Android常用的設計模式<二>:觀察者模式
在上一篇Android常用的設計模式<一>:Builder模式中,介紹了Builder設計模式,本文將介紹另一種常用的設計模式,觀察者模式。觀察者模式涉及到兩個基本概念,觀察者和事件,思路很簡單,事件對象將所有關心該事件的觀察者對象添加在自己維護的一個Vector容器中,完成註冊,當事件被觸發時,通知列表中的對象,這些對象依次執行自己的動作。
通過調用回調函數也可以在事件被觸發時完成不同的動作,他們的區別在於,回調函數的方式是一對一的方式,而觀察者是一對多的方式。當一個事件對應多個觀察者對象,或者需要動態添加或刪除某個觀察者對象時,無論在邏輯的理解上還是代碼的清晰度上,觀察者模式都要優於回調函數方式。
Java對觀察者模式的支持是通過Observable類和Observer接口實現的,事件通過繼承Observable類實現對觀察者的註冊和解除註冊功能,並在事件被觸發時調用setChanged函數和notifyObservers函數通知觀察者。
給個具體的例子,定義一個地震警報事件(EarthQuakeAlert),有三個觀察者關注這個事件,大學(CollegeObserver),科研機構(InstitutionObserver)以及市民(CitizenObserver)。
首先定義EarthQuakeAlert:
public class EarthQuakeAlert extends Observable {
private String mLocation;
private String mTime;
public void setDetail(String time, String location) {
mTime = time;
mLocation = location;
setChanged();
notifyObservers(this);
}
public String getLocation() {
return mLocation;
}
public String getTime() {
return mTime;
}
}
EarthQuakeAlert繼承Observable,通過addObserver和deleteObserver添加或刪除觀察者,並在事件被觸發時調用父類Observable的setChanged函數和notifyObservers函數通知所有的觀察者。
然後定義三個觀察者:
CollegeObserver.java
public class CollegeObserver implements Observer {
@Override
public void update(Observable o, Object arg) {
EarthQuakeAlert alert = (EarthQuakeAlert) o;
String info = String.format("College received earthquake alert, location is %s, time is %s",
alert.getLocation(), alert.getTime());
System.out.println(info);
}
}
InstitutionObserver.java
public class InstitutionObserver implements Observer {
@Override
public void update(Observable o, Object arg) {
EarthQuakeAlert alert = (EarthQuakeAlert) o;
String info = String.format("Institution received earthquake alert, location is %s, time is %s",
alert.getLocation(), alert.getTime());
System.out.println(info);
}
}
CitizenObserver.java
public class CitizenObserver implements Observer {
@Override
public void update(Observable o, Object arg) {
EarthQuakeAlert alert = (EarthQuakeAlert) o;
String info = String.format("Citizens received earthquake alert, location is %s, time is %s",
alert.getLocation(), alert.getTime());
System.out.println(info);
}
}
使用過程如下:
public class ObserverPatternDemo {
public static void main(String[] args) {
// 定義事件
EarthQuakeAlert alert = new EarthQuakeAlert();
// 定義三個觀察者
CollegeObserver collegeObserver = new CollegeObserver();
InstitutionObserver institutionObserver = new InstitutionObserver();
CitizenObserver citizenObserver = new CitizenObserver();
// 添加三個觀察者
alert.addObserver(collegeObserver);
alert.addObserver(institutionObserver);
alert.addObserver(citizenObserver);
// 觸發事件
alert.setDetail("2008-5-12", "SiChuan, China");
// 刪除一個觀察者
alert.deleteObserver(collegeObserver);
// 觸發事件
alert.setDetail("2015-8-8", "Washington, USA");
}
}
輸出如下:
Citizens received earthquake alert, location is SiChuan, China, time is 2008-5-12
Institution received earthquake alert, location is SiChuan, China, time is 2008-5-12
College received earthquake alert, location is SiChuan, China, time is 2008-5-12
Citizens received earthquake alert, location is Washington, USA, time is 2015-8-8
Institution received earthquake alert, location is Washington, USA, time is 2015-8-8
我們看一下Observable和Observer代碼,原理都比較簡單。
Observable類代碼如下:
public class Observable {
private boolean changed = false;
// Vector存放觀察者
private Vector<Observer> obs;
public Observable() {
obs = new Vector<>();
}
// 添加新的觀察者
public synchronized void addObserver(Observer o) {
if (o == null)
throw new NullPointerException();
if (!obs.contains(o)) {
obs.addElement(o);
}
}
// 刪除觀察者
public synchronized void deleteObserver(Observer o) {
obs.removeElement(o);
}
// 通知事件發生,無參數
public void notifyObservers() {
notifyObservers(null);
}
// 通知事件發生,帶參數
public void notifyObservers(Object arg) {
Object[] arrLocal;
// 將所有觀察者拷貝到一個臨時數組裏(快照),然後對該數組中的觀察者執行update函數
// 這樣可以避免遍歷Vector過程中有新的觀察者加入或刪除
synchronized (this) {
if (!changed)
return;
arrLocal = obs.toArray();
clearChanged();
}
// 調用每個觀察者的update方法
for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}
// 刪除所有觀察者
public synchronized void deleteObservers() {
obs.removeAllElements();
}
// 設置標誌位,否則notifyObservers不起作用
protected synchronized void setChanged() {
changed = true;
}
// 返回觀察者數量
public synchronized int countObservers() {
return obs.size();
}
}
Observer接口很簡單,只有一個update函數,觀察者實現這個函數,完成自己接收到事件後的操作。
public interface Observer {
void update(Observable o, Object arg);
}
觀察者模式在android裏面BroadCast,Eventbus等方面有所使用。