examples: Add chainhead follow subscription example

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

examples/examples/chainhead_subscription.rs

@@ -0,0 +1,73 @@
+// Copyright 2019-2022 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+//! To run this example, a local polkadot node should be running. Example verified against polkadot v0.9.28-9ffe6e9e3da.
+//!
+//! E.g.
+//! ```bash
+//! curl "https://github.com/paritytech/polkadot/releases/download/v0.9.28/polkadot" --output /usr/local/bin/polkadot --location
+//! polkadot --dev --tmp
+//! ```
+
+use codec::Encode;
+use futures::StreamExt;
+use sp_keyring::AccountKeyring;
+use subxt::{
+    OnlineClient,
+    SubstrateConfig
+};
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tracing_subscriber::fmt::init();
+
+    // Create a client to use:
+    let api = OnlineClient::<SubstrateConfig>::new().await?;
+
+    let genesis = api.rpc().chainhead_genesis_hash().await?;
+    println!("Genesis: {:?}", genesis);
+
+    let mut follow_sub = api.blocks().subscribe_chainhead_finalized(true).await?;
+    println!("Following subscription...");
+
+    // Handle all subscriptions from the `chainHead_follow`.
+    while let Some(block) = follow_sub.next().await {
+        let block = block?;
+
+        let body = block.body().await?;
+        println!("[hash={:?}] body={:?}", block.hash(), body);
+
+        let header = block.header().await?;
+        println!("[hash={:?}] header={:?}", block.hash(), header);
+
+        // let active_era_addr = substrate::storage().staking().active_era();
+        // let era = block.storage(&active_era_addr).await?.unwrap();
+        // println!(
+        //     "[hash={:?}] storage index: {:?}, start: {:?}",
+        //     block.hash(),
+        //     era.index,
+        //     era.start
+        // );
+
+        let call_params = AccountKeyring::Alice.to_account_id().encode();
+        let call = block
+            .call("AccountNonceApi_account_nonce".into(), Some(&call_params))
+            .await?;
+        println!("[hash={:?}] call={:?}", block.hash(), call);
+
+        println!("Events:");
+        let events = block.events().await?;
+        for event in events.iter() {
+            let event = event?;
+            let pallet = event.pallet_name();
+            let variant = event.variant_name();
+            println!("    {pallet}::{variant} event");
+        }
+
+        // Unpin the block as last step.
+        block.unpin().await?;
+    }
+
+    Ok(())
+}