HeadFirst 设计模式（二）观察者模式

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观察者模式（Observer Pattern）

在对象之间建立一对多的依赖，这样一来，当一个对象改变状态，依赖它的对象都会收到通知，并自动更新。

设计原则

  1. 找出应用中可能需要变化之处，把它们独立出来，不要和那些不需要变化的代码混在一起
  2. 针对接口编程，而不是针对实现编程
  3. 多用组合，少用继承
  4. 为了交互对象之间的松耦合设计而努力

气象观测站

有一个气象观测站，可以获取到温度、湿度、气压等数据。每隔一定时间都会更新数据。

有三个布告板，根据不同的规则显示气象数据。（当前温度湿度布告板、气温统计布告板、气压变化布告板）

当气象观测站更新测量数据后，需要通知三个布告板，更新布告板上的数据。

问题所在：

  1. 针对具体实现编程，而非接口
  2. 对于每个新的布告板，都需要修改代码
  3. 无法在运行时动态地增加或删除布告板
  4. 尚未封装改变的部分

利用松耦合原则重新设计

定义接口：

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public interface Observer {
    public void update(float temp, float humidity, float pressure);
}
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public interface Subject {
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void noticeObservers();
}
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public interface DisplayElement {
    public void display();
}

定义气象站：

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public class WeatherData implements Subject {

    private List<Observer> 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(0);
        if (i >= 0) {
            observers.remove(i);
        }
    }

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

    public void measurementsChanged() {
        noticeObservers();
    }

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

定义布告板：

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public 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 display() {
        System.out.println("CurrentConditionsDisplay temperature:" + temperature + " humidity:" + humidity);
    }

    @Override
    public void update(float temp, float humidity, float pressure) {
        this.temperature = temp;
        this.humidity = humidity;
        display();
    }
}
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public class StatisticsDisplay implements Observer, DisplayElement {
	private float maxTemp = 0.0f;
	private float minTemp = 200;
	private float tempSum= 0.0f;
	private int numReadings;
	private WeatherData weatherData;

	public StatisticsDisplay(WeatherData weatherData) {
		this.weatherData = weatherData;
		weatherData.registerObserver(this);
	}

	public void update(float temp, float humidity, float pressure) {
		tempSum += temp;
		numReadings++;

		if (temp > maxTemp) {
			maxTemp = temp;
		}

		if (temp < minTemp) {
			minTemp = temp;
		}

		display();
	}

	public void display() {
		System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
			+ "/" + maxTemp + "/" + minTemp);
	}
}
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public class ForecastDisplay implements Observer, DisplayElement {
	private float currentPressure = 29.92f;
	private float lastPressure;
	private WeatherData weatherData;

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

	public void update(float temp, float humidity, float pressure) {
        lastPressure = currentPressure;
		currentPressure = pressure;

		display();
	}

	public void display() {
		System.out.print("Forecast: ");
		if (currentPressure > lastPressure) {
			System.out.println("Improving weather on the way!");
		} else if (currentPressure == lastPressure) {
			System.out.println("More of the same");
		} else if (currentPressure < lastPressure) {
			System.out.println("Watch out for cooler, rainy weather");
		}
	}
}

测试代码：

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public class Main {
    public static void main(String[] args) {
        WeatherData weatherData = new WeatherData();
        CurrentConditionsDisplay currentDisplay =
                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);
    }
}

运行结果：

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CurrentConditionsDisplay temperature:80.0 humidity:65.0
Avg/Max/Min temperature = 80.0/80.0/80.0
Forecast: Improving weather on the way!
CurrentConditionsDisplay temperature:82.0 humidity:70.0
Avg/Max/Min temperature = 81.0/82.0/80.0
Forecast: Watch out for cooler, rainy weather
CurrentConditionsDisplay temperature:78.0 humidity:90.0
Avg/Max/Min temperature = 80.0/82.0/78.0
Forecast: More of the same

上述实现中，每次数据更新后，都会主动推送数据到布告板。

其实有的布告板不需要那么多的数据，当然布告板可以通过拉取的方式获取自己想要的数据。

使用 JAVA 内置的观察者模式

java 内置了 Observer 接口和 Observable 类来实现观察者模式。

定义气象站：

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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 float getTemperature() {
		return temperature;
	}

	public float getHumidity() {
		return humidity;
	}

	public float getPressure() {
		return pressure;
	}
}

定义布告板：

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

	public CurrentConditionsDisplay(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();
			display();
		}
	}

	public void display() {
		System.out.println("Current conditions: " + temperature
			+ "F degrees and " + humidity + "% humidity");
	}
}
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public class StatisticsDisplay implements Observer, DisplayElement {
	private float maxTemp = 0.0f;
	private float minTemp = 200;
	private float tempSum= 0.0f;
	private int numReadings;

	public StatisticsDisplay(Observable observable) {
		observable.addObserver(this);
	}

	public void update(Observable observable, Object arg) {
		if (observable instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)observable;
			float temp = weatherData.getTemperature();
			tempSum += temp;
			numReadings++;

			if (temp > maxTemp) {
				maxTemp = temp;
			}

			if (temp < minTemp) {
				minTemp = temp;
			}

			display();
		}
	}

	public void display() {
		System.out.println("Avg/Max/Min temperature = " + (tempSum / numReadings)
			+ "/" + maxTemp + "/" + minTemp);
	}
}
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public class ForecastDisplay implements Observer, DisplayElement {
	private float currentPressure = 29.92f;
	private float lastPressure;

	public ForecastDisplay(Observable observable) {
		observable.addObserver(this);
	}

	public void update(Observable observable, Object arg) {
		if (observable instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)observable;
			lastPressure = currentPressure;
			currentPressure = weatherData.getPressure();
			display();
		}
	}

	public void display() {
		System.out.print("Forecast: ");
		if (currentPressure > lastPressure) {
			System.out.println("Improving weather on the way!");
		} else if (currentPressure == lastPressure) {
			System.out.println("More of the same");
		} else if (currentPressure < lastPressure) {
			System.out.println("Watch out for cooler, rainy weather");
		}
	}
}

测试代码：

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

        CurrentConditionsDisplay currentConditions = 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);
    }
}

运行结果：

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Forecast: Improving weather on the way!
Avg/Max/Min temperature = 80.0/80.0/80.0
Current conditions: 80.0F degrees and 65.0% humidity
Forecast: Watch out for cooler, rainy weather
Avg/Max/Min temperature = 81.0/82.0/80.0
Current conditions: 82.0F degrees and 70.0% humidity
Forecast: More of the same
Avg/Max/Min temperature = 80.0/82.0/78.0
Current conditions: 78.0F degrees and 90.0% humidity

这里使用 notifyObservers() 方法，通知布告板，布告板收到通知后主动拉取数据。

也可以改为 notifyObservers(Object arg) 方法，主动推送数据。

Observalbe 类中，setChanged() 方法用于标志是否需要发送通知。（假如气象站每秒更新一次数据，而布告板只需要每小时更新一次数据，此时，可以通过 setChanged() 方法来灵活地控制通知时间）

总结

为了交互对象之间的松耦合设计而努力。

在对象之间建立一对多的依赖，这样一来，当一个对象改变状态，依赖它的对象都会收到通知，并自动更新。