1

If I make a request to get events in a particular block using web UI, I get a nice human-readable response:

enter image description here

I've made an RPC call to get the same data

{
    "id": 1,
    "jsonrpc": "2.0",
    "method": "state_getStorageAt", 
    "params": ["0x26aa394eea5630e07c48ae0c9558cef780d41e5e16056765bc8461851072c9d7","0x5056c4cb701e192962e5a76c895273e69188edea1badf9b145b884a9416c8443"]
}

... and I got an output

{
    "jsonrpc": "2.0",
    "result": "0x1000000000000000585f8f090000000002000000010000000508ba6ef6480a2c4173bbbf0c27355c0e3c6d7a874a21cc2464e97d3fd5d92e245b1b5a7307000000000000000000000000000001000000080380eb30ca364f5c349d573b62cfadadba95af2e94f62fd8af4e15b90b971a4da633803b0648373051a1e2158c547112dcf63802d0d52b301a33dae8b9c6114e90f012b8516d57525358775a7476385956586574346e43424a396776646d4a6e3564364b34385a6a6148704763515a75676300000100000000001027000000000000000000",
    "id": 1
}

I understand that it is a set of data encrypted with SCALE codec.

For example 0803 is a pallet and method indexes and eb30ca364f5c349d573b62cfadadba95af2e94f62fd8af4e15b90b971a4da633 is attached data.

What is the best way to decode the complete result with Rust? Thanks

1 Answer 1

3

There's two ways of decoding events in rust: statically typed and dynamically typed.

Statically you can use subxt to decode events from this SCALE encoding - it's done here for example. Basically every runtime has a concrete Event type and that's what you're decoding into.

let event_bytes = hex::decode("0x1000000000000000585f8f090000000002000000010000000508ba6ef6480a2c4173bbbf0c27355c0e3c6d7a874a21cc2464e97d3fd5d92e245b1b5a7307000000000000000000000000000001000000080380eb30ca364f5c349d573b62cfadadba95af2e94f62fd8af4e15b90b971a4da633803b0648373051a1e2158c547112dcf63802d0d52b301a33dae8b9c6114e90f012b8516d57525358775a7476385956586574346e43424a396776646d4a6e3564364b34385a6a6148704763515a75676300000100000000001027000000000000000000").unwrap();
let event = <polkadot::Event as Decode>::decode(&mut event_bytes.as_slice()).unwrap();

and I have polkadot defined as:

#[subxt::subxt(runtime_metadata_path = "polkadot_metadata.scale")]
pub mod polkadot {}

(rather than pointing it at a metadata file you can download it by giving it a url #[subxt::subxt(runtime_metadata_path = "wss://kusama-rpc.polkadot.io:443")]. If you point that at your node's url then it should decode your event.)

Dynamically the tooling needs generalising, but it is possible and has been done by cargo-contract.

If you need to interpret all events across all parachains then go dynamic. If you are focused on a few particular parachains then static typing is a lot safer and easier.

2
  • Thanks for the answer. Is there an option to decode it using parity_scale_codec::Decode? If I understand correctly Decode is just a Trait and I need pre-defined Struct to use it. Where can I find the structure of an event? May 17 at 7:41
  • The above are all using that Decode trait but the key is that you need to give it the type to re-hydrate (to deserialise into). That's the Event type that is defined on the runtime associated with the parachain / relay chain that your decoding the event for.
    – Squirrel
    May 18 at 8:55

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