updates the value to null and adds it to the Stream.
Even with null-safety coming, is still an important feature to support, as
call() doesn't accept null values. For instance,
InputDecoration.errorText has to be null to not show the "error state". [...]
Returns a StreamSubscription similar to listen, but with the
added benefit that it primes the stream with the current value, rather
than waiting for the next value. This should not be called in onInit
or anywhere else during the build process.
Following certain practices on Rx data, we might want to react to certain
listeners when a value has been provided, even if the value is the same.
At the moment, we ignore part of the process if we .call(value) with
the same value since it holds the value and there's no real
need triggering the entire process for the same value inside, but
there are other situations where we might be interested in
triggering this. [...]