Make it impossible to pass invalid `SessionObserver` ids.

- Add an associated type to the `SessionNotifier` trait for the returned Id's instead of using usize.
- Create a new Id type for the `DirectSessionNotifier`'s implementation, wrapping the previously used usize.
- Derive necessary traits of the new wrapper, make internal value inaccessible and Id's not publically constructable.

src/backend/session/direct.rs

@@ -322,13 +322,18 @@ impl Drop for DirectSession {
     }
 }
 
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub struct Id(usize);
+
 impl SessionNotifier for DirectSessionNotifier {
-    fn register<S: SessionObserver + 'static>(&mut self, signal: S) -> usize {
+    type Id = Id;
+
+    fn register<S: SessionObserver + 'static>(&mut self, signal: S) -> Id {
         self.signals.push(Some(Box::new(signal)));
-        self.signals.len() - 1
+        Id(self.signals.len() - 1)
     }
-    fn unregister(&mut self, signal: usize) {
-        self.signals[signal] = None;
+    fn unregister(&mut self, signal: Id) {
+        self.signals[signal.0] = None;
     }
 
     fn is_active(&self) -> bool {

src/backend/session/mod.rs

@@ -47,12 +47,15 @@ pub trait Session {
 /// gets paused or becomes active again. Any object implementing the `SessionObserver` trait
 /// may be registered.
 pub trait SessionNotifier {
+    /// Id type of registered observers
+    type Id: PartialEq + Eq;
+
     /// Registers a given `SessionObserver`.
     ///
     /// Returns an id of the inserted observer, can be used to remove it again.
-    fn register<S: SessionObserver + 'static>(&mut self, signal: S) -> usize;
+    fn register<S: SessionObserver + 'static>(&mut self, signal: S) -> Self::Id;
     /// Removes an observer by its given id from `SessionNotifier::register`.
-    fn unregister(&mut self, signal: usize);
+    fn unregister(&mut self, signal: Self::Id);
 
     /// Check if this session is currently active
     fn is_active(&self) -> bool;