How to pass an array of checkboxes to a knockout custom binding?

Question

i'm using knockout 2.2.1. I have a set of 3 check boxes to concatenate to get the corresponding values all together:

<fieldset data-role="controlgroup" id="top-colours" data-bind="topColoursLabel: { topColoursRed,topColoursBlue,topColoursGreen }">
    <legend>Top Colours:</legend>
    <input type="checkbox" name="top-colours-red" data-bind="checked: topColoursRed" id="tc-check-1" />
    <label for="tc-check-1">Red stripes</label>
    <input type="checkbox" name="top-colours-blue" data-bind="checked: topColoursBlue" id="tc-check-2" />
    <label for="tc-check-2">Blue stripes</label>
    <input type="checkbox" name="top-colours-green" data-bind="checked: topColoursGreen" id="tc-check-3" />
    <label for="tc-check-3">Green stripes</label>
</fieldset>

The result shall be for example: "Red stripes, Blue stripes". My viewmodel is as follows:

function ColoursViewModel() {
    var self = this;
    self.template = "coloursView";
    self.topColoursRed = ko.observable(false);
    self.topColoursBlue = ko.observable(false);
    self.topColoursGreen = ko.observable(false);
    self.topColoursDescription = ko.observable("");
}

How shall be the custom bindings to achieve this? I try something like that:

ko.bindingHandlers.topColoursLabel = {
    update: function (element, valueAccessor, allBindingsAccessor, viewModel, bindingContext) {
        ko.utils.unwrapObservable(valueAccessor());
        // ...
        var checkText = '...';
        viewModel.topColoursDescription(checkText);
    }
};

I'm not able to find out how to pass the array to my custom bindings to subscribe to the values of the 3 check boxes, because I'm noob to knockout.

It seems to me, that a declaration like:

data-bind="topColoursLabel: { topColoursRed,topColoursBlue,topColoursGreen }"

would be great to achieve this, but i'm searching the right way to do that.

Note: i cannot use a computed observable here, because i need to get some other properties from element - i mean the label-for text - so a custom binding is needed.

Can someone help?

UPDATED jsFiddle: http://jsfiddle.net/Sx87j/

Answer 1

Actually, custom binding handler is not what you really need. You should implement your self.coloursDescription as computed observable which will track checkbox changes and return currently selected stripes:

self.topColoursDescription = ko.computed(function(){
    var colors = [];
    if (self.topColoursRed())   colors.push('Red stripes');
    if (self.topColoursBlue())  colors.push('Blue stripes');
    if (self.topColoursGreen()) colors.push('Green stripes');
    return colors.join(', ');
});

Also remove all tracks of your custom bindings from markup and you will get something like this: http://jsfiddle.net/h7Bmb/8/

Update

I can make your updated fiddle to work with top colours. Making it work with bottom colors too looks a bit complicated with your current approach.

Enumerate all linked color observables in your binding:

<fieldset data-role="controlgroup" id="top-colours" data-bind="topColoursLabel: [ topColoursRed, topColoursBlue, topColoursGreen ]">

Change your custom binding code (the line where ko.utils.unwrapObservable is called):

ko.utils.arrayForEach(valueAccessor(), ko.utils.unwrapObservable);

Example: http://jsfiddle.net/Sx87j/1/