Using Events

pallets/simple-event, pallets/generic-event

Having a transaction included in a block does not guarantee that the function executed successfully. As we saw in the previous recipe, many calls can cause errors, but still be included in a block. To verify that functions have executed successfully, emit an event at the bottom of the function body.

Events notify the off-chain world of successful state transitions.

Some Prerequisites

When using events, we have to include the Event type in our configuration trait. Although the syntax is a bit complex, it is the same every time. If you are a skilled Rust programmer you will recognize this as a series of trait bounds. If you don't recognize this feature of Rust yet, don't worry; it is the same every time, so you can just copy it and move on.

pub trait Trait: system::Trait {
	type Event: From<Event> + Into<<Self as system::Trait>::Event>;
}

Next we have to add a line inside of the decl_module! macro which generates the deposit_event function we'll use later when emitting our events. Even experienced Rust programmers will not recognize this syntax because it is unique to this macro. Just copy it each time.

decl_module! {
	pub struct Module<T: Trait> for enum Call where origin: T::Origin {

		// This line is new
		fn deposit_event() = default;

		// --snip--
	}
}

Declaring Events

To declare an event, use the decl_event! macro. Like any rust enum, Events have names, and can optionally carry data with them. The syntax is slightly different depending on whether the events carry data of primitive types, or generic types from the pallet's configuration trait. These two techniques are demonstrated in the simple-event and generic-event pallets respectively.

Simple Events

The simplest example of an event uses the following syntax

decl_event!(
	pub enum Event {
		EmitInput(u32),
	}
);

Events with Generic Types

Sometimes events might contain types from the pallet's Configuration Trait. In this case, it is necessary to specify additional syntax

decl_event!(
	pub enum Event<T> where AccountId = <T as system::Trait>::AccountId {
		EmitInput(AccountId, u32),
	}
);

This example also demonstrates how the where clause can be used to specify type aliasing for more readable code.

Emitting Events

Events are emitted from dispatchable calls using the deposit_event method.

Simple Events

The event is emitted at the bottom of the do_something function body.

Self::deposit_event(Event::EmitInput(new_number));

Events with Generic Types

The syntax for deposit_event now takes the RawEvent type because it is generic over the pallet's configuration trait.

Self::deposit_event(RawEvent::EmitInput(user, new_number));

Constructing the Runtime

For the first time in the recipes, our pallet has an associated type in its configuration trait. We must specify this type when implementing its trait. In the case of the Event type, this is entirely straight forward, and looks the same for both simple events and generic events.

impl simple_event::Trait for Runtime {
	type Event = Event;
}

Events, like dispatchable calls and storage items, requires a slight change to the line in construct_runtime!. Notice that the <T> is necessary for generic events.

construct_runtime!(
	pub enum Runtime where
		Block = Block,
		NodeBlock = opaque::Block,
		UncheckedExtrinsic = UncheckedExtrinsic
	{
		// --snip--
		GenericEvent: generic_event::{Module, Call, Event<T>},
		SimpleEvent: simple_event::{Module, Call, Event},
	}
);