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Observer Pattern

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This commit is contained in:
kim
2021-01-17 23:32:44 +09:00
parent 8f907de6ff
commit 8455511ac8
3 changed files with 218 additions and 0 deletions
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22
design-pattern/src/Observer/example03/Application.java Normal file
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package Observer.example03;
public class Application {
public static void main(String[] args) {
// 1. 제네릭
// 2. 델리케이트
// 3. 내부에 옵저버를 넣는다.
Button button = new Button();
button.addObserver(new Observable.Observer<String>() {
@Override
public void update(Observable<String> o, String arg) {
System.out.println(o + " is Clicked");
}
});
button.onClick();
button.onClick();
button.onClick();
}
}

23
design-pattern/src/Observer/example03/Button.java Normal file
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package Observer.example03;
public class Button {
private Observable<String> observable;
public Button() {
observable = new Observable<>();
}
public void addObserver(Observable.Observer<String> o) {
observable.addObserver(o);
}
public void notifyObservers() {
observable.notifyObservers(null);
}
public void onClick() {
observable.setChanged();
notifyObservers();
}
}

173
design-pattern/src/Observer/example03/Observable.java Normal file
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package Observer.example03;
import java.util.Vector;
public class Observable<T> {
private boolean changed = false;
private Vector<Observer<T>> obs;
/** Construct an Observable with zero Observers. */
public Observable() {
obs = new Vector<>();
}
/**
* Adds an observer to the set of observers for this object, provided
* that it is not the same as some observer already in the set.
* The order in which notifications will be delivered to multiple
* observers is not specified. See the class comment.
*
* @param o an observer to be added.
* @throws NullPointerException if the parameter o is null.
*/
public synchronized void addObserver(Observer<T> o) {
if (o == null)
throw new NullPointerException();
if (!obs.contains(o)) {
obs.addElement(o);
}
}
/**
* Deletes an observer from the set of observers of this object.
* Passing {@code null} to this method will have no effect.
* @param o the observer to be deleted.
*/
public synchronized void deleteObserver(Observer<T> o) {
obs.removeElement(o);
}
/**
* If this object has changed, as indicated by the
* {@code hasChanged} method, then notify all of its observers
* and then call the {@code clearChanged} method to
* indicate that this object has no longer changed.
* <p>
* Each observer has its {@code update} method called with two
* arguments: this observable object and {@code null}. In other
* words, this method is equivalent to:
* <blockquote>{@code
* notifyObservers(null)}</blockquote>
*
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers() {
notifyObservers(null);
}
/**
* If this object has changed, as indicated by the
* {@code hasChanged} method, then notify all of its observers
* and then call the {@code clearChanged} method to indicate
* that this object has no longer changed.
* <p>
* Each observer has its {@code update} method called with two
* arguments: this observable object and the {@code arg} argument.
*
* @param arg any object.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers(T arg) {
/*
* a temporary array buffer, used as a snapshot of the state of
* current Observers.
*/
Vector<Observer<T>> arrLocal;
synchronized (this) {
/* We don't want the Observer doing callbacks into
* arbitrary code while holding its own Monitor.
* The code where we extract each Observable from
* the Vector and store the state of the Observer
* needs synchronization, but notifying observers
* does not (should not). The worst result of any
* potential race-condition here is that:
* 1) a newly-added Observer will miss a
* notification in progress
* 2) a recently unregistered Observer will be
* wrongly notified when it doesn't care
*/
if (!changed)
return;
arrLocal = (Vector<Observer<T>>) obs.clone();
clearChanged();
}
for (Observer<T> observer: arrLocal) {
observer.update(this, arg);
}
}
/**
* Clears the observer list so that this object no longer has any observers.
*/
public synchronized void deleteObservers() {
obs.removeAllElements();
}
/**
* Marks this {@code Observable} object as having been changed; the
* {@code hasChanged} method will now return {@code true}.
*/
public synchronized void setChanged() {
changed = true;
}
/**
* Indicates that this object has no longer changed, or that it has
* already notified all of its observers of its most recent change,
* so that the {@code hasChanged} method will now return {@code false}.
* This method is called automatically by the
* {@code notifyObservers} methods.
*
* @see java.util.Observable#notifyObservers()
* @see java.util.Observable#notifyObservers(java.lang.Object)
*/
protected synchronized void clearChanged() {
changed = false;
}
/**
* Tests if this object has changed.
*
* @return {@code true} if and only if the {@code setChanged}
* method has been called more recently than the
* {@code clearChanged} method on this object;
* {@code false} otherwise.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#setChanged()
*/
public synchronized boolean hasChanged() {
return changed;
}
/**
* Returns the number of observers of this {@code Observable} object.
*
* @return the number of observers of this object.
*/
public synchronized int countObservers() {
return obs.size();
}
public interface Observer<T> {
/**
* This method is called whenever the observed object is changed. An
* application calls an {@code Observable} object's
* {@code notifyObservers} method to have all the object's
* observers notified of the change.
*
* @param o the observable object.
* @param arg an argument passed to the {@code notifyObservers}
* method.
*/
void update(Observable<T> o, T arg);
}
}