Events with Method References

rust.rs

#[derive(CandidType, Deserialize)]
struct NotifyArgs {
    pub callback: candid::Func,
    pub transaction_id: candid::Nat,
    pub metadata: String,
}

#[derive(CandidType, Deserialize)]
enum TransferError {
    InsufficientBalance,
    AmountTooLarge,
    CallFailed,
    Unknown
}

#[derive(CandidType, Deserialize)]
enum TransferResult {
    Ok(candid::Nat),
    Err(TransferError),
}

#[derive(CandidType, Deserialize)]
struct TransferRequest {
    pub from: Option<candid::Principal>,
    pub to: candid::Principal,
    pub amount: candid::Nat,
    pub notify: Option<NotifyArgs>,
}

#[update]
async fn transfer(transfer_request : TransferRequest) -> TransferResult {
    // Transfer logic above...
    let a = transfer_request.notify.borrow();
    ic_cdk::print(a.is_some().to_string());
    match transfer_request.notify {
        Some(v) => {
            ic_cdk::print("Here");
            let _ = call::<(String,),()>(v.callback.principal, v.callback.method.as_str(), (v.metadata, )).await;
        }
        _ => {}
    }
    return TransferResult::Ok(candid::Nat::from(0));
}