EventHub

EventHub is an event communication mechanism based on the publish-subscribe pattern. It decouples senders and subscribers through event names, supporting efficient data transfer and state synchronization