6.22. Observer mode

6.22.1. Introduction ¶

意图： Define an one-to-many dependency between objects. When the state of an object changes, all objects that depend on it are notified and updated automatically.

主要解决： The problem of notifying other objects of a change in the state of one object, and taking into account ease of use and low coupling to ensure a high degree of collaboration.

何时使用： The state of an object (target object) changes, and all dependent objects (observer objects) will be notified for broadcast notification.

如何解决： Using object-oriented technology, this dependency can be weakened.

关键代码： In the abstract class there is an ArrayList for observers.

应用实例： 1. During the auction, the auctioneer observes the highest bid and then notifies other bidders of the bid. 2. In the Journey to the West, Wukong asked the Bodhisattva to surrender the Red Bodhisattva. The Bodhisattva sprinkled water to attract an old tortoise. The tortoise was the observer. He observed the Bodhisattva’s act of sprinkling water.

优点： 1. The observer and the observed are abstract and coupled. 2. Establish a set of trigger mechanism.

缺点： 1. If an observed object has many direct and indirect observers, it will take a lot of time to notify all the observers. 2. If there is a circular dependency between the observer and the observation target, the observation target will trigger a circular call between them, which may cause the system to crash. 3. The observer model has no corresponding mechanism to let the observer know how the observed object has changed, but only knows that the observed object has changed.

使用场景：

• An abstract model has two aspects, one of which depends on the other. Encapsulate these aspects in separate objects so that they can be changed and reused independently.

• The change of one object will lead to the change of one or more other objects, but we don’t know how many objects will change, which can reduce the coupling between objects.

• An object must notify other objects without knowing who those objects are.

• You need to create a trigger chain in the system, the behavior of An object will affect B object, and the behavior of B object will affect C object. You can use observer mode to create a chained trigger mechanism

注意事项： 2. Avoid circular references. 3. If executed sequentially, an observer error will cause the system to jam, usually in an asynchronous way.

6.22.2. Realize ¶

The observer mode uses three classes, Subject, Observer, and Client. The Subject object has methods to bind the observer to and unbind the observer from the Client object. We create Subject Class, Observer Abstract classes and extension of abstract classes Observer The entity class of the

ObserverPatternDemo Our demo class uses the Subject And entity class objects to demonstrate the observer pattern.

6.22.3. Step 1 ¶

Create the Subject class.

importjava.util.ArrayList;importjava.util.List;publicclassSubject{privateList<Observer>observers=newArrayList<Observer>();privateintstate;publicintgetState(){returnstate;}publicvoidsetState(intstate){this.state=state;notifyAllObservers();}publicvoidattach(Observerobserver){observers.add(observer);}publicvoidnotifyAllObservers(){for(Observerobserver:observers){observer.update();}}} 

6.22.4. Step 2 ¶

Create the Observer class.

publicabstractclassObserver{protectedSubjectsubject;publicabstractvoidupdate();} 

6.22.5. Step 3 ¶

Create an entity observer class.

publicclassBinaryObserverextendsObserver{publicBinaryObserver(Subjectsubject){this.subject=subject;this.subject.attach(this);}@Overridepublicvoidupdate(){System.out.println("Binary String:"+Integer.toBinaryString(subject.getState()));}} 

publicclassOctalObserverextendsObserver{publicOctalObserver(Subjectsubject){this.subject=subject;this.subject.attach(this);}@Overridepublicvoidupdate(){System.out.println("Octal String:"+Integer.toOctalString(subject.getState()));}} 

publicclassHexaObserverextendsObserver{publicHexaObserver(Subjectsubject){this.subject=subject;this.subject.attach(this);}@Overridepublicvoidupdate(){System.out.println("Hex String:"+Integer.toHexString(subject.getState()).toUpperCase());}} 

6.22.6. Step 4 ¶

Use Subject And physical observer objects.

publicclassObserverPatternDemo{publicstaticvoidmain(String[]args){Subjectsubject=newSubject();newHexaObserver(subject);newOctalObserver(subject);newBinaryObserver(subject);System.out.println("First state change: 15");subject.setState(15);System.out.println("Second state change: 10");subject.setState(10);}} 

6.22.7. Step 5 ¶

Execute the program and output the result:

First state change: 15 Hex String: F Octal String: 17 Binary String: 1111 Second state change: 10 Hex String: A Octal String: 12 Binary String: 1010