add example

subxt/examples/setup_rpc_chainhead_backend_lowlevel.rs

@@ -0,0 +1,51 @@
+//! Example to utilize the low-level api of ChainHeadBackend rpc backend to subscribe to finalized blocks.
+
+#![allow(missing_docs)]
+
+use futures::StreamExt;
+use subxt::backend::chain_head::{ChainHeadBackend, ChainHeadBackendBuilder};
+use subxt::backend::rpc::RpcClient;
+use subxt::{OnlineClient, PolkadotConfig};
+use tokio::spawn;
+
+// Generate an interface that we can use from the node's metadata.
+#[subxt::subxt(runtime_metadata_path = "../artifacts/polkadot_metadata_small.scale")]
+pub mod polkadot {}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tracing_subscriber::fmt::init();
+
+    let rpc = RpcClient::from_url("ws://localhost:9944".to_string()).await?;
+    let (backend, mut driver) = ChainHeadBackendBuilder::default().build(rpc.clone());
+
+    // Note: That it is required to poll the driver until it's done (i.e returns None)
+    // to ensure that the backend is shutdown properly.
+    spawn(async move {
+        while let Some(res) = driver.next().await {
+            if let Err(err) = res {
+                tracing::debug!(target: "subxt", "chainHead backend error={err}");
+            }
+        }
+
+        tracing::debug!(target: "subxt", "chainHead backend was closed");
+    });
+
+    let api = OnlineClient::from_backend::<ChainHeadBackend<PolkadotConfig>>(std::sync::Arc::new(
+        backend,
+    ))
+    .await?;
+
+    let mut blocks_sub = api.blocks().subscribe_finalized().await?.take(100);
+
+    while let Some(block) = blocks_sub.next().await {
+        let block = block?;
+
+        let block_number = block.number();
+        let block_hash = block.hash();
+
+        println!("Block #{block_number} ({block_hash})");
+    }
+
+    Ok(())
+}