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.

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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.

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// Invalid: An event cannot be declared globally

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//

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event GlobalEvent(field: Int)

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access(all)

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contract Events {

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// Event with explicit argument labels

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//

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

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

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// because it would be moved and lost

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//

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event ResourceEvent(resourceField: @Vault)

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}

Emitting events

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

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access(all)

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contract Events {

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event FooEvent(x: Int, y: Int)

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// Event with argument labels

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

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fun events() {

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emit FooEvent(x: 1, y: 2)

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// Emit event with explicit argument labels

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

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

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emit BarEvent(labelA: 1, labelB: 2)

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}

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}

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.