How to query events?

Question

I need to get a list of all events of a specific custom type. In the documentation, it said that I can query the System pallet's storage for events. I am looking for an example (in Rust) of how to get a list of specific events. I am planning to do this in an OCW and I need this for the current block but also need it for all blocks.

Listening to events The Substrate RPC does not directly expose an endpoint for querying events. If you used the default implementation, you can see the list of events for the current block by querying the storage of the System pallet. Otherwise, the Polkadot-JS API supports a WebSocket subscription on runtime events.

I tried this:

    let evts = <frame_system::Pallet<T>>::read_events_no_consensus();
    for evt in evts {
        frame_support::log::info!("evt={:?}", evt.event);
    }

which works BUT it prints:

evt=<wasm:stripped>
evt=<wasm:stripped>
evt=<wasm:stripped>
evt=<wasm:stripped>
evt=<wasm:stripped>
evt=<wasm:stripped>

How can I dump the fields of each event so I can understand what events occurred?

For efficiency, how can I get just the events for the current block?

Answer 1

You can do frame_system::Pallet::<T>::events() to fetch all the events emitted. Example

If you are working with mock runtime then you can directly use System::events() (System being the instantiation of pallet_frame_system that you generated using the construct_runtime!() macro). You can then iterate over all the events and match with the ones that you are concerned about.

In the below example, I'm trying to extract the address of the smart contract I deployed by matching with the Instantiated event from the pallet-contracts.

    let evts = System::events();
    let deployed_address = evts
        .iter()
        .rev()
        .find_map(|rec| {
            if let Event::Contracts(pallet_contracts::Event::Instantiated {
                deployer: _,
                contract,
            }) = &rec.event
            {
                Some(contract)
            } else {
                None
            }
        })
        .expect("unable to find deployed contract");

Hope this answers your question :)

Answer 2

You can use the Libuptest crate for Rust to extract the block events from a block. For example: If you know the blocknr or blockhash you can use libuptest to extract all extrinsics from a block in a Vec, code example: https://uptest-sc.github.io/cli/event_sub.html#code-example