Track promises and fire event when all finish

promise-tracker.js

function PromiseTracker(changeFunc) {
    var tracked = [];
    var events = [];

    function fireOnChange(isActive) {
        for (var i = 0; i < events.length; i++)
            events[i](isActive);
    }

    function removePromise(promise) {
        tracked.splice(tracked.indexOf(promise), 1);

        if (tracked.length === 0) {
            fireOnChange(false);
        }
    }

    this.addPromise = function(promise) {
        if (promise.state() != "pending") return promise;

        tracked.push(promise);

        promise.always(function() {
            removePromise(promise);
        });

        if (tracked.length == 1) {
            fireOnChange(true);
        }
        
        return promise;
    };

    this.numTracked = function() {
        return tracked.length;
    };

    this.onChange = function(func) {
        events.push(func);
    };

    this.onActive = function(func) {
        events.push(function(isActive) {
            if (isActive) {
                func(isActive);
            }
        });
    };

    this.onInactive = function(func) {
        events.push(function(isActive) {
            if (!isActive) {
                func(isActive);
            }
        });
    };

    if (changeFunc) {
        this.onChange(changeFunc);
    }
}

PromiseTracker.toggleElementActiveClass = function(el, className) {
    el = $(el);
    return function(isActive) {
        el.toggleClass(className, isActive);
    };
};

PromiseTracker.toggleElementInactiveClass = function(el, className) {
    el = $(el);
    return function(isActive) {
        el.toggleClass(className, !isActive);
    };
};

example.html

<style>
#loader { display:none; }
#loader.loading { display: block }
</style>

<div id="loader">Loading</div>
<button id="button">Click</button>

<script>
function timeoutPromise(time) {
    var def = $.Deferred();

    setTimeout(function() {

        def.resolve("Done");

    }, time);

    return def.promise();
}

$(function() {

    var tracker = new PromiseTracker(PromiseTracker.toggleElementActiveClass("#loader", "loading"));

    $("#button").click(function() {

        tracker.addPromise(timeoutPromise(1000));
        tracker.addPromise(timeoutPromise(2000));

    })

});

</script>