Custom binding not firing unless I unwrap valueAccessor()

[Damien Sawyer]

Hi,

I'm trying to learn KO and am a bit confused about something in a
custom binder.

Please have a look at the simple sample:
http://jsfiddle.net/damiensawyer/kkPhy/

If I comment out the following line, then the binding does not respond
to changes in the text boxes.
var valueUnwrapped = ko.utils.unwrapObservable(valueAccessor());

My understanding is that my custom binding should be data bound to
'Selected' via the label without any other dependencies. Can someone
please advise what I'm missing?

Thanks in advance,


Damien

[rpn]

Hi Damien-

Bindings are implemented using dependentObservables.  What this means is that any observables that have their value accessed while the binding is evaluated will become a dependency.  If that observable is updated, then the "update" function of the binding will run again.   So, there are some cases in a custom binding where you may have to access the value of an observable and then throw it away, just to create the dependency.

Additionally, a binding will have its update function run again, if another binding in the same data-bind attribute is triggered as well.

[Damien]

Thanks very much for that.