示

import java.util.ArrayList;
import java.util.List;
import java.util.Observable;
import java.util.Observer;

class WeatherData2 extends Observable{
    private float temperature;
    private float humidity;
    private float pressure;

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

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

    float getTemperature() {
        return temperature;
    }

    float getHumidity() {
        return humidity;
    }

    float getPressure(){
        return pressure;
    }

}

class CurrentConditionsDisplay2 implements Observer,DisplayElement{

    private float temperature;
    private float humidity;
    private Observable observable;

    public CurrentConditionsDisplay2(Observable observable){
        this.observable = observable;
        observable.addObserver(this);
    }

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

    @Override
    public void update(Observable o, Object arg) {
        if (o instanceof WeatherData2){
            WeatherData2 weatherData2 = (WeatherData2) o;
            this.temperature = weatherData2.getTemperature();
            this.humidity = weatherData2.getHumidity();
            display();
        }
    }
}

class StatisticsDisplay2 implements Observer, DisplayElement{

    private Observable observable;
    private List<Float> temperatureList;
    private List<Float> humidityList;
    private List<Float> pressureList;

    public StatisticsDisplay2(Observable observable){
        this.observable = observable;
        temperatureList = new ArrayList<Float>();
        humidityList = new ArrayList<Float>();
        pressureList = new ArrayList<Float>();
        observable.addObserver(this);
    }

    @Override
    public void display() {
        System.out.println("Statistics Average Data: " + calAverage(temperatureList)
                + "F degrees and " + calAverage(humidityList) + "% humidity and average pressure is "
                + calAverage(pressureList));
    }

    private Float calAverage(List<Float> list){
        Float sum = 0.0f;
        for (Float item : list){
            sum += item;
        }
        return sum / (float) list.size();
    }

    @Override
    public void update(Observable o, Object arg) {
        if (o instanceof WeatherData2){
            WeatherData2 weatherData2 = (WeatherData2) o;
            temperatureList.add(weatherData2.getTemperature());
            humidityList.add(weatherData2.getHumidity());
            pressureList.add(weatherData2.getPressure());
            display();
        }
    }
}

class ForecastDisplay2 implements Observer, DisplayElement{

    private float temperature;
    private float humidity;
    private float pressure;

    private Observable observable;

    public ForecastDisplay2(Observable observable){
        this.observable = observable;
        observable.addObserver(this);
    }

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

    @Override
    public void update(Observable o, Object arg) {
        if (o instanceof WeatherData2){
            WeatherData2 weatherData2 = (WeatherData2) o;
            this.temperature = weatherData2.getTemperature() + (float) 1;
            this.humidity = weatherData2.getHumidity() - (float) 1;
            this.pressure = weatherData2.getPressure() +  0.5f;
            display();
        }
    }
}

public class WeatherStation2 {
    public static void main(String[] args) {
        WeatherData2 weatherData2 = new WeatherData2();

        CurrentConditionsDisplay2 currentConditionsDisplay = new CurrentConditionsDisplay2(weatherData2);
        StatisticsDisplay2 statisticsDisplay = new StatisticsDisplay2(weatherData2);
        ForecastDisplay2 forecastDisplay = new ForecastDisplay2(weatherData2);


        weatherData2.setMeasurements(80,65,30.4f);
        weatherData2.setMeasurements(82,70,29.2f);
        weatherData2.setMeasurements(78,90,29.2f);

    }
}

• 测试结果

Forecast conditions: 81.0F degrees and 64.0% humidity and pressure is 30.9
Statistics Average Data: 80.0F degrees and 65.0% humidity and average pressure is 30.4
Current conditions: 80.0F degrees and 65.0% humidity
Forecast conditions: 83.0F degrees and 69.0% humidity and pressure is 29.7
Statistics Average Data: 81.0F degrees and 67.5% humidity and average pressure is 29.8
Current conditions: 82.0F degrees and 70.0% humidity
Forecast conditions: 79.0F degrees and 89.0% humidity and pressure is 29.7
Statistics Average Data: 80.0F degrees and 75.0% humidity and average pressure is 29.6
Current conditions: 78.0F degrees and 90.0% humidity

结论

两种方式都可以实现观察者模式，各有特色。两种模式都能实现“推”和“拉”的方式。

• 自定义观察者：可以实现用接口开发，而不是实现开发，实现方式比较自由，但需要自定义的代码多。
• 内置观察类：不能实现接口开发，必须用继承的方式开发，但不需要自己实现注册、取消等代码。