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use {Async, Future, Poll};
use stream::Stream;
/// Future for the `flatten_stream` combinator, flattening a
/// future-of-a-stream to get just the result of the final stream as a stream.
///
/// This is created by the `Future::flatten_stream` method.
#[must_use = "streams do nothing unless polled"]
pub struct FlattenStream<F>
where F: Future,
<F as Future>::Item: Stream<Error=F::Error>,
{
state: State<F>
}
pub fn new<F>(f: F) -> FlattenStream<F>
where F: Future,
<F as Future>::Item: Stream<Error=F::Error>,
{
FlattenStream {
state: State::Future(f)
}
}
enum State<F>
where F: Future,
<F as Future>::Item: Stream<Error=F::Error>,
{
// future is not yet called or called and not ready
Future(F),
// future resolved to Stream
Stream(F::Item),
// EOF after future resolved to error
Eof,
// after EOF after future resolved to error
Done,
}
impl<F> Stream for FlattenStream<F>
where F: Future,
<F as Future>::Item: Stream<Error=F::Error>,
{
type Item = <F::Item as Stream>::Item;
type Error = <F::Item as Stream>::Error;
fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
loop {
let (next_state, ret_opt) = match self.state {
State::Future(ref mut f) => {
match f.poll() {
Ok(Async::NotReady) => {
// State is not changed, early return.
return Ok(Async::NotReady)
},
Ok(Async::Ready(stream)) => {
// Future resolved to stream.
// We do not return, but poll that
// stream in the next loop iteration.
(State::Stream(stream), None)
}
Err(e) => {
(State::Eof, Some(Err(e)))
}
}
}
State::Stream(ref mut s) => {
// Just forward call to the stream,
// do not track its state.
return s.poll();
}
State::Eof => {
(State::Done, Some(Ok(Async::Ready(None))))
}
State::Done => {
panic!("poll called after eof");
}
};
self.state = next_state;
if let Some(ret) = ret_opt {
return ret;
}
}
}
}