# Emitter

> ![icon-note.gif](public_sys-resources/icon-note.gif) **NOTE**<br>
> The initial APIs of this module are supported since API version 7.

## Modules to Import

```javascript
import emitter from '@ohos.events.emitter'
```

## Required Permissions

None

## EventPriority

Enumerates the event emit priority levels.

| Name     | Value  | Description                                             |
| --------- | ---- | ------------------------------------------------- |
| IMMEDIATE | 0    | The event will be emitted immediately.<br>**System capability**: SystemCapability.Notification.Emitter |
| HIGH      | 1    | The event will be emitted before low-priority events.<br>**System capability**: SystemCapability.Notification.Emitter |
| LOW       | 2    | The event will be emitted before idle-priority events. By default, an event is in LOW priority.<br>**System capability**: SystemCapability.Notification.Emitter |
| IDLE      | 3    | The event will be emitted after all the other events.<br>**System capability**: SystemCapability.Notification.Emitter |

## emitter.on

on(event: [InnerEvent](#innerevent), callback: Callback\<[EventData](#eventdata)\>): void

Subscribes to an event in persistent manner. This API uses a callback to return the event.

**System capability**: SystemCapability.Notification.Emitter

**Parameters**

| Name  | Type                               | Mandatory | Description                    |
| -------- | ----------------------------------- | ---- | ------------------------ |
| event    | [InnerEvent](#innerevent)           | Yes  | Event to subscribe to in persistent manner. |
| callback | Callback\<[EventData](#eventdata)\> | Yes  | Callback used to return the event. |

**Example**

```javascript
var innerEvent = {
    eventId: 1
};
var callback = (eventData) => {
    console.info('callback');
};
emitter.on(innerEvent, callback);
```

## emitter.once

once(event: [InnerEvent](#innerevent), callback: Callback\<[EventData](#eventdata)\>): void

Subscribes to an event in one-shot manner and unsubscribes from it after the event callback is received.

**System capability**: SystemCapability.Notification.Emitter

**Parameters**

| Name  | Type                               | Mandatory | Description                    |
| -------- | ----------------------------------- | ---- | ------------------------ |
| event    | [InnerEvent](#innerevent)           | Yes  | Event to subscribe to in one-shot manner. |
| callback | Callback\<[EventData](#eventdata)\> | Yes  | Callback used to return the event. |

**Example**

```javascript
var innerEvent = {
    eventId: 1
};
var callback = (eventData) => {
    console.info('once callback');
};
emitter.once(innerEvent, callback);
```

## emitter.off

off(eventId: number): void

Unsubscribes from an event.

**System capability**: SystemCapability.Notification.Emitter

**Parameters**

| Name | Type  | Mandatory | Description |
| ------- | ------ | ---- | ------ |
| eventId | number | Yes | Event ID. |

**Example**

```javascript
emitter.off(1);
```

## emitter.emit

emit(event: InnerEvent, data?: EventData): void

Emits an event to the event queue.

**System capability**: SystemCapability.Notification.Emitter

**Parameters**

| Name| Type                     | Mandatory| Description |
| ------ | ------------------------- | ---- | -------------- |
| event  | [InnerEvent](#innerevent) | Yes | Event to emit. |
| data   | [EventData](#eventdata)   | No | Data carried by the event. |

**Example**

```javascript
var eventData = {
    data: {
        "content": "c",
        "id": 1,
    }};
var innerEvent = {
    eventId: 1,
    priority: emitter.EventPriority.HIGH
};
emitter.emit(innerEvent, eventData);
```

## InnerEvent

Describes an intra-process event.

| Name    | Type                       | Readable| Writable| Description                              |
| -------- | ------------------------------- | ---- | ---- | ---------------------------------- |
| eventId  | number                          | Yes  | Yes | Event ID, which is used to identify an event.<br>**System capability**: SystemCapability.Notification.Emitter |
| priority | [EventPriority](#eventpriority) | Yes  | Yes | Emit priority of the event.<br>**System capability**: SystemCapability.Notification.Emitter |

## EventData

Describes the data passed in the event.

| Name| Type          | Readable| Writable| Description          |
| ---- | ------------------ | ---- | ---- | -------------- |
| data | [key: string]: any | Yes | Yes | Data carried by the event. The data type can be String, Integer, or Boolean.<br>**System capability**: SystemCapability.Notification.Emitter |