观察者模式

在观察者模式用于事件处理和授权。一个主题维护着一组*观察者。*然后，当事件发生时，受试者通知这些观察者。如果你曾经使用过 addEventListener，那么你已经使用了 Observer 模式。

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
    };
}

用法示例：

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