I need to know (in JS) when my model (using knockout.js) or rather a propery has changed.

How do I do that?

Here some code:

function DrawingToolViewModel() {
    var self = this;
    self.drawMode = ko.observable('Line');
}
model = new DrawingToolViewModel();
ko.applyBindings(model);

Now the assigned HTML element to drawMode will be updated by the model and back, whatever changes. That's fine, but how can I react in JS if something in the model has changed?

EDIT

My question wasn't clear enough, sorry. I know observables but I want to subscribe to ALL properties without doing that for every single property. More like "notify me if something in the model has changed"

I need to know (in JS) when my model (using knockout.js) or rather a propery has changed.

How do I do that?

Here some code:

function DrawingToolViewModel() {
    var self = this;
    self.drawMode = ko.observable('Line');
}
model = new DrawingToolViewModel();
ko.applyBindings(model);

Now the assigned HTML element to drawMode will be updated by the model and back, whatever changes. That's fine, but how can I react in JS if something in the model has changed?

EDIT

My question wasn't clear enough, sorry. I know observables but I want to subscribe to ALL properties without doing that for every single property. More like "notify me if something in the model has changed"

• javascript
• knockout.js

• Do you need to know what property was changed? And do you need that to apply to the object deeply, or just the first layer of properties? – Peter T. LaComb Jr. Commented Apr 3, 2012 at 14:53
• The property doesn't mater and just the first layer, nothing deeply. – Marc Commented Apr 3, 2012 at 15:35

2 Answers 2

If you want to register your own subscriptions to be notified of changes to observables, you can call their subscribe function, for example:

myViewModel.personName.subscribe(function(newValue) {
    alert("The person's new name is " + newValue);
});

More details @ knockoutjs.

Summarizing the ments below

To get notified on every change in the ViewModel, check Ryan Niemeyer article and John papa's changeTracker on NuGet

If you want to be notified when a specific property changes then there are several ways of doing what you want. One way is to use the subscribe function:

model.drawMode.subscribe(function(newValue) {    
   // your js goes in here
});

EDIT

However, if you want to be notified when ANY property changes on your view model then I would take a look at this post for creating a 'dirty flag':

http://www.knockmeout/2011/05/creating-smart-dirty-flag-in-knockoutjs.html

This effectively tracks any changes to your view model so you should be able to adapt it to your needs.