How can I use knockout's $parent/$root pseudovariables from inside a .computed() observable?

Question

Inside a knockout.js binding expression, I can use the $data, $parent, and $root pseudovariables. How can I get the equivalent of those pseudovariables when I\'m using a ko.

Answer 1

In my experience the approach in @RP Niemeyer's answer is fine if Items live for the duration of the application. But if not, it can lead to memory leaks, because Item's computed observable sets up a reverse dependency from the ViewModel. Again, that's ok if you never get rid of any Item objects. But if you do try to get rid of Items they won't get garbage collected because knockout will still have that reverse dependency reference.

You could make sure to dispose() of the computed, maybe in a cleanup() method on Item that gets called when the item goes away, but you have to remember to do that whenever removing Items.

Instead, why not make Item a little less smart and have ViewModel tell it when it is selected? Just make Item's isSelected() a regular old observable and then in ViewModel subscribe to selectedItem and update inside that subscription.

Or, use @RP Niemeyer's pub/sub solution. (To be fair, this solution came about after his answer here.) You'll still need to clean up, though, because it creates reverse dependencies, too. But at least there's less coupling.

See the answer to my recent question on this same topic for more details.

Answer 2

The pseudovariables are only available in the context of data binding. The view model itself ideally should not know about or have any dependencies on the view that is displaying it.

So, when adding computed observables in the view model, you have no knowledge of how it will be bound (like what is going to be $root). A view model or part of a view model could even be bound separately to multiple areas of the page at different levels, so the pseudo-variables would be different depending on the element that you are starting with.

It depends on what you are trying to accomplish, but if you want your child to have an isSelected computed observable that indicates whether this item is the same as the selected item on the parent view model, then you will need to find a way to make the parent available to the child.

One option is to pass the parent into the constructor function of your child. You do not even need to add the pointer to the parent as a property of the child and can just use it in your computed observable directly.

Something like:

var Item = function(name, parent) {
   this.name = ko.observable(name);  
   this.isSelected = ko.computed(function() {
       return this === parent.selectedItem();        
   }, this);
};

var ViewModel = function() {
   this.selectedItem = ko.observable();
   this.items = ko.observableArray([
       new Item("one", this),
       new Item("two", this),
       new Item("three", this)
       ]);
};

Sample here: http://jsfiddle.net/rniemeyer/BuH7N/

If all you care about is the selected status, then you can tweak it to pass a reference to the selectedItem observable to the child constructor like: http://jsfiddle.net/rniemeyer/R5MtC/

If your parent view model is stored in a global variable, then you could consider not passing it to the child and using it directly like: http://jsfiddle.net/rniemeyer/3drUL/. I prefer to pass the reference to the child though.