feat(display): input routed via display server to focused app; input Subscribe; uses caps badged + Delegate

crates/servers/display-server/src/focus.rs

@@ -0,0 +1,56 @@
+//! Who gets input: the display server is the input server's only client
+//! and passes events on to the app in focus, the one that subscribed last.
+//! Apps out of focus get nothing.
+
+/// Events received and not yet polled by the app in focus.
+const QUEUED: usize = 32;
+
+pub struct Focus<N: Copy> {
+    /// The app in focus: its badge and the notification it gave to be told.
+    holder: Option<(usize, N)>,
+    events: [[usize; 4]; QUEUED],
+    head: usize,
+    len: usize,
+}
+
+impl<N: Copy> Focus<N> {
+    pub const fn new() -> Self {
+        Self { holder: None, events: [[0; 4]; QUEUED], head: 0, len: 0 }
+    }
+
+    /// `badge` takes focus; what the last holder had not read is dropped.
+    pub fn subscribe(&mut self, badge: usize, notify: N) {
+        self.holder = Some((badge, notify));
+        self.len = 0;
+    }
+
+    /// Queues an event (as the input server sends it) for the holder;
+    /// returns whom to tell. With no holder the event goes nowhere.
+    pub fn push(&mut self, event: [usize; 4]) -> Option<N> {
+        let (_, notify) = self.holder?;
+        if self.len == QUEUED {
+            self.head = (self.head + 1) % QUEUED;
+            self.len -= 1;
+        }
+        self.events[(self.head + self.len) % QUEUED] = event;
+        self.len += 1;
+        Some(notify)
+    }
+
+    /// The next event for `badge`, if it is in focus and one is queued.
+    pub fn poll(&mut self, badge: usize) -> Option<[usize; 4]> {
+        if self.holder.is_none_or(|(b, _)| b != badge) || self.len == 0 {
+            return None;
+        }
+        let event = self.events[self.head];
+        self.head = (self.head + 1) % QUEUED;
+        self.len -= 1;
+        Some(event)
+    }
+}
+
+impl<N: Copy> Default for Focus<N> {
+    fn default() -> Self {
+        Self::new()
+    }
+}