Implement Runtime and Executor in iced_core

They can be leveraged by shells to easily execute commands
and track subscriptions.

core/src/runtime.rs

@@ -0,0 +1,74 @@
+mod executor;
+
+pub use executor::Executor;
+
+use crate::{subscription, Command, Subscription};
+
+use futures::Sink;
+use std::marker::PhantomData;
+
+#[derive(Debug)]
+pub struct Runtime<Hasher, Event, Executor, Receiver, Message> {
+    executor: Executor,
+    subscriptions: subscription::Tracker<Hasher, Event>,
+    receiver: Receiver,
+    _message: PhantomData<Message>,
+}
+
+impl<Hasher, Event, Executor, Receiver, Message>
+    Runtime<Hasher, Event, Executor, Receiver, Message>
+where
+    Hasher: std::hash::Hasher + Default,
+    Event: Send + Clone + 'static,
+    Executor: self::Executor,
+    Receiver: Sink<Message, Error = core::convert::Infallible>
+        + Unpin
+        + Send
+        + Clone
+        + 'static,
+    Message: Send + 'static,
+{
+    pub fn new(receiver: Receiver) -> Self {
+        Self {
+            executor: Executor::new(),
+            subscriptions: subscription::Tracker::new(),
+            receiver,
+            _message: PhantomData,
+        }
+    }
+
+    pub fn spawn(&mut self, command: Command<Message>) {
+        use futures::{FutureExt, SinkExt};
+
+        let futures = command.futures();
+
+        for future in futures {
+            let mut receiver = self.receiver.clone();
+
+            self.executor.spawn(future.then(|message| {
+                async move {
+                    let _ = receiver.send(message).await;
+
+                    ()
+                }
+            }));
+        }
+    }
+
+    pub fn track(
+        &mut self,
+        subscription: Subscription<Hasher, Event, Message>,
+    ) {
+        let futures = self
+            .subscriptions
+            .update(subscription, self.receiver.clone());
+
+        for future in futures {
+            self.executor.spawn(future);
+        }
+    }
+
+    pub fn broadcast(&mut self, event: Event) {
+        self.subscriptions.broadcast(event);
+    }
+}