How can I get Knockout JS to data-bind on keypress instead of lost-focus?

How can I get Knockout JS to data-bind on keypress instead of lost-focus?

This example of knockout js works so when you edit a field and press TAB, the viewmodel data and hence the text below the fields is updated.
How can I change this code so that the viewmodel data is updated every keypress?



knockout js




First name:

Last name:

Hello, !


Solutions/Answers:

Solution 1:

<body>
        <p>First name: <input data-bind="value: firstName, valueUpdate: 'afterkeydown'" /></p>
        <p>Last name: <input data-bind="value: lastName, valueUpdate: 'afterkeydown'" /></p>
        <h2>Hello, <span data-bind="text: fullName"> </span>!</h2>
</body>

From the documentation

Additional parameters

  • valueUpdate

    If your binding also includes a parameter called valueUpdate, this
    defines which browser event KO should use to detect changes. The
    following string values are the most commonly useful choices:

    • “change” (default) – updates your view model when the user
      moves the focus to a different control, or in the case of
      elements, immediately after any change

    • “keyup” – updates your view model when the user releases a key

    • “keypress” – updates your view model when the user has typed a
      key. Unlike keyup, this updates repeatedly while the user holds a key
      down

    • “afterkeydown” – updates your view model as soon as the user
      begins typing a character. This works by catching the browser’s
      keydown event and handling the event asynchronously.

Of these options, “afterkeydown” is the best choice if you want to
keep your view model updated in real-time.

Solution 2:

In version 3.2 you can simply use textinput binding.:

<input data-bind="textInput: userName" />

It does two important things:

  • make immediate updates
  • handles browser differences for cut, drag, autocomplete …

So no need for additional modules, custom controls and other things.

Solution 3:

If you want it to do updates on afterkeydown “by default,” you could inject the valueUpdate binding in the value binding handler. Just supply a new allBindingsAccessor for the handler to use that includes afterkeydown.

(function () {
    var valueHandler = ko.bindingHandlers.value;
    var getInjectValueUpdate = function (allBindingsAccessor) {
        var AFTERKEYDOWN = 'afterkeydown';
        return function () {
            var allBindings = ko.utils.extend({}, allBindingsAccessor()),
                valueUpdate = allBindings.valueUpdate;

            if (valueUpdate === undefined) {
                return ko.utils.extend(allBindings, { valueUpdate: AFTERKEYDOWN });
            } else if (typeof valueUpdate === 'string' && valueUpdate !== AFTERKEYDOWN) {
                return ko.utils.extend(allBindings, { valueUpdate: [valueUpdate, AFTERKEYDOWN] });
            } else if (typeof valueUpdate === 'array' && ko.utils.arrayIndexOf(valueUpdate, AFTERKEYDOWN) === -1) {
                valueUpdate = ko.utils.arrayPushAll([AFTERKEYDOWN], valueUpdate);
                return ko.utils.extend(allBindings, {valueUpdate: valueUpdate});
            }
            return allBindings;
        };
    };
    ko.bindingHandlers.value = {
        // only needed for init
        'init': function (element, valueAccessor, allBindingsAccessor) {
            allBindingsAccessor = getInjectValueUpdate(allBindingsAccessor);
            return valueHandler.init(element, valueAccessor, allBindingsAccessor);
        },
        'update': valueHandler.update
    };
} ());

If you’re not comfortable “overriding” the value binding, you could give the overriding custom binding a different name and use that binding handler.

ko.bindingHandlers.realtimeValue = { 'init':..., 'update':... };

demo

A solution like this would be suitable for Knockout version 2.x. The Knockout team has fleshed out a more complete binding for text-like inputs through the textInput binding in Knockout version 3 and up. It was designed to handle all text input methods for text inputs and textarea. It will even handle real time updating which effectively renders this approach obsolete.

Solution 4:

Jeff Mercado’s answer is fantastic, but unfortunately broken with Knockout 3.

But I found the answer suggested by the ko devs while working through Knockout 3 changes. See the bottom comments at https://github.com/knockout/knockout/pull/932. Their code:

//automatically add valueUpdate="afterkeydown" on every value binding
(function () {
    var getInjectValueUpdate = function (allBindings) {
        return {
            has: function (bindingKey) {
                return (bindingKey == 'valueUpdate') || allBindings.has(bindingKey);
            },
            get: function (bindingKey) {
                var binding = allBindings.get(bindingKey);
                if (bindingKey == 'valueUpdate') {
                    binding = binding ? [].concat(binding, 'afterkeydown') : 'afterkeydown';
                }
                return binding;
            }
        };
    };

    var valueInitHandler = ko.bindingHandlers.value.init;
    ko.bindingHandlers.value.init = function (element, valueAccessor, allBindings, viewModel, bindingContext) {
        return valueInitHandler(element, valueAccessor, getInjectValueUpdate(allBindings), viewModel, bindingContext);
    };
}());

http://jsfiddle.net/mbest/GKJnt/

Edit
Ko 3.2.0 now has a more complete solution with the new “textInput” binding. See SalvidorDali’s answer

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