Feature request: subscribing to storage changes

[sander2]

Substrate provides state_subscribeStorage to subscribe to storage changes. It would be great if subxt could expose this. I'm not sure how the api would look - maybe something like api.subscribe_storage().timestamp().now(). It shouldn't be super difficult to implement, I think, although I'm not sure how listening only to finalized changes could best be implemented.

[jsdw]

This does come up from time to time, but one blocker for this is that the new RPC interface doesn't currently provide such a method, so I can imagine that the steps will be:

• Subxt to implement new RPC interface (see https://github.com/paritytech/json-rpc-interface-spec/)
• Subxt to only use new interface in higher level APIS
• Push to add new methods to this interface to improve experience in Subxt (eg subscribing to storage)

[dzmitry-lahoda]

I do exactly this with smoldot light client. I used subxt only as decoder, not as RPC caller or subscriber.

    client
        .json_rpc_request(
r#"{"id":1,"jsonrpc":"2.0","method":"state_subscribeStorage","params": { "list": ["0x26aa394eea5630e07c48ae0c9558cef780d41e5e16056765bc8461851072c9d7", "0x26aa394eea5630e07c48ae0c9558cef7"] }}"#,
chain_id,
        )
        .unwrap();

So would be ideal if subxt to provide named prefix for each storage (pallet/storage/key1/key2). I took 0x26aa394eea5630e07c48ae0c9558cef780d41e5e16056765bc8461851072c9d7 from pd.js frontend.

#960

[jsdw]

@lexnv I wonder whether the recent V2 RPC spec changes have added something that lets us subscribe to storage changes? Maybe it's something to raise an issue on the RPC spec repo for because it could be pretty cool to have this sort of thing in Subxt :)