How to create knockout observable subscription that fires only once?

Question

In my model I have a property that is loaded asynchronously. I want it to generate another model property after it loads.

I was thinking about subscription that could fire after the 1st property changes, generates the 2nd property and then get disposed - I don't know how can I dispose subscription from inside itself.

Is there a way to fire one time event after observable property changes?

Answer 1

To dispose subscription from inside itself just create reference to it:

var subscription = yourObservable.subscribe(function (newValue) {
    ...
    subsription.dispose();
});

If you don't want to write this code every time, you can extend observable with new method:

ko.subscribable.fn.subscribeOnce = function (handler, owner, eventName) {
    var subscription = this.subscribe(function (newValue) {
        subscription.dispose();
        handler(newValue);
    }, owner, eventName);
    return subscription;
};

...

// usage
var subscription = yourObservable.subsribeOnce(yourHandler);

And dont' forget to dispose subscription on component disposal in case it was not fired.