Unexpected equalityComparer behavior

[Antonio Salazar Cardozo]

Hey there,

We ran into some unexpected behaviors with equalityComparer in Knockout 3.4

both with deferred updates on and off, and realized this was actually a combination

of unexpected behavior from pre-3.4 and new unexpected behavior from 3.4 with

deferred updates.

These are exemplified here:

 - https://jsfiddle.net/r39vwz3z/3/#run (Knockout 3.3)

 - https://jsfiddle.net/r39vwz3z/4/#run (3.4, deferred updates off)

 - https://jsfiddle.net/r39vwz3z/5/#run (3.4, deferred updates on)

The fiddle in question sets up an observable array and binds to it with the `checked`

binding. It also creates a computed that depends on the list, and subscribes directly

to the list. Each one of these appends to a visible log to indicate that it has fired and

what value of the list it's seeing. Finally, we set a custom equality comparer and log

what it receives.

In 3.3 and 3.4 with no deferred updates, the equality comparer is never invoked. My

suspicion is that we've actually just been setting the comparer incorrectly all along.

In 3.4 with deferred updates, the comparer is used—but both its parameters are the

final version of the array, rather than having the version before the check that's currently

going into effect and the version after. So no matter how clever one wants to get, the

equality comparer will always return true for a deep comparison.

Not sure what the right move is here. Should I file an issue? Or is this expected

behavior? Is the setup of equalityComparer that I use incorrect anyway, regardless

of how deferred updates affect it?

Thanks,

Antonio

PS: We used to use knockout-deferred-updates with 3.3, which behaves the same

as 3.3 and 3.4 with no deferred updates; see https://jsfiddle.net/r39vwz3z/6/#run .

[Michael Best]

To get notified of the changes to an array, you can use the arrayChange event. Unfortunately, this isn't documented on knockoutjs.com, but it is described on Steve's blog: http://blog.stevensanderson.com/2013/10/08/knockout-3-0-release-candidate-available/

equalityComparer cannot be used to track array changes as you've discovered. The fact that it is called in some instances is just an artifact of the implementation.

-- Michael

[Antonio Salazar Cardozo]

Sorry for the absurdly late reply here—I see what you’re saying. Thanks for

the extra details. For what it’s worth, I think the most confusing thing about

this is that it works inconsistently—it would be useful for the discovery process

if it at least consistently didn’t work :)

Thanks again for the detailed reply!

--

Antonio Salazar Cardozo

On twitter @lightfiend

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[Michael Best]

I've created an issue in Github: https://github.com/knockout/knockout/issues/2098