Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
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Structure of design pattern
// Base publisher class. It should include the subscription management code and // notification methods. class EventManager is field listeners: hash map of eventTypes and EventListeners method subscribe(eventType, listener) is listeners.add(eventType, listener) method unsubscribe(eventType, listener) is listeners.remove(eventType, listener) method notify(eventType, a) is foreach listeners.of(eventType) as listener listener.update(a) // Concrete publisher, which contains real business logic interesting for some // subscribers. We could extend this class from a base publisher, but that is // not always possible in real life, since a publisher it might already have a // parent class. In this case, you can patch the subscription logic in with // composition, just like we did it here. class Editor is field events: EventManager field file: File constructor Editor() is events = new EventManager() // Business logic methods can notify subscribers about the changes. method openFile(filename) is this.file = new File(filename) events.notify('open', filename) method saveFile() is file.write() events.notify('save', file.name) // ... // Common subscribers interface. By the way, modern programming languages allow // to simplify this code and use functions as subscribers. interface EventListener is method update(a) // List of concrete listeners. They react to publisher updates by doing some // useful work. class LogOpenListener is field log: File constructor LogOpenListener(filename) is this.log = new File(filename) method update(filename) is log.write("Opened: " + filename) class EmailNotificationListener is field email: string constructor EmailNotificationListener(email) is this.email = email method update(filename) is system.email(email, "Someone has changed the file: " + filename) // Application can configure publishers and subscribers even in run time. class Application is method config() is editor = new TextEditor() editor.events.subscribe("open", new LogOpenListener("/path/to/log/file.txt")) editor.events.subscribe("save", new EmailNotificationListener("firstname.lastname@example.org"))