Events are special values that can be emitted during the execution of a program.
An event type can be declared with the event
keyword.
1event 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.
1// Invalid: An event cannot be declared globally2//3event GlobalEvent(field: Int)45pub contract Events {6// Event with explicit argument labels7//8event BarEvent(labelA fieldA: Int, labelB fieldB: Int)910// Invalid: A resource type is not allowed to be used11// because it would be moved and lost12//13event ResourceEvent(resourceField: @Vault)14}
To emit an event from a program, use the emit
statement:
1pub contract Events {2event FooEvent(x: Int, y: Int)34// Event with argument labels5event BarEvent(labelA fieldA: Int, labelB fieldB: Int)67fun events() {8emit FooEvent(x: 1, y: 2)910// Emit event with explicit argument labels11// Note that the emitted event will only contain the field names,12// not the argument labels used at the invocation site.13emit BarEvent(labelA: 1, labelB: 2)14}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.