Revert an invalid value in knockout without subscriptions firing

Question

Is it possible to revert a value change to a view model with something other than a custom binding handler (maybe an extender) without the subscriptions firing?

For example, say you have a numeric field that only allows values up to 100. If someone types 101, we want the value to drop back to the previous value and most importantly not fire any subscriptions on the reverted value.

I'm trying to find a generic way of accomplishing this without having to write a custom binding handler that inherently would require duplication of core knockout code to handle text fields, select fields, etc.

Answer 1

Yes, it can be done with an extender, like this:

ko.extenders.numeric = function(target, properties) {
    var result = ko.computed({
        read: target,
        write: function(newValue) {
            var current = target();
            var valueToWrite = newValue;
            if(properties) {
                if(properties.maxNum && properties.maxNum < newValue) {
                    valueToWrite = current;
                }
                if(properties.minNum && properties.minNum > newValue) {
                    valueToWrite = current;
                }
            }

            if(valueToWrite !== current) {
                target(valueToWrite);
            } else {
                target.notifySubscribers(valueToWrite);
            }
        }
    });

    result(target());
    return result;
};

And this is how you use it:

self.number = ko.observable().extend({numeric: { minNum: 50, maxNum: 100} });

You can test that in the fiddle I've created.

You can comment the target.notifySubscribers(valueToWrite) line but what will happen is that if you change that value from outside (like in an input element), the value will not be updated back to the previous one.