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

problem ：

Inside a knockout.js binding expression, I can use the $data , $parent , and $root pseudovariables . stay knockout.js Binding expression , I can use $data , $parent and $root Pseudo variable .How can I get the equivalent of those pseudovariables when I'm using a ko.computed observable declared in JavaScript? When I use in JavaScript Declarative ko.computed observable when , How to obtain the equivalent of these pseudo variables ？

I've got a parent viewmodel with a collection of children, and the parent viewmodel has a selectedChild observable. I have a parent view model with subsets , The parent view model has a selectedChild observable.Given that, I can use databinding expressions to add a CSS class to whichever child is currently selected: In view of this , I can use data binding expressions to CSS Class is added to the currently selected child ：

<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?" But my view model will become more complex , I think “ Have I been chosen ？”to be able to do more than just adding a single CSS class to a single element. What we can do is not just to put a single CSS Class is added 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 really want to create one on the subview model isSelected Compute properties , So I can add other computational 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: I try to write references $data and $root JavaScript, Because elimination may define these variables , And when it calls my computed When assigning a function , They will put them in range in some way ：

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

But no such luck: inside my evaluator function , both $data and $root are undefined . But there is no such luck ： In my assessment function , $data and $root All are undefined .

I've also tried using ko.contextFor inside my evaluator, since it does give access to $data and $root . I also try to use it in my evaluator ko.contextFor , Because it can really access $data and $root .Unfortunately, inside my evaluator function, contextFor also always returns undefined . Unfortunately , In my evaluation function , contextFor Always return 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.)（ in any case , I have no high hopes for this strategy - If I have to chase it like this , So it's not clear how the knockout can track dependencies .）

I could always manually set a property on each child viewmodel that refers back to the parent viewmodel. I can always manually set a property on each subview model , This view returns to the parent view model .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. But I know the knockout is capable of doing this for me , I want to explore whether I can use its mechanism at least before I write it myself .

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 : It seems that it should be possible to convert the above binding expression to computed observable - After all , This is what elimination has done ：

The other neat trick is that declarative bindings are simply implemented as computed observables. Another clever trick is that declarative binding is only for computation observable Realization .

But how do I go about dealing with the $data and $root pseudovariables when I'm writing my own computed observable? But when I write my own computational observables , How to deal with it $data and $root Pseudo variable ？

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Solution ：

Reference resources ： https://stackoom.com/en/question/aFqu