觀察者模式（Observer Pattern）
在對象之間建立一對多的依賴，這樣一來，當一個對象改變狀態，依賴它的對象都會收到通知，並自動更新。
設計原則
- 找出應用中可能需要變化之處，把它們獨立出來，不要和那些不需要變化的代碼混在一起
- 針對接口編程，而不是針對實現編程
- 多用組合，少用繼承
- 爲了交互對象之間的鬆耦合設計而努力
氣象觀測站
有一個氣象觀測站，可以獲取到溫度、溼度、氣壓等數據。每隔一定時間都會更新數據。
有三個佈告板，根據不同的規則顯示氣象數據。（當前溫度溼度佈告板、氣溫統計佈告板、氣壓變化佈告板）
當氣象觀測站更新測量數據後，需要通知三個佈告板，更新佈告板上的數據。
問題所在：
- 針對具體實現編程，而非接口
- 對於每個新的佈告板，都需要修改代碼
- 無法在運行時動態地增加或刪除佈告板
- 尚未封裝改變的部分
利用鬆耦合原則重新設計
定義接口：
public interface Observer {
public void update(float temp, float humidity, float pressure);
}
public interface Subject {
public void registerObserver(Observer o);
public void removeObserver(Observer o);
public void noticeObservers();
}
public interface DisplayElement {
public void display();
}
定義氣象站：
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();
}
}
定義佈告板：
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();
}
}
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);
}
}
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");
}
}
}
測試代碼：
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);
}
}
運行結果：
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
上述實現中，每次數據更新後，都會主動推送數據到佈告板。
其實有的佈告板不需要那麼多的數據，當然佈告板可以通過拉取的方式獲取自己想要的數據。
使用 JAVA 內置的觀察者模式
java 內置了 Observer 接口和 Observable 類來實現觀察者模式。
定義氣象站：
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;
}
}
定義佈告板：
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");
}
}
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);
}
}
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");
}
}
}
測試代碼：
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);
}
}
運行結果：
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
這裏使用 notifyObservers() 方法，通知佈告板，佈告板收到通知後主動拉取數據。
也可以改爲 notifyObservers(Object arg) 方法，主動推送數據。
Observalbe 類中，setChanged() 方法用於標誌是否需要發送通知。（假如氣象站每秒更新一次數據，而佈告板只需要每小時更新一次數據，此時，可以通過 setChanged() 方法來靈活地控制通知時間）
總結
爲了交互對象之間的鬆耦合設計而努力。
在對象之間建立一對多的依賴，這樣一來，當一個對象改變狀態，依賴它的對象都會收到通知，並自動更新。