Publish events from a Rust contract

An event can contain up to 4 topics, alongside the data it is publishing. The data can be any value or type you want. However, the topics must not contain:

• Vec
• Map
• Bytes or BytesN longer than 32 bytes
• contracttype

// This function does nothing beside publishing events. Topics we are using are
// some `u32` integers for the sake of simplicity here.
pub fn events_function(env: Env) {
    // A symbol will be our `data` we want published
    my_data = Symbol::new(&env, "data_to_publish");

    // an event with 0 topics
    env.events().publish((), my_data.clone());

    // an event with 1 topic (Notice the extra comma after the topic in the
    // tuple? That comma is required in Rust to make a one-element tuple)
    env.events().publish((1u32,), my_data.clone());

    // an event with 2 topics
    env.events().publish((1u32, 2u32), my_data.clone());

    // an event with 3 topics
    env.events().publish((1u32, 2u32, 3u32), my_data.clone());

    // an event with 4 topics
    env.events().publish((1u32, 2u32, 3u32, 4u32), my_data.clone());
}

A more realistic example can be found in the way the token interface works. For example, the interface requires an event to be published every time the transfer function is invoked, with the following information:

pub fn transfer(env: Env, from: Address, to: Address, amount: i128) {
    // transfer logic omitted here
    env.events().publish(
        (symbol_short!("transfer"), from, to),
        amount
    );
}