Question

Is there anyway I can tell the knockout mapping plugin to subscribe to all property changes call a certain function?

I realize I can manually subscribe to the property change event in this manner:

var viewModel = {
    name: ko.observable('foo'),
}

// subscribe manually here
viewModel.name.subscribe(function(newValue){
   // do work
})

I would like to be able to generically subscribe though, since my view models may vary, I don’t want to hardcode the property names. I created a function that does this, but it may not be the best approach. It works over all browsers except IE7 and below.

Here I take a viewmodel as an argument and try to reflect on it subscribing to the properties:

function subscribeToKO(data) {

        $.each(data, function (property, value) {
            if (getType(value) == "Object")
                data[property] = subscribeToKO(value);
            else if (getType(value) == "Array") {
                $.each(value, function (index, item) {
                    item = subscribeToKO(item);
                });
            }
            else {
                if (value.subscribe) {
                    value.subscribe(function (newValue) {
                        // do work                                         
                    });
                }
            }
        });
        return data;
    }

Like I said this works, but since I am using the mapping pluging I was hoping there was a hook I could use to provide it with a function that will generically subscribe to property changes.

Something like:

mapping = {
   create: function(options){
       options.data.subscribe(function(newValue){
            // do work ???
       });
   }
}

ko.mapping.fromJS(viewModel, mapping);

Any ideas?

Answer 1

Here’s a generic approach based on Ryan Niemeyer’s dirty flag.
Click here for the JsFiddle.

Html:

<ol>
<li>
    Telephone : <input data-bind="value: telephone"/>
</li>
<li>
    Address : <input data-bind="value: address"/>
</li>
</ol>​

Javascript:

var model = {
    telephone: ko.observable('0294658963'),
    address: ko.observable('167 New Crest Rd')

};
// knockout extension for creating a changed flag (similar to Ryan's dirty flag except it resets itself after every change)
ko.changedFlag = function(root) {
    var result = function() {};
    var initialState = ko.observable(ko.toJSON(root));

    result.isChanged = ko.dependentObservable(function() {
        var changed = initialState() !== ko.toJSON(root);
        if (changed) result.reset();
        return changed;
    });

    result.reset = function() {
        initialState(ko.toJSON(root));
    };

    return result;
};
// add changed flag property to the model
model.changedFlag = new ko.changedFlag(model);
// subscribe to changes
model.changedFlag.isChanged.subscribe(function(isChanged) {
    if (isChanged)  alert("model changed");
});
ko.applyBindings(model);​