How can I get two computed values to bind to each other?

Question

What I'm trying to do is keep two text box's math in sync. The fields I'm working with is subtotal, taxTotal, tax rate, and sale total. What I want to happen is:

• If the taxRate is updated by the user, then the taxTotal will update with the correct amount (subTotal * taxRate).
• If the taxTotal is updated by the user, then the taxRate should be updated with the correct amount (taxTotal / subTotal * 100)

I didn't get very far with this, and I think it's because I keep thinking in terms of properties with backing fields (like in C#) and I'm having trouble trying to figure out the logic to keep everything bound and observable within the knockout framework (managing dirty states etc).

Does anyone have a solution for this? As far as I got led me to using pureComputed instead of computed. e.g.

this.subTotal = ko.observable(0.00);

this.taxRate = ko.pureComputed({
     read: function(){

     },
     write: function(){

     },
     owner: this
});

this.taxTotal = ko.pureComputed({
    read: function(){

     },
     write: function(){

     },
     owner: this
});

this.saleTotal = ko.computed(function(){
    return this.subTotal ()+ this.taxTotal();
});

Answer 1

This was a fun one to figure out. One rule defines the read function for the taxTotal computed, the other defines the write function. The other two variables are just observables. I left out the 100 multiplier because it wasn't symmetric. It needs to be in both functions or neither.

var viewModel = (function () {
    var subTotal = ko.observable(10),
        taxRate = ko.observable(5);


    var taxTotal = ko.computed({
        read: function () {
            return subTotal() * taxRate();
        },
        write: function (newValue) {
            taxRate(newValue / subTotal());
        }
    });

    return {
        taxRate: taxRate,
        taxTotal: taxTotal,
        subTotal: subTotal
    };
}());

ko.applyBindings(viewModel);

http://jsfiddle.net/ypsdh53q/