create<T> function
Creates an Observable that uses the provided callback
to emit elements
to the provided Subscriber on each subscribe.
On each subscription the callback
is run and a Disposable is returned.
The callback
can synchronously or asynchronously generate events on the
passed-in subscriber, as well as register dispose actions by using
Subscriber.add.
For example:
final observable = create
Implementation
Observable<T> create<T>(Callback1<Subscriber<T>> callback) =>
CreateObservable<T>(callback);