How to identify received contract event?

Question

My contract emits several [ink(event)] events. My application subscribes to the events and receives Contracts_ContractEmitted data (from centrifuge go-substrate-rpc-client). How may I understand which of my events caused this?

From my experiments, I see that event.Topic[0] somehow identifies the event - the same event has the same values, different have differents. If so, how is it encoded?

Answer 1

What helped me to get a glimpse of this is to cargo expand a simple lib.rs file.

I added two events to the flipper contract:

    #[ink(event)]
    pub struct Created {
        value: bool,
    }

    #[ink(event)]
    pub struct Flipped {
        #[ink(topic)]
        value: bool,
    }

I executed cargo expand --lib > expanded_lib.rs and somewhere you can find the code where the topics are pushed (push_topic) to the TopicsBuilderBackend.

    const _: () = {
        impl ::ink::env::Topics for Created {
           type RemainingTopics = [::ink::env::topics::state::HasRemainingTopics; 1usize];
            fn topics<E, B>(
                &self,
                builder: ::ink::env::topics::TopicsBuilder<
                    ::ink::env::topics::state::Uninit,
                    E,
                    B,
                >,
            ) -> <B as ::ink::env::topics::TopicsBuilderBackend<E>>::Output
            where
                E: ::ink::env::Environment,
                B: ::ink::env::topics::TopicsBuilderBackend<E>,
            {
                builder
                    .build::<Self>()
                    .push_topic::<
                        ::ink::env::topics::PrefixedValue<[u8; 13usize]>,
                    >(
                        &::ink::env::topics::PrefixedValue {
                            value: b"Test::Created",
                            prefix: b"",
                        },
                    )
                    .finish()
            }
        }
    };
    const _: () = {
        impl ::ink::env::Topics for Flipped {
            type RemainingTopics = [::ink::env::topics::state::HasRemainingTopics; 2usize];
            fn topics<E, B>(
                &self,
                builder: ::ink::env::topics::TopicsBuilder<
                    ::ink::env::topics::state::Uninit,
                    E,
                    B,
                >,
            ) -> <B as ::ink::env::topics::TopicsBuilderBackend<E>>::Output
            where
                E: ::ink::env::Environment,
                B: ::ink::env::topics::TopicsBuilderBackend<E>,
            {
                builder
                    .build::<Self>()
                    .push_topic::<
                        ::ink::env::topics::PrefixedValue<[u8; 13usize]>,
                    >(
                        &::ink::env::topics::PrefixedValue {
                            value: b"Test::Flipped",
                            prefix: b"",
                        },
                    )
                    .push_topic::<
                        ::ink::env::topics::PrefixedValue<bool>,
                    >(
                        &::ink::env::topics::PrefixedValue {
                            value: &self.value,
                            prefix: b"Test::Flipped::value",
                        },
                    )
                    .finish()
            }
        }
    };

The first topic is the PrefixedValue of the event itself:

PrefixedValue { value: b"Test::Created", prefix: b"" }

The second topic is the actual #[ink(topic)] in the struct:

PrefixedValue { value: &self.value, prefix: b"Test::Flipped::value" }

When we add the anonymous attribute we get something like:

    const _: () = {
        impl ::ink::env::Topics for Created {
            type RemainingTopics = ::ink::env::topics::state::NoRemainingTopics;
            fn topics<E, B>(
                &self,
                builder: ::ink::env::topics::TopicsBuilder<
                    ::ink::env::topics::state::Uninit,
                    E,
                    B,
                >,
            ) -> <B as ::ink::env::topics::TopicsBuilderBackend<E>>::Output
            where
                E: ::ink::env::Environment,
                B: ::ink::env::topics::TopicsBuilderBackend<E>,
            {
                builder.build::<Self>().finish()
            }
        }
    };

Where you can see that no topics are pushed to TopicsBuilderBackend.

Additional info: erc20 tests for emitted events.

Hope this helps.