Fix rtu-server-address example (#267)

examples/rtu-server-address.rs

@@ -11,27 +11,35 @@ struct Service {
     slave: Slave,
 }
 
-impl tokio_modbus::server::Service for Service {
-    type Request = SlaveRequest<'static>;
-    type Response = Response;
-    type Exception = Exception;
-    type Future = future::Ready<Result<Self::Response, Self::Exception>>;
-
-    fn call(&self, req: Self::Request) -> Self::Future {
-        if req.slave != self.slave.into() {
-            return future::ready(Err(Exception::IllegalFunction));
+impl Service {
+    fn handle(&self, req: SlaveRequest<'_>) -> Result<Option<Response>, Exception> {
+        let SlaveRequest { slave, request } = req;
+        if slave != self.slave.into() {
+            // Filtering: Ignore requests with mismatching slave IDs.
+            return Ok(None);
         }
-        match req.request {
+        match request {
             Request::ReadInputRegisters(_addr, cnt) => {
                 let mut registers = vec![0; cnt.into()];
                 registers[2] = 0x77;
-                future::ready(Ok(Response::ReadInputRegisters(registers)))
+                Ok(Some(Response::ReadInputRegisters(registers)))
             }
-            _ => future::ready(Err(Exception::IllegalFunction)),
+            _ => Err(Exception::IllegalFunction),
         }
     }
 }
 
+impl tokio_modbus::server::Service for Service {
+    type Request = SlaveRequest<'static>;
+    type Response = Option<Response>;
+    type Exception = Exception;
+    type Future = future::Ready<Result<Self::Response, Self::Exception>>;
+
+    fn call(&self, req: Self::Request) -> Self::Future {
+        future::ready(self.handle(req))
+    }
+}
+
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let slave = Slave(12);