Question

I have a view model with an observableArray (named ‘all’) of objects. One of the properties of that object is an observable name selected. I want some code to execute whenever the selected property of the of the child object in the array changes. I tried manually subscribing to all via all.subscribe() but that code only fires when items are added or removed. I updated the code to do it like this:

all.subscribe(function () {
    ko.utils.arrayForEach(all(), function (item) {
        item.selected.subscribe(function () {
            //code to fire when selected changes
        });
    });
});

Is this the right way to do this or is there a better way?

Answer 1

This is close to correct. Observable array subscriptions are only for when items are added or removed, not modified. So if you want to subscribe to an item itself, you’ll need to, well, subscribe to the item itself:

Key point: An observableArray tracks which objects are in the array, not the state of those objects

Simply putting an object into an observableArray doesn’t make all of that object’s properties themselves observable. Of course, you can make those properties observable if you wish, but that’s an independent choice. An observableArray just tracks which objects it holds, and notifies listeners when objects are added or removed.

(from Knockout documentation)

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I say "close to correct" since you will want to remove all the old subscriptions. Currently, if the observable array starts as [a, b] you are subscribing to [a, b], but then if c gets added you have two subscriptions for a and b plus one for c.