Derive Macro EventMetadata

#[derive(EventMetadata)]

Derives the [ink::EventMetadata] trait for the given struct, which provides metadata about the event definition.

Requires that the struct also implements the [ink::Event] trait, so this derive is usually used in combination with the [Event] derive.

Metadata is not embedded into the contract binary, it is generated from a separate compilation of the contract with the std feature, therefore this derive must be conditionally compiled e.g. #[cfg_attr(feature = "std", derive(::ink::EventMetadata))] (see example below).

For convenience there is the [event] attribute macro that will expand to all the necessary derives for an event implementation, including this one.

Example

use ink::{
    Event,
    env::DefaultEnvironment,
};
use scale::Encode;

#[cfg_attr(feature = "std", derive(::ink::EventMetadata))]
#[derive(Event, Encode)]
struct MyEvent {
    a: u32,
    #[ink(topic)]
    b: [u8; 32],
}

assert_eq!(<MyEvent as ink::metadata::EventMetadata>::event_spec().args().len(), 2);

The generated code will also register this implementation with the global static distributed slice [ink::metadata::EVENTS], in order that the metadata of all events used in a contract can be collected.