can't find information on creating observables, particularly for array changes

I am looking for information on how to observe array changes (that is, changes to the membership of arrays). The Aurelia docs contain almost nothing on this subject.

There is one section on `@observable`, buried under the Cheat Sheet under Custom Elements (??), but this doesn't say anything about watching for array changes.

One can find the `CollectionObserver` interface in the API, but only if you know to look for it, and even if you find it gives you very little useful information. For example, the argument to the `subscribe` method is defined with type `any`, which tells you nothing about how to handle the subscription.

I can't find anything beside that. Out there in internet land there isn't much more that I've so far been able to find.

Can we please have some documentation on observing arrays (and creating observables in general would be gravy)? Or am I missing something? Thanks.

Start with the existing @observable section in the Cheat Sheet under Custom Elements and the CollectionObserver API entry. Document how to observe array membership changes, explain the subscribe callback argument, and cover creating observables generally; done means a newcomer can find and use this guidance without relying on undocumented API details.

Written by the indexing model from the issue text.