Knockout with multiple valueUpdates

Question

I'm a newbie when it comes to using knockout, I really want to maximize it's usage in the web that I am developing. But I am having concerns when it comes to the browsers capability to listen to multiple user events. I believe that knockout implements a property called valueupdate. So far, I have only used 'afterkeydown' and 'blur'. But I need my browser to listen to multiple events at the same time. Below are the user events that my browser should listen to:

1. On page load, I want my input fields to start validating on page load.
2. On blur, I want my input fields to validate after it losts focus.
3. After key down, I also want my fields to re-validate them selves after the user presses a key.
4. On submit, though my fields are not wrapped in a form, I still want to validate my fields, after the user clicks the "Save" button.

I'm thinking of something like this:

<input data-bind="value:someObservable, valueUpdate:'blur' + 'afterKeyDown' + 'onLoad' + 'onClickOfSaveButton'" />

--> Something like that, though I know this will cause a syntax error, but I hope you get my point.

I know, this question is kinda confusing, but if you want more details, please don't hesitate to specify it on your comment. I really need help here. Thanks.

Answer 1

Always happy to see a 'newbie' decide to make the switch to KnockoutJS. Anything that can help rid the world of verbose, ad hoc data-binding solutions is a wonderful thing.

To answer your question regarding how to key off multiple events to update your data-binding value, you would write:

<input data-bind="value:someObservable, valueUpdate: ['blur', 'afterKeyDown', 'onLoad']"/>

In my case, I was using a bootstrap typeahead to select a value, but the data-binding was not being refreshed after the user made a selection from the dropdown suggestion box. After looking at the source, I found this little hidden gem that was not mentioned on the KO docs.

I cringe telling anyone to "go read the source", but if you're dealing with a fringe case that isn't covered in the docs, don't be scared to! I've uncovered many undocumented features by doing so

source: https://github.com/knockout/knockout/blob/master/src/binding/defaultBindings/value.js

ko.bindingHandlers['value'] = {
    'init': function (element, valueAccessor, allBindingsAccessor) {
        // Always catch "change" event; possibly other events too if asked
        var eventsToCatch = ["change"];
        var requestedEventsToCatch = allBindingsAccessor()["valueUpdate"];
        var propertyChangedFired = false;
        if (requestedEventsToCatch) {
            // Allow both individual event names, and arrays of event names
            if (typeof requestedEventsToCatch == "string")       
                requestedEventsToCatch = [requestedEventsToCatch];
            ko.utils.arrayPushAll(eventsToCatch, requestedEventsToCatch);
            eventsToCatch = ko.utils.arrayGetDistinctValues(eventsToCatch);
        }
    .... truncated ....

Depending on your familiarity with writing bindings, it may be fairly apparent that the valueUpdate binding handles an array of events. Here they're getting the supplemental binding "valueUpdate" and storing it in var requestedEventsToCatch . The next comment marks the logic to handle an array of events to update the value on -- indeed they cast even a single event such as "afterKeyDown" to ["afterKeyDown"]

Hope this helps!