Observer

在上一篇Android常用的設(shè)計(jì)模式<一>：Builder模式中，介紹了Builder設(shè)計(jì)模式，本文將介紹另一種常用的設(shè)計(jì)模式，觀察者模式。觀察者模式涉及到兩個(gè)基本概念，觀察者和事件，思路很簡單，事件對(duì)象將所有關(guān)心該事件的觀察者對(duì)象添加在自己維護(hù)的一個(gè)Vector容器中，完成注冊(cè)，當(dāng)事件被觸發(fā)時(shí)，通知列表中的對(duì)象，這些對(duì)象依次執(zhí)行自己的動(dòng)作。

通過調(diào)用回調(diào)函數(shù)也可以在事件被觸發(fā)時(shí)完成不同的動(dòng)作，他們的區(qū)別在于，回調(diào)函數(shù)的方式是一對(duì)一的方式，而觀察者是一對(duì)多的方式。當(dāng)一個(gè)事件對(duì)應(yīng)多個(gè)觀察者對(duì)象，或者需要?jiǎng)討B(tài)添加或刪除某個(gè)觀察者對(duì)象時(shí)，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(yōu)于回調(diào)函數(shù)方式。

Java對(duì)觀察者模式的支持是通過Observable類和Observer接口實(shí)現(xiàn)的，事件通過繼承Observable類實(shí)現(xiàn)對(duì)觀察者的注冊(cè)和解除注冊(cè)功能，并在事件被觸發(fā)時(shí)調(diào)用setChanged函數(shù)和notifyObservers函數(shù)通知觀察者。

給個(gè)具體的例子，定義一個(gè)地震警報(bào)事件(EarthQuakeAlert)，有三個(gè)觀察者關(guān)注這個(gè)事件，大學(xué)(CollegeObserver)，科研機(jī)構(gòu)(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，并在事件被觸發(fā)時(shí)調(diào)用父類Observable的setChanged函數(shù)和notifyObservers函數(shù)通知所有的觀察者。

然后定義三個(gè)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(gè)觀察者
        CollegeObserver collegeObserver = new CollegeObserver();
        InstitutionObserver institutionObserver = new InstitutionObserver();
        CitizenObserver citizenObserver = new CitizenObserver();
        // 添加三個(gè)觀察者
        alert.addObserver(collegeObserver);
        alert.addObserver(institutionObserver);
        alert.addObserver(citizenObserver);
        // 觸發(fā)事件
        alert.setDetail("2008-5-12", "SiChuan, China");
        // 刪除一個(gè)觀察者
        alert.deleteObserver(collegeObserver);
        // 觸發(fā)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(fā)生，無參數(shù)
    public void notifyObservers() {
        notifyObservers(null);
    }
    // 通知事件發(fā)生，帶參數(shù)
    public void notifyObservers(Object arg) {
        Object[] arrLocal;

        // 將所有觀察者拷貝到一個(gè)臨時(shí)數(shù)組里（快照），然后對(duì)該數(shù)組中的觀察者執(zhí)行update函數(shù)
        // 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(diào)用每個(gè)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(shè)置標(biāo)志位，否則notifyObservers不起作用
    protected synchronized void setChanged() {
        changed = true;
    }
    // 返回觀察者數(shù)量
    public synchronized int countObservers() {
        return obs.size();
    }
}

Observer接口很簡單，只有一個(gè)update函數(shù)，觀察者實(shí)現(xiàn)這個(gè)函數(shù)，完成自己接收到事件后的操作。

public interface Observer {
    void update(Observable o, Object arg);
}

觀察者模式在android里面BroadCast，Eventbus等方面有所使用。