Defers creation and materialization of a
Source until there is demand.
Invokes the user supplied factory when the first downstream demand arrives. When the returned future completes successfully the source switches over to the new source and emits downstream just like if it had been created up front.
Note that asynchronous boundaries (and other operators) in the stream may do pre-fetching which counter acts the laziness and will trigger the factory immediately.
See also lazySource.
emits depends on the wrapped
completes depends on the wrapped