Page 143 - JavaScript

Chapter 12: Behavioral Design Patterns

Examples

Observer pattern

The Observer pattern is used for event handling and delegation. A subject maintains a collection
of observers. The subject then notifies these observers whenever an event occurs. If you've ever
used addEventListener then you've utilized the Observer pattern.

function Subject() {
this.observers = []; // Observers listening to the subject

this.registerObserver = function(observer) {
// Add an observer if it isn't already being tracked
if (this.observers.indexOf(observer) === -1) {
this.observers.push(observer);
}
};

this.unregisterObserver = function(observer) {
// Removes a previously registered observer
var index = this.observers.indexOf(observer);
if (index > -1) {
this.observers.splice(index, 1);
}
};

this.notifyObservers = function(message) {
// Send a message to all observers
this.observers.forEach(function(observer) {
observer.notify(message);
});
};
}

function Observer() {
this.notify = function(message) {
// Every observer must implement this function
};
}

Example usage:

function Employee(name) {
this.name = name;

// Implement `notify` so the subject can pass us messages
this.notify = function(meetingTime) {
console.log(this.name + ': There is a meeting at ' + meetingTime);
};
}

var bob = new Employee('Bob');

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