Events

Events are special values that can be emitted during the execution of a program.

An event type can be declared with the event keyword.

_10

event FooEvent(x: Int, y: Int)

The syntax of an event declaration is similar to that of a function declaration; events contain named parameters, each of which has an optional argument label.

Event parameters may only have a valid event parameter type. Valid types are boolean, string, integer, arrays and dictionaries of these types, and structures where all fields have a valid event parameter type. Resource types are not allowed, because when a resource is used as an argument, it is moved.

Events can only be declared within a contract body. Events cannot be declared globally or within resource or struct types.

_14

// Invalid: An event cannot be declared globally

_14

//

_14

event GlobalEvent(field: Int)

_14

_14

access(all) contract Events {

_14

// Event with explicit argument labels

_14

//

_14

event BarEvent(labelA fieldA: Int, labelB fieldB: Int)

_14

_14

// Invalid: A resource type is not allowed to be used

_14

// because it would be moved and lost

_14

//

_14

event ResourceEvent(resourceField: @Vault)

_14

}

Emitting events

To emit an event from a program, use the emit statement:

_15

access(all) contract Events {

_15

event FooEvent(x: Int, y: Int)

_15

_15

// Event with argument labels

_15

event BarEvent(labelA fieldA: Int, labelB fieldB: Int)

_15

_15

fun events() {

_15

emit FooEvent(x: 1, y: 2)

_15

_15

// Emit event with explicit argument labels

_15

// Note that the emitted event will only contain the field names,

_15

// not the argument labels used at the invocation site.

_15

emit BarEvent(labelA: 1, labelB: 2)

_15

}

_15

}

Emitting events has the following restrictions:

• Events can only be invoked in an emit statement.

  This means events cannot be assigned to variables or used as function parameters.

• Events can only be emitted from the location in which they are declared.