Question

I’m implementing a simple pub/sub in javascript to help me fully understand how this pattern works:

//obj to hold references to all subscribers
pubSubCache = {};

//subscribe function - push the topic and function in to pubSubCache
subscribe = function (topic, fn) {
    pubSubCache[topic] = fn;
};

//publish function
publish = function (topic, obj) {
    var func;
    console.log(obj);
    console.log(pubSubCache);
    // If topic is found in the cache
    if (pubSubCache.hasOwnProperty(topic)) {
        //Loop over the properties of the pubsub obj - the properties are functions
        //for each of the funcitons subscribed to the topic - call that function and feed it the obj           
        for (func in pubSubCache[topic]) {
            //this console.log returns a long list of functions - overloadsetter,overloadgetter,extend etc
            //I expected to see the 'therapist' function here...
            console.log(func);
            //error occurs here - it should be calling the therapist function
            pubSubCache[topic][func](obj);
        }
    }
}; 


function therapist (data) {
    alert(data.response);
}

subscribe('patient/unhappy', therapist);
publish('patient/unhappy',{response:'Let me prescribe you some pills'})

​

I’m almost there but seem to have a strange problem in my code. The publisher function searches through the object that holds all the references to the subscribers and successfully finds a match. Then when I try to do a for in loop to get a reference to the function that is subscribed I get back this long list of functions instead of the function I want:

overloadSetter
overloadGetter
extend
implement
hide
protect
$family
$constructor

I initially thought these functions were from the prototype of the function but they are not.

Any ideas? Hope this makes sense.

Answer 1

In your subscribe, you surely want to allow for multiple subscriptions to a topic. The cache entry for each topic should therefor be an array :

 //subscribe function - push the topic and function in to pubSubCache
subscribe = function (topic, fn) { // if new topic init to empty array
    if (!pubSubCache.hasOwnProperty (topic)) 
      pubSubCache[topic] = [];   
    pubSubCache[topic].push (fn);
};

In publish, you need to call EACH of the functions in the topic cache :

//publish function
publish = function (topic, obj) {
  if (pubSubCache.hasOwnProperty (topic)) {
    for (var f = pubSubCache[topic].length; f--;) {
      pubSubCache[topic][f](obj);
    }
  }
}; 

See the fiddle : http://jsfiddle.net/cRTRL/1/