Events sub (#126)

* Make event subscription logic more generic.

* Fix build.

* Add test-node.

* Update deps.

* Address review comments.

src/frame/balances.rs

@@ -114,10 +114,12 @@ mod tests {
             Error,
             RuntimeError,
         },
+        events::EventsDecoder,
         signer::{
             PairSigner,
             Signer,
         },
+        subscription::EventSubscription,
         system::AccountStoreExt,
         tests::{
             test_client,
@@ -201,4 +203,28 @@ mod tests {
             panic!("expected an error");
         }
     }
+
+    #[async_std::test]
+    async fn test_transfer_subscription() {
+        env_logger::try_init().ok();
+        let alice = PairSigner::new(AccountKeyring::Alice.pair());
+        let bob = AccountKeyring::Bob.to_account_id();
+        let (client, _) = test_client().await;
+        let sub = client.subscribe_events().await.unwrap();
+        let mut decoder = EventsDecoder::<TestRuntime>::new(client.metadata().clone());
+        decoder.with_balances();
+        let mut sub = EventSubscription::<TestRuntime>::new(sub, decoder);
+        sub.filter_event::<TransferEvent<_>>();
+        client.transfer(&alice, &bob, 10_000).await.unwrap();
+        let raw = sub.next().await.unwrap().unwrap();
+        let event = TransferEvent::<TestRuntime>::decode(&mut &raw.data[..]).unwrap();
+        assert_eq!(
+            event,
+            TransferEvent {
+                from: alice.account_id().clone(),
+                to: bob.clone(),
+                amount: 10_000,
+            }
+        );
+    }
 }