# Controller-Registry

A prototype for an alternative to custom elements.

## Concept

Similar to a custom element, a controller defines custom behaviours for HTML
elements and is managed automatically by a registry.

Like custom built-in elements, controllers are controlled via an attribute on
the element.

Unlike custom elements, controllers are external objects or functions that are
attached to an object, meaning several different controllers can be managing the
same element at once, and the class of the element does not change in the
process.

Controllers can be added and removed as necessary.

## API

The library exports a global `ControllerRegistry` attached to the document root
with a similar API to the `CustomElementRegistry` class.

Controllers can be registered under any name as either a callback which gets
called when the controller is added to an element or a constructor which gets
called with `new` and passed a revokable proxy to the element.

If the controller is a function, the second argument is a promise that resolves
when the controller is removed again. This promise has an additional property
`"signal"` which returns an `AbortSignal`. This means the promise can be passed
directly as the third argument to `addEventListener` function calls.

The registry also exposes a `list` function which, given an element, returns an
object similar to a `DomTokenList` for easier management of the controller list.

The `controller` attribute is a space-separated list of controller names as
registered in the registry.