JavaScript Behavioral Design Patterns Observer pattern
Example
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');
var jane = new Employee('Jane');
var meetingAlerts = new Subject();
meetingAlerts.registerObserver(bob);
meetingAlerts.registerObserver(jane);
meetingAlerts.notifyObservers('4pm');
// Output:
// Bob: There is a meeting at 4pm
// Jane: There is a meeting at 4pm
