Computed created each time dirty flag status is checked

[mdonatas]

If you take this example:
viewModel.dirtyFlag().isDirty();

viewModel.dirtyFlag is a function which creates computed that hashes the model. So each and every single time when you do that viewModel.dirtyFlag().isDirty(); a new computed is created. So after 10 times editing a field you already have 11 computeds doing the hashing. This is a gigantic performance kill.

[mdonatas]

We ended up with this implementation for our needs. Nice thing about it is that hashing of the model (_objectToTrack) is performed only when it actually changes

; (function (ko) {
    ko.DirtyFlag = function (objectToTrack, isInitiallyDirty, hashFunction) {

        hashFunction = hashFunction || ko.toJSON;

        var
            _objectToTrack = objectToTrack,
            _lastCleanState = ko.observable(hashFunction(_objectToTrack)),
            _isInitiallyDirty = ko.observable(isInitiallyDirty),

            _isDirty = ko.observable(),
            _isDirtyComputed = ko.computed(function () {
                return _isInitiallyDirty() || hashFunction(_objectToTrack) !== _lastCleanState();
            });

        // Set initial _isDirty value
        _isDirty(_isDirtyComputed.peek());

        // Subscribe to computed to update _isDirty observable
        _isDirtyComputed.subscribe(function (value) {
            _isDirty(value);
        });

        // Function for resetting state
        function _reset() {
            _lastCleanState(hashFunction(_objectToTrack));
            _isInitiallyDirty(false);
        };

        var result = function () {
            // Set isDirty to either a simple observable or a computed one depending of whether there is a need to force reevaluation
            // i.g. when the model is made of POJOs
            this.isDirty = function (forceEvaluation) {
                if (forceEvaluation) {
                    return _isDirtyComputed();
                }
                else {
                    return _isDirty();
                }
            };

            this.reset = _reset;

            return this;
        };

        return result;
    };
})(ko);

[jamesarnall]

It looks like this is a dead thread, but I came here while trying to solve a performance problem, which I ended up fixing by hacking DirtyFlag a bit. When I changed line 63 from

self.isDirty = ko.computed(function () {

to

self.isDirty = ko.pureComputed(function () {

--everything worked just fine. Perhaps someone will find this useful :)