Implement command::channel helper

It is analogous to `subscription::channel`.

runtime/src/command.rs

@@ -7,6 +7,7 @@ use crate::core::widget;
 use crate::futures::futures;
 use crate::futures::MaybeSend;
 
+use futures::channel::mpsc;
 use futures::Stream;
 use std::fmt;
 use std::future::Future;
@@ -118,3 +119,23 @@ impl<T> fmt::Debug for Command<T> {
         command.fmt(f)
     }
 }
+
+/// Creates a [`Command`] that produces the `Message`s published from a [`Future`]
+/// to an [`mpsc::Sender`] with the given bounds.
+pub fn channel<Fut, Message>(
+    size: usize,
+    f: impl FnOnce(mpsc::Sender<Message>) -> Fut + MaybeSend + 'static,
+) -> Command<Message>
+where
+    Fut: Future<Output = ()> + MaybeSend + 'static,
+    Message: 'static + MaybeSend,
+{
+    use futures::future;
+    use futures::stream::{self, StreamExt};
+
+    let (sender, receiver) = mpsc::channel(size);
+
+    let runner = stream::once(f(sender)).filter_map(|_| future::ready(None));
+
+    Command::single(Action::Stream(Box::pin(stream::select(receiver, runner))))
+}

src/lib.rs

@@ -190,7 +190,6 @@ pub use crate::core::{
     color, Alignment, Background, BorderRadius, Color, ContentFit, Degrees,
     Gradient, Length, Padding, Pixels, Point, Radians, Rectangle, Size, Vector,
 };
-pub use crate::runtime::Command;
 
 pub mod clipboard {
     //! Access the clipboard.
@@ -239,6 +238,11 @@ pub mod mouse {
     };
 }
 
+pub mod command {
+    //! Run asynchronous actions.
+    pub use crate::runtime::command::{channel, Command};
+}
+
 pub mod subscription {
     //! Listen to external events in your application.
     pub use iced_futures::subscription::{
@@ -287,6 +291,7 @@ pub mod widget {
 }
 
 pub use application::Application;
+pub use command::Command;
 pub use error::Error;
 pub use event::Event;
 pub use executor::Executor;