How can I force a throttled observable to update immediately?

I show or hide a "Loading" indicator on my UI by binding its visibility to an observable named waiting, which is defined like this:

// Viewmodel
var outstandingRequests = ko.observable(0);

// true if any requests are outstanding
var waiting = ko.computed(function() {
    return outstandingRequests() > 0;
}.extend({ throttle: 500 });

// These are called when AJAX requests begin or end
function ajaxBegin() {
    outstandingRequests(++outstandingRequests());
}
function ajaxEnd() {
    outstandingRequests(--outstandingRequests());
}

<!-- View -->
<div data-bind="visible: waiting">Please wait, loading...</div>

I'm throttling the waiting observable because I don't want the loading message to appear unless the request is taking a long time (>500ms in this case), to increase the perceived speed of the application. The problem is that once a long-running request finishes, the loading indicator doesn't disappear until an additional 500ms has passed. Instead, when the last outstanding request finishes, I want waiting to flip to false immediately.

My first attempt at a fix involved using valueHasMutated(), but the update is still delayed.

function ajaxEnd() {
    outstandingRequests(--outstandingRequests());
    // If that was the last request, we want the loading widget to disappear NOW.
    outstandingRequests.valueHasMutated(); // Nope, 'waiting' still 500ms to update :(
}

How can I bypass the throttle extension and force waiting to update immediately?

Solution

What you really want is to delay the notifications from the waiting observable when it becomes true. This can be done by intercepting the notifySubscribers function of the observable:

var originalNotifySubscribers = this.isWaiting.notifySubscribers,
    timeoutInstance;
this.isWaiting.notifySubscribers = function(value, event) {
    clearTimeout(timeoutInstance);
    if ((event === 'change' || event === undefined) && value) {
        timeoutInstance = setTimeout(function() {
            originalNotifySubscribers.call(this, value, event);
        }.bind(this), 500);
    } else {
        originalNotifySubscribers.call(this, value, event);
    }
};

jsFiddle: http://jsfiddle.net/mbest/Pk6mH/

EDIT: I just thought of another, possibly better, solution for your particular case. Since the waiting observable only depends on one other observable, you can create a manual subscription that updates the waiting observable:

var timeoutInstance;
this.isLoading.subscribe(function(value) {
    clearTimeout(timeoutInstance);
    if (value) {
        timeoutInstance = setTimeout(function() {
            this.isWaiting(true);
        }.bind(this), 500);
    } else {
        this.isWaiting(false);
    }
}, this);

jsFiddle: http://jsfiddle.net/mbest/wCJHT/