pezkuwi-subxt/subxt/examples/unstable_light_client_parachains.rs

use futures::StreamExt;
use std::{iter, num::NonZeroU32};
use subxt::{
    client::{LightClient, RawLightClient},
    PolkadotConfig,
};

// 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 {}

const POLKADOT_SPEC: &str = include_str!("../../artifacts/demo_chain_specs/polkadot.json");
const ASSET_HUB_SPEC: &str =
    include_str!("../../artifacts/demo_chain_specs/polkadot_asset_hub.json");

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // The smoldot logs are informative:
    tracing_subscriber::fmt::init();

    // Connecting to a parachain is a multi step process.

    // Step 1. Construct a new smoldot client.
    let mut client =
        subxt_lightclient::smoldot::Client::new(subxt_lightclient::smoldot::DefaultPlatform::new(
            "subxt-example-light-client".into(),
            "version-0".into(),
        ));

    // Step 2. Connect to the relay chain of the parachain. For this example, the Polkadot relay chain.
    let polkadot_connection = client
        .add_chain(subxt_lightclient::smoldot::AddChainConfig {
            specification: POLKADOT_SPEC,
            json_rpc: subxt_lightclient::smoldot::AddChainConfigJsonRpc::Enabled {
                max_pending_requests: NonZeroU32::new(128).unwrap(),
                max_subscriptions: 1024,
            },
            potential_relay_chains: iter::empty(),
            database_content: "",
            user_data: (),
        })
        .expect("Light client chain added with valid spec; qed");
    let polkadot_json_rpc_responses = polkadot_connection
        .json_rpc_responses
        .expect("Light client configured with json rpc enabled; qed");
    let polkadot_chain_id = polkadot_connection.chain_id;

    // Step 3. Connect to the parachain. For this example, the Asset hub parachain.
    let assethub_connection = client
        .add_chain(subxt_lightclient::smoldot::AddChainConfig {
            specification: ASSET_HUB_SPEC,
            json_rpc: subxt_lightclient::smoldot::AddChainConfigJsonRpc::Enabled {
                max_pending_requests: NonZeroU32::new(128).unwrap(),
                max_subscriptions: 1024,
            },
            // The chain specification of the asset hub parachain mentions that the identifier
            // of its relay chain is `polkadot`.
            potential_relay_chains: [polkadot_chain_id].into_iter(),
            database_content: "",
            user_data: (),
        })
        .expect("Light client chain added with valid spec; qed");
    let parachain_json_rpc_responses = assethub_connection
        .json_rpc_responses
        .expect("Light client configured with json rpc enabled; qed");
    let parachain_chain_id = assethub_connection.chain_id;

    // Step 4. Turn the smoldot client into a raw client.
    let raw_light_client = RawLightClient::builder()
        .add_chain(polkadot_chain_id, polkadot_json_rpc_responses)
        .add_chain(parachain_chain_id, parachain_json_rpc_responses)
        .build(client)
        .await?;

    // Step 5. Obtain a client to target the relay chain and the parachain.
    let polkadot_api: LightClient<PolkadotConfig> =
        raw_light_client.for_chain(polkadot_chain_id).await?;
    let parachain_api: LightClient<PolkadotConfig> =
        raw_light_client.for_chain(parachain_chain_id).await?;

    // Step 6. Subscribe to the finalized blocks of the chains.
    let polkadot_sub = polkadot_api
        .blocks()
        .subscribe_finalized()
        .await?
        .map(|block| ("Polkadot", block));
    let parachain_sub = parachain_api
        .blocks()
        .subscribe_finalized()
        .await?
        .map(|block| ("AssetHub", block));
    let mut stream_combinator = futures::stream::select(polkadot_sub, parachain_sub);

    while let Some((chain, block)) = stream_combinator.next().await {
        let block = block?;

        println!("     Chain {:?} hash={:?}", chain, block.hash());
    }

    Ok(())
}