This article mainly introduces the example of using the Observer Pattern in Java Design Pattern to develop a WeChat public account. The WeChat SDK and other parts of Java will not be detailed here. Only focus on the key parts and the embodiment of the advantages of the observer mode during the development process. Friends in need can refer to

Do you still remember how gangsters cooperate in committing crimes in police movies? When a gang is committing a theft, there are always one or two people standing guard at the door. If there is any sign of trouble, the accomplices inside will be immediately notified to make an emergency retreat. Maybe the person who is cheating doesn’t necessarily know every accomplice inside; and there may be new guys inside who don’t know the one who is cheating. But it doesn't matter. It won't affect the communication between them, because they have already agreed on the secret code.
Haha, the relationship between the watcher and the thief mentioned above is a living example of the observer pattern in reality.

The Observer pattern is also known as the Publish/Subscribe pattern. GOF defines the observer pattern as follows: defines a one-to-many dependency relationship between objects . When the status of an object changes, all objects that depend on it get Notified and updated automatically.
Here let’s talk about an important principle of Object-oriented design - the single responsibility principle. Each object of the system should therefore focus on a discrete abstraction within the problem domain. So ideally, an object does only one thing. This brings many benefits in development: it provides reusability and maintainability, and is also a good basis for reconstruction.
So almost all design patterns are based on this basic Design principle. I think the origin of the observer pattern should be in the processing of GUI and business data, because most of the examples explaining the observer pattern now are on this subject. But the application of the observer pattern is by no means limited to this aspect.

Okay, the understanding of the definition always needs to be analyzed by examples. Today's WeChat service account is quite popular. Let's introduce the observer mode to everyone using the WeChat service account as the background.
Look at a picture:

Each user has 3 lines in the above picture, which have been omitted to make the picture clear.
As shown in the picture above, the service account is our subject, and the user is the observer. Now let’s clarify the functions:
1. The service account is the topic, and the business is pushing messages
2. Observers only need to subscribe to the topic, and new messages will be sent
3. When you don’t want this When receiving topic messages, unsubscribe
4. As long as the service account is still there, there will always be people subscribing
Okay, now let’s take a look at the class diagram of the observer pattern:

The next step is code time. We simulate a WeChat 3D lottery service account and some subscribers.
First start writing our theme Interface , and observer interface:

package?com.zhy.pattern.observer;?
?
/**?
?*?主題接口，所有的主題必須實(shí)現(xiàn)此接口?
?*?
?*?@author?zhy?
?*?
?*/?
public?interface?Subject?
{?
??/**?
???*?注冊一個觀察著?
???*?
???*?@param?observer?
???*/?
??public?void?registerObserver(Observer?observer);?
?
??/**?
???*?移除一個觀察者?
???*?
???*?@param?observer?
???*/?
??public?void?removeObserver(Observer?observer);?
?
??/**?
???*?通知所有的觀察著?
???*/?
??public?void?notifyObservers();?
?
}?
package?com.zhy.pattern.observer;?
?
/**?
?*?@author?zhy?所有的觀察者需要實(shí)現(xiàn)此接口?
?*/?
public?interface?Observer?
{?
??public?void?update(String?msg);?
?
}

Next, the implementation class of the 3D service account:

package?com.zhy.pattern.observer;?
?
import?java.util.ArrayList;?
import?java.util.List;?
?
public?class?ObjectFor3D?implements?Subject?
{?
??private?List<observer>?observers?=?new?ArrayList<observer>();?
??/**?
???*?3D彩票的號碼?
???*/?
??private?String?msg;?
?
??@Override?
??public?void?registerObserver(Observer?observer)?
??{?
????observers.add(observer);?
??}?
?
??@Override?
??public?void?removeObserver(Observer?observer)?
??{?
????int?index?=?observers.indexOf(observer);?
????if?(index?>=?0)?
????{?
??????observers.remove(index);?
????}?
??}?
?
??@Override?
??public?void?notifyObservers()?
??{?
????for?(Observer?observer?:?observers)?
????{?
??????observer.update(msg);?
????}?
??}?
?
??/**?
???*?主題更新消息?
???*?
???*?@param?msg?
???*/?
??public?void?setMsg(String?msg)?
??{?
????this.msg?=?msg;?
?????
????notifyObservers();?
??}?
?
}</observer></observer>

Simulate two users :

package?com.zhy.pattern.observer;?
?
public?class?Observer1?implements?Observer?
{?
?
??private?Subject?subject;?
?
??public?Observer1(Subject?subject)?
??{?
????this.subject?=?subject;?
????subject.registerObserver(this);?
??}?
?
??@Override?
??public?void?update(String?msg)?
??{?
????System.out.println("observer1?得到?3D?號碼?-->"?+?msg?+?",?我要記下來。");?
??}?
?
}

package?com.zhy.pattern.observer;?
?
public?class?Observer2?implements?Observer?
{?
??private?Subject?subject?;??
???
??public?Observer2(Subject?subject)?
??{?
????this.subject?=?subject?;?
????subject.registerObserver(this);?
??}?
???
??@Override?
??public?void?update(String?msg)?
??{?
????System.out.println("observer2?得到?3D?號碼?-->"?+?msg?+?"我要告訴舍友們。");?
??}?
???
???
?
}

