1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117

use std::{error::Error, fmt, future::Future};

use futures_channel::{mpsc, oneshot};
use futures_util::{
    future::{self, Either, FutureExt},
    stream::StreamExt,
    task::SpawnExt,
};

/// Construct a new actor, requires an `Executor` and an initial state.  Returns a reference that can be cheaply
/// cloned and passed between threads.  A specific implementation is expected to wrap this return value and implement
/// the required custom logic.
pub fn actor<A, S, R, E>(executor: impl SpawnExt, initial_state: S) -> ActorSender<A, R, E>
where
    A: Action<State = S, Result = R, Error = E> + Send + 'static,
    S: Send + 'static,
    R: Send + 'static,
    E: Send + 'static,
{
    let (tx, rx) = mpsc::unbounded();
    executor
        .spawn(actor_future(rx, initial_state))
        .expect("Cannot schedule actor on executor");
    ActorSender(tx)
}

type ActorSenderInner<A, R, E> = mpsc::UnboundedSender<(A, oneshot::Sender<Result<R, E>>)>;

#[derive(Debug)]
pub struct ActorSender<A, R, E>(ActorSenderInner<A, R, E>);

impl<A, R, E> Clone for ActorSender<A, R, E> {
    fn clone(&self) -> Self {
        ActorSender(self.0.clone())
    }
}

impl<A, R, E> ActorSender<A, R, E>
where
    A: Send + 'static,
    R: Send + 'static,
    E: Send + From<ActorError> + 'static,
{
    /// Invokes a specific action on the actor.  Returns a future that completes when the actor has
    /// performed the action.
    ///
    /// Specifically not an `async` function to allow it to be used correctly in a fire-and-forget
    /// manner, although this is not advised.
    pub fn invoke(&self, action: A) -> impl Future<Output = ActResult<R, E>> {
        let (tx, rx) = oneshot::channel();
        if let Err(e) = self.0.unbounded_send((action, tx)) {
            Either::Right(future::err(ActorError::from(e).into()))
        } else {
            Either::Left(rx.then(|result| match result {
                Ok(Ok(r)) => future::ok(r),
                Ok(Err(e)) => future::err(e),
                Err(e) => future::err(ActorError::from(e).into()),
            }))
        }
    }
}

async fn actor_future<A, S, R, E>(
    mut receiver: mpsc::UnboundedReceiver<(A, oneshot::Sender<Result<R, E>>)>,
    mut state: S,
) where
    A: Action<State = S, Result = R, Error = E>,
{
    while let Some((a, tx)) = receiver.next().await {
        let _ = tx.send(a.act(&mut state));
    }
}

pub trait Action {
    type State;
    type Result;
    type Error;

    fn act(self, s: &mut Self::State) -> Result<Self::Result, Self::Error>;
}

pub type ActResult<R, E> = Result<R, E>;

#[derive(Debug)]
pub enum ActorError {
    /// Cannot send message to the actor
    InvokeError,
    /// Response was cancelled before being received.
    WaitError,
}

impl Error for ActorError {
    fn description(&self) -> &str {
        match self {
            ActorError::InvokeError => "Cannot send message to actor",
            ActorError::WaitError => "Cannot wait for an answer",
        }
    }
}

impl fmt::Display for ActorError {
    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
        write!(f, "{}", self.description())
    }
}

impl<T> From<mpsc::TrySendError<T>> for ActorError {
    fn from(_from: mpsc::TrySendError<T>) -> Self {
        ActorError::InvokeError
    }
}

impl From<oneshot::Canceled> for ActorError {
    fn from(_from: oneshot::Canceled) -> Self {
        ActorError::WaitError
    }
}