doOnListen method
Invokes the given callback function when the Single is first listened to.
Example
Single.value(1)
.doOnListen(() => print('Is someone there?'))
.listen(null); // prints 'Is someone there?'
Implementation
Single<T> doOnListen(void Function() onListen) => forwardSingleWithSink(
() => _DoStreamSingleSink(onListenCallback: onListen), true);