Develop data-resources API for widgets

[tomdye]

Enhancement

As part of the work on paginated virtual scrolling we discovered the need for a consistent data-aware widget pattern. This would take the form of a resource middleware that exposes an API to the widget to get the data it requires.

The resource handling side of things will be handled by framework for dojo 7. This issue is related to the widget API side of things.

Initial POC for this using a basic menu / typeahead can be seen here in code sandbox.

This works by creating a resource which is then passed down from the typeahead to it's menu. the resource itself is reactive and will work similar to icache by invalidating the widget when data becomes available.
Thus far the API exposed by the resources middleware is:

interface ResourceMiddleware {
  getOrRead: ({ offset, query, pageSize }: GetOrReadProps) => any;
  setQuery: (newQuery?: string) => void;
  setOffset: (newOffset: number) => void;
  setPageSize: (newPageSize: number) => void;
  gotoPage: (pageNumber: any) => void;
  next: () => void;
  previous: () => void;
  current: () => GetOrReadProps;
}

[tomdye]

This was completed via dojo/framework#672