event subscription example (paritytech#140)

* added example

* fmt

examples/transfer_subscribe.rs

@@ -0,0 +1,54 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of substrate-subxt.
+//
+// subxt is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// subxt is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
+
+use sp_keyring::AccountKeyring;
+use substrate_subxt::{
+    balances::{
+        BalancesEventsDecoder,
+        TransferCallExt,
+        TransferEvent,
+    },
+    sp_core::Decode,
+    ClientBuilder,
+    DefaultNodeRuntime,
+    EventSubscription,
+    EventsDecoder,
+    PairSigner,
+};
+
+#[async_std::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    env_logger::init();
+
+    let signer = PairSigner::new(AccountKeyring::Alice.pair());
+    let dest = AccountKeyring::Bob.to_account_id().into();
+
+    let client = ClientBuilder::<DefaultNodeRuntime>::new().build().await?;
+    let sub = client.subscribe_events().await?;
+    let mut decoder = EventsDecoder::<DefaultNodeRuntime>::new(client.metadata().clone());
+    decoder.with_balances();
+    let mut sub = EventSubscription::<DefaultNodeRuntime>::new(sub, decoder);
+    sub.filter_event::<TransferEvent<_>>();
+    client.transfer(&signer, &dest, 10_000).await?;
+    let raw = sub.next().await.unwrap().unwrap();
+    let event = TransferEvent::<DefaultNodeRuntime>::decode(&mut &raw.data[..]);
+    if let Ok(e) = event {
+        println!("Balance transfer success: value: {:?}", e.amount);
+    } else {
+        println!("Failed to subscribe to Balances::Transfer Event");
+    }
+    Ok(())
+}