Use droppable signal subscription

Instead of returning an opaque ID for manual unsubscription, let's
instead return a struct which holds on the connection and automatically
unsubscribes when dropped.

This allows controlling the lifecycle of a signal subscription through
the standard lifetime management of Rust which is a lot more convenient.

Author: swsnr

gio/src/dbus_connection.rs

@@ -117,8 +117,20 @@ pub struct ActionGroupExportId(NonZeroU32);
 pub struct MenuModelExportId(NonZeroU32);
 #[derive(Debug, Eq, PartialEq)]
 pub struct FilterId(NonZeroU32);
+
 #[derive(Debug, Eq, PartialEq)]
-pub struct SignalSubscriptionId(NonZeroU32);
+struct SignalSubscriptionId(NonZeroU32);
+
+#[derive(Debug)]
+pub struct SignalSubscription(DBusConnection, Option<SignalSubscriptionId>);
+
+impl Drop for SignalSubscription {
+    fn drop(&mut self) {
+        if let Some(id) = self.1.take() {
+            self.0.signal_unsubscribe(id);
+        }
+    }
+}
 
 // rustdoc-stripper-ignore-next
 /// Build a registered DBus object, by handling different parts of DBus.
@@ -450,7 +462,7 @@ impl DBusConnection {
         arg0: Option<&str>,
         flags: DBusSignalFlags,
         callback: P,
-    ) -> SignalSubscriptionId {
+    ) -> SignalSubscription {
         let callback_data: Box_<P> = Box_::new(callback);
         unsafe extern "C" fn callback_func<
             P: Fn(&DBusConnection, &str, &str, &str, &str, &glib::Variant) + 'static,
@@ -502,12 +514,15 @@ impl DBusConnection {
                 Box_::into_raw(super_callback0) as *mut _,
                 destroy_call9,
             );
-            SignalSubscriptionId(NonZeroU32::new_unchecked(id))
+            SignalSubscription(
+                self.clone(),
+                Some(SignalSubscriptionId(NonZeroU32::new_unchecked(id))),
+            )
         }
     }
 
     #[doc(alias = "g_dbus_connection_signal_unsubscribe")]
-    pub fn signal_unsubscribe(&self, subscription_id: SignalSubscriptionId) {
+    fn signal_unsubscribe(&self, subscription_id: SignalSubscriptionId) {
         unsafe {
             ffi::g_dbus_connection_signal_unsubscribe(
                 self.to_glib_none().0,

gio/src/lib.rs

@@ -33,7 +33,7 @@ pub use self::dbus::*;
 mod dbus_connection;
 pub use self::dbus_connection::{
     ActionGroupExportId, FilterId, MenuModelExportId, RegistrationBuilder, RegistrationId,
-    SignalSubscriptionId, WatcherId,
+    SignalSubscription, WatcherId,
 };
 mod dbus_message;
 mod dbus_method_invocation;