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.computed observable declared in JavaScript?

I’ve got a parent viewmodel with a collection of children, and the parent viewmodel has a selectedChild observable. Given that, I can use databinding expressions to add a CSS class to whichever child is currently selected:

<ul data-bind="foreach: children">
    <li data-bind="text: name,
                   css: {selected: $data === $root.selectedChild()},
                   click: $root.selectChild"></li>
</ul>
<script>
vm = {
    selectedChild: ko.observable(),
    children: [{name: 'Bob'}, {name: 'Ned'}],
    selectChild: function(child) { vm.selectedChild(child); }
};
ko.applyBindings(vm);
</script>

But my viewmodels are going to get more complex, and I’d like “am I selected?” to be able to do more than just adding a single CSS class to a single element. I really want to make an isSelected computed property on the child viewmodel, so I can then add other computed properties that depend on it.

I’ve tried just writing JavaScript that refers to $data and $root, on the off-chance that knockout might define those variables and somehow have them be in scope when it calls my computed evaluator function:

{
  name: 'Bob',
  isSelected: ko.computed(function(){ return $data === $root.selectedChild(); })
}

But no such luck: inside my evaluator function, both $data and $root are undefined.

I’ve also tried using ko.contextFor inside my evaluator, since it does give access to $data and $root. Unfortunately, inside my evaluator function, contextFor also always returns undefined. (I didn’t hold out high hopes for this strategy anyway — it’s not clear how well knockout would be able to track the dependencies if I had to go behind its back like this.)

I could always manually set a property on each child viewmodel that refers back to the parent viewmodel. But I know that knockout has the ability to do this for me, and I’d like to at least explore whether I can use its mechanisms before I go writing my own.

It seems like it should be possible to translate the above binding expression to a computed observable — after all, that’s what knockout already does:

The other neat trick is that declarative bindings are simply implemented as computed observables.

But how do I go about dealing with the $data and $root pseudovariables when I’m writing my own computed observable?

Answer 1

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.