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//! ump server running in an async task.

use std::ops::ControlFlow;

use tokio::task::{self, JoinHandle};

use async_trait::async_trait;

use super::{channel, Client, ReplyContext};

/// Message processing trait for an async handler.
#[async_trait]
pub trait Handler<S, R, E, RV> {
  /// Optional initialization callback.
  ///
  /// This is called on the dispatcher task before the main message dispatch
  /// loop is entered.
  #[allow(unused_variables)]
  fn init(&mut self, weak_client: ump::WeakClient<S, R, E>) {}

  /// Message processing callback.
  ///
  /// The callback must return `ControlFlow::Continue(())` to keep the
  /// dispatcher loop going.  Returning `ControlFlow::Break(RV)` will cause the
  /// dispatcher loop to abort and returns the value in `RV` from the task.
  async fn proc_req(
    &mut self,
    msg: S,
    rctx: ReplyContext<R, E>
  ) -> ControlFlow<RV, ()>;

  /// Optional termination callback.
  ///
  /// This is called on the dispatcher task just after the main message
  /// processing loop has been terminated.
  ///
  /// The `rv` argument is set to the return value returned from the dispatcher
  /// loop.  It will be set to `Some()` value if a request handler returned
  /// `ControlFlow::Break(RV)`.  If will be set to `None` if the dispatch loop
  /// terminated because the queue is empty and all of the linked clients have
  /// been dropped.
  ///
  /// The value returned from this callback is returned from the dispatcher
  /// task when it is joined.
  ///
  /// The default implementation simply returns the `rv` parameter.
  fn term(&mut self, rv: Option<RV>) -> Option<RV> {
    rv
  }
}

/// Run a task which will process incoming messages from an ump server
/// end-point.
///
/// See top module's documentation for an overview of the [dispatch
/// loop](crate#dispatch-loop).
pub fn spawn<S, R, E, RV>(
  mut handler: impl Handler<S, R, E, RV> + Send + 'static
) -> (Client<S, R, E>, JoinHandle<Option<RV>>)
where
  S: 'static + Send,
  R: 'static + Send,
  E: 'static + Send,
  RV: 'static + Send
{
  let (server, client) = channel();
  let weak_client = client.weak();
  let jh = task::spawn(async move {
    handler.init(weak_client);
    let ret = loop {
      let (msg, rctx) = match server.async_wait().await {
        Ok(d) => d,
        Err(_) => break None
      };
      match handler.proc_req(msg, rctx).await {
        ControlFlow::Continue(_) => {}
        ControlFlow::Break(rv) => break Some(rv)
      }
    };
    handler.term(ret)
  });

  (client, jh)
}

// vim: set ft=rust et sw=2 ts=2 sts=2 cinoptions=2 tw=79 :