Add ko.isPureComputed function and tests

[zhaoyu0810]

Added ko.isPureComputed function

[brianmhunt]

This strategy only works in debug builds, since _state will be a Symbol when compiled.

A better strategy may be to check if the pure functions are the same as the regular e.g. instance.getVersion is the same as the ko.computed/subscribable version. (the overloaded version is here: https://github.com/zhaoyu0810/knockout/blob/master/src/subscribables/dependentObservable.js#L407)

i.e.

/*some test for */isComputed && instance.getVersion !== ko.computed.fn.getVersion

[zhaoyu0810]

Hi Brian,

Thank you very much! How about below code?

ko.isPureComputed = function (instance) {
    if (!instance) {
        return false;
    }

    return instance.getVersion === pureComputedOverrides.getVersion && ko.isComputed(instance);
}

[brianmhunt]

Awesome. Maybe it can condense it with e.g.

ko.isPureComputed = function(instance) {
   return ko.hasPrototype(instance, ko.computed) &&
     instance.getVersion === pureComputedOverrides.getVersion
}

This avoids the overhead of an extra function call to ko.isComputed, and we can get rid of the extra !instance test since it's subsumed by hasPrototype.

[zhaoyu0810]

Agree. Thank you!

[brianmhunt]

I'm ok with this. @knockout/owners ?

[brianmhunt]

I think there should also IMHO be assertions that the ko.isPureComputed fails for regular computeds and observables.

[zhaoyu0810]

Thanks Brian. I added tests for computeds and observables.

[brianmhunt]

Thanks @zhaoyu0810 - looks good to me.

[SteveSanderson]

Thanks @zhaoyu0810! Since @brianmhunt has also looked at this, I've merged it.

I made a slight tweak to the implementation of isPureComputed so that it would still work if some future subclass of pure computed overrode the getVersion method. Hopefully it still satisfies your requirements.

[zhaoyu0810]

Thank you very much! @brianmhunt , @SteveSanderson