It can be seen that the service account maintains all users who subscribe to messages from it. When there are new messages in the service account, all users are notified. The entire architecture is a loose coupling. The implementation of the theme does not depend on the user. When a new user is added, the code of the theme does not need to be changed; how the user processes the obtained data has nothing to do with the theme;
Finally, take a look at the test code:

package?com.zhy.pattern.observer.test;?
?
import?com.zhy.pattern.observer.ObjectFor3D;?
import?com.zhy.pattern.observer.Observer;?
import?com.zhy.pattern.observer.Observer1;?
import?com.zhy.pattern.observer.Observer2;?
import?com.zhy.pattern.observer.Subject;?
?
public?class?Test?
{?
??public?static?void?main(String[]?args)?
??{?
????//模擬一個3D的服務(wù)號?
????ObjectFor3D?subjectFor3d?=?new?ObjectFor3D();?
????//客戶1?
????Observer?observer1?=?new?Observer1(subjectFor3d);?
????Observer?observer2?=?new?Observer2(subjectFor3d);?
?
????subjectFor3d.setMsg("20140420的3D號碼是：127"?);?
????subjectFor3d.setMsg("20140421的3D號碼是：333"?);?
?????
??}?
}

Output results:

observer1?得到?3D?號碼?-->20140420的3D號碼是：127,?我要記下來。?
observer2?得到?3D?號碼?-->20140420的3D號碼是：127我要告訴舍友們。?
observer1?得到?3D?號碼?-->20140421的3D號碼是：333,?我要記下來。?
observer2?得到?3D?號碼?-->20140421的3D號碼是：333我要告訴舍友們。

There are many places in JDK or Andorid that implement the observer mode, such as XXXView.addXXXListenter, and of course XXXView.setOnXXXListener It is not necessarily the observer mode, because the observer mode is a one-to-many relationship, and setXXXListener is a 1-to-1 relationship and should be called a callback.

恭喜你學(xué)會了觀察者模式，上面的觀察者模式使我們從無到有的寫出，當(dāng)然了java中已經(jīng)幫我們實(shí)現(xiàn)了觀察者模式，借助于java.util.Observable和java.util.Observer。
下面我們使用Java內(nèi)置的類實(shí)現(xiàn)觀察者模式：

首先是一個3D彩票服務(wù)號主題：

package?com.zhy.pattern.observer.java;?
?
import?java.util.Observable;?
?
public?class?SubjectFor3d?extends?Observable?
{?
??private?String?msg?;??
???
???
??public?String?getMsg()?
??{?
????return?msg;?
??}?
?
?
??/**?
???*?主題更新消息?
???*?
???*?@param?msg?
???*/?
??public?void?setMsg(String?msg)?
??{?
????this.msg?=?msg?;?
????setChanged();?
????notifyObservers();?
??}?
}

下面是一個雙色球的服務(wù)號主題：

package?com.zhy.pattern.observer.java;?
?
import?java.util.Observable;?
?
public?class?SubjectForSSQ?extends?Observable?
{?
??private?String?msg?;??
???
???
??public?String?getMsg()?
??{?
????return?msg;?
??}?
?
?
??/**?
???*?主題更新消息?
???*?
???*?@param?msg?
???*/?
??public?void?setMsg(String?msg)?
??{?
????this.msg?=?msg?;?
????setChanged();?
????notifyObservers();?
??}?
}

最后是我們的使用者：

package?com.zhy.pattern.observer.java;?
?
import?java.util.Observable;?
import?java.util.Observer;?
?
public?class?Observer1?implements?Observer?
{?
?
??public?void?registerSubject(Observable?observable)?
??{?
????observable.addObserver(this);?
??}?
?
??@Override?
??public?void?update(Observable?o,?Object?arg)?
??{?
????if?(o?instanceof?SubjectFor3d)?
????{?
??????SubjectFor3d?subjectFor3d?=?(SubjectFor3d)?o;?
??????System.out.println("subjectFor3d's?msg?--?>"?+?subjectFor3d.getMsg());?
????}?
?
????if?(o?instanceof?SubjectForSSQ)?
????{?
??????SubjectForSSQ?subjectForSSQ?=?(SubjectForSSQ)?o;?
??????System.out.println("subjectForSSQ's?msg?--?>"?+?subjectForSSQ.getMsg());?
????}?
??}?
}

看一個測試代碼：

package?com.zhy.pattern.observer.java;?
?
public?class?Test?
{?
??public?static?void?main(String[]?args)?
??{?
????SubjectFor3d?subjectFor3d?=?new?SubjectFor3d()?;?
????SubjectForSSQ?subjectForSSQ?=?new?SubjectForSSQ()?;?
?????
????Observer1?observer1?=?new?Observer1();?
????observer1.registerSubject(subjectFor3d);?
????observer1.registerSubject(subjectForSSQ);?
?????
?????
????subjectFor3d.setMsg("hello?3d'nums?:?110?");?
????subjectForSSQ.setMsg("ssq'nums?:?12,13,31,5,4,3?15");?
?????
??}?
}

測試結(jié)果：

subjectFor3d's?msg?--?>hello?3d'nums?:?110??
subjectForSSQ's?msg?--?>ssq'nums?:?12,13,31,5,4,3?15

可以看出，使用Java內(nèi)置的類實(shí)現(xiàn)觀察者模式，代碼非常簡潔，對了addObserver,removeObserver,notifyObservers都已經(jīng)為我們實(shí)現(xiàn)了，所有可以看出Observable（主題）是一個類，而不是一個接口，基本上書上都對于Java的如此設(shè)計抱有反面的態(tài)度，覺得Java內(nèi)置的觀察者模式，違法了面向接口編程這個原則，但是如果轉(zhuǎn)念想一想，的確你拿一個主題在這寫觀察者模式（我們自己的實(shí)現(xiàn)），接口的思想很好，但是如果現(xiàn)在繼續(xù)添加很多個主題，每個主題的ddObserver,removeObserver,notifyObservers代碼基本都是相同的吧，接口是無法實(shí)現(xiàn)代碼復(fù)用的，而且也沒有辦法使用組合的模式實(shí)現(xiàn)這三個方法的復(fù)用，所以我覺得這里把這三個方法在類中實(shí)現(xiàn)是合理的。

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