Update signature of AppService::register_event_handler

It no longer requires a mutable `AppService` and can also be chained,
like `Client::register_event_handler`.

crates/matrix-sdk-appservice/examples/appservice_autojoin.rs

@@ -39,7 +39,7 @@ pub async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let server_name = "localhost";
     let registration = AppServiceRegistration::try_from_yaml_file("./tests/registration.yaml")?;
 
-    let mut appservice = AppService::new(homeserver_url, server_name, registration).await?;
+    let appservice = AppService::new(homeserver_url, server_name, registration).await?;
     appservice
         .register_event_handler({
             let appservice = appservice.clone();

crates/matrix-sdk-appservice/src/lib.rs

@@ -380,7 +380,7 @@ impl AppService {
     ///
     /// [`join` room `timeline` events]: https://spec.matrix.org/unstable/client-server-api/#get_matrixclientr0sync
     /// [MSC2409]: https://github.com/matrix-org/matrix-doc/pull/2409
-    pub async fn register_event_handler<Ev, Ctx, H>(&mut self, handler: H) -> Result<()>
+    pub async fn register_event_handler<Ev, Ctx, H>(&self, handler: H) -> Result<&Self>
     where
         Ev: SyncEvent + DeserializeOwned + Send + 'static,
         H: EventHandler<Ev, Ctx>,
@@ -389,7 +389,7 @@ impl AppService {
         let client = self.get_cached_client(None)?;
         client.register_event_handler(handler).await;
 
-        Ok(())
+        Ok(self)
     }
 
     /// Register a responder for queries about the existence of a user with a

crates/matrix-sdk-appservice/tests/tests.rs

@@ -199,7 +199,7 @@ async fn test_no_access_token() -> Result<()> {
 
 #[async_test]
 async fn test_event_handler() -> Result<()> {
-    let mut appservice = appservice(None).await?;
+    let appservice = appservice(None).await?;
 
     #[allow(clippy::mutex_atomic)]
     let on_state_member = Arc::new(Mutex::new(false));