Creating custom bindings that control descendant bindings

Note: This is an advanced technique, typically used only when creating libraries of reusable bindings. It’s not something you’ll normally need to do when building applications with Knockout.

By default, bindings only affect the element to which they are applied. But what if you want to affect all descendant elements too? This is possible. Your binding can tell Knockout not to bind descendants at all, and then your custom binding can do whatever it likes to bind them in a different way.

To do this, simply return {controlsDescendantBindings:true} from your binding’s init function.

Example: Controlling whether or not descendant bindings are applied

For a very simple example, here’s a custom binding called allowBindings that allows descendant bindings to be applied only if its value is true. If the value is false, then allowBindings tells Knockout that it is responsible for descendant bindings so they won’t be bound as usual.

ko.bindingHandlers.allowBindings = {
    init: function(elem, valueAccessor) {
        // Let bindings proceed as normal *only if* my value is false
        var shouldAllowBindings = ko.unwrap(valueAccessor());
        return { controlsDescendantBindings: !shouldAllowBindings };
    }
};

To see this take effect, here’s a sample usage:

<div data-bind="allowBindings: true">
    <!-- This will display Replacement, because bindings are applied -->
    <div data-bind="text: 'Replacement'">Original</div>
</div>

<div data-bind="allowBindings: false">
    <!-- This will display Original, because bindings are not applied -->
    <div data-bind="text: 'Replacement'">Original</div>
</div>

Example: Supplying additional values to descendant bindings

Normally, bindings that use controlsDescendantBindings will also call ko.applyBindingsToDescendants(someBindingContext, element) to apply the descendant bindings against some modified binding context. For example, you could have a binding called withProperties that attaches some extra properties to the binding context that will then be available to all descendant bindings:

ko.bindingHandlers.withProperties = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // Make a modified binding context, with a extra properties, and apply it to descendant elements
        var innerBindingContext = bindingContext.extend(valueAccessor);
        ko.applyBindingsToDescendants(innerBindingContext, element);

        // Also tell KO *not* to bind the descendants itself, otherwise they will be bound twice
        return { controlsDescendantBindings: true };
    }
};

As you can see, binding contexts have an extend function that produces a clone with extra properties. The extend function accepts either an object with the properties to copy or a function that returns such an object. The function syntax is preferred so that future changes in the binding value are always updated in the binding context. This process doesn’t affect the original binding context, so there is no danger of affecting sibling-level elements - it will only affect descendants.

Here’s an example of using the above custom binding:

<div data-bind="withProperties: { emotion: 'happy' }">
    Today I feel <span data-bind="text: emotion"></span>. <!-- Displays: happy -->
</div>
<div data-bind="withProperties: { emotion: 'whimsical' }">
    Today I feel <span data-bind="text: emotion"></span>. <!-- Displays: whimsical -->
</div>

Example: Adding extra levels in the binding context hierarchy

Bindings such as with and foreach create extra levels in the binding context hierarchy. This means that their descendants can access data at outer levels by using $parent, $parents, $root, or $parentContext.

If you want to do this in custom bindings, then instead of using bindingContext.extend(), use bindingContext.createChildContext(someData). This returns a new binding context whose viewmodel is someData and whose $parentContext is bindingContext. If you want, you can then extend the child context with extra properties using ko.utils.extend. For example,

ko.bindingHandlers.withProperties = {
    init: function(element, valueAccessor, allBindings, viewModel, bindingContext) {
        // Make a modified binding context, with a extra properties, and apply it to descendant elements
        var childBindingContext = bindingContext.createChildContext(
            bindingContext.$rawData, 
            null, // Optionally, pass a string here as an alias for the data item in descendant contexts
            function(context) {
                ko.utils.extend(context, valueAccessor());
            });
        ko.applyBindingsToDescendants(childBindingContext, element);

        // Also tell KO *not* to bind the descendants itself, otherwise they will be bound twice
        return { controlsDescendantBindings: true };
    }
};

This updated withProperties binding could now be used in a nested way, with each level of nesting able to access the parent level via $parentContext:

<div data-bind="withProperties: { displayMode: 'twoColumn' }">
    The outer display mode is <span data-bind="text: displayMode"></span>.
    <div data-bind="withProperties: { displayMode: 'doubleWidth' }">
        The inner display mode is <span data-bind="text: displayMode"></span>, but I haven't forgotten
        that the outer display mode is <span data-bind="text: $parentContext.displayMode"></span>.
    </div>
</div>

By modifying binding contexts and controlling descendant bindings, you have a powerful and advanced tool to create custom binding mechanisms of your own.