config: Add calloop feature to provide a calloop source (#102)

This should be useful for integrating cosmic-config into things using
calloop, like cosmic-comp.

cosmic-config/Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 
 [dependencies]
 atomicwrites = "0.4.0"
+calloop = { version = "0.10.5", optional = true }
 dirs = "4.0.0"
 notify = "5.1.0"
 ron = "0.8.0"

cosmic-config/src/calloop.rs

@@ -0,0 +1,63 @@
+// TODO If possible, calloop could poll inotify/kqueue without a thread
+
+use calloop::channel;
+
+use crate::{Config, Error};
+
+pub struct ConfigWatchSource {
+    channel: channel::Channel<(Config, Vec<String>)>,
+    _watcher: notify::RecommendedWatcher,
+}
+
+impl ConfigWatchSource {
+    pub fn new(config: &Config) -> Result<Self, Error> {
+        let (sender, channel) = channel::sync_channel(32);
+        let _watcher = config.watch(move |config, keys| {
+            let _ = sender.send((config.clone(), keys.to_owned()));
+        })?;
+        Ok(Self { channel, _watcher })
+    }
+}
+
+impl calloop::EventSource for ConfigWatchSource {
+    type Event = (Config, Vec<String>);
+    type Metadata = ();
+    type Ret = ();
+    type Error = calloop::channel::ChannelError;
+
+    fn process_events<F>(
+        &mut self,
+        readiness: calloop::Readiness,
+        token: calloop::Token,
+        mut cb: F,
+    ) -> Result<calloop::PostAction, Self::Error>
+    where
+        F: FnMut(Self::Event, &mut Self::Metadata) -> Self::Ret,
+    {
+        self.channel
+            .process_events(readiness, token, |event, ()| match event {
+                calloop::channel::Event::Msg(msg) => cb(msg, &mut ()),
+                calloop::channel::Event::Closed => {}
+            })
+    }
+
+    fn register(
+        &mut self,
+        poll: &mut calloop::Poll,
+        token_factory: &mut calloop::TokenFactory,
+    ) -> Result<(), calloop::Error> {
+        self.channel.register(poll, token_factory)
+    }
+
+    fn reregister(
+        &mut self,
+        poll: &mut calloop::Poll,
+        token_factory: &mut calloop::TokenFactory,
+    ) -> Result<(), calloop::Error> {
+        self.channel.reregister(poll, token_factory)
+    }
+
+    fn unregister(&mut self, poll: &mut calloop::Poll) -> Result<(), calloop::Error> {
+        self.channel.unregister(poll)
+    }
+}

cosmic-config/src/lib.rs

@@ -7,6 +7,9 @@ use std::{
     sync::Mutex,
 };
 
+#[cfg(feature = "calloop")]
+pub mod calloop;
+
 #[derive(Debug)]
 pub enum Error {
     AtomicWrites(atomicwrites::Error<std::io::Error>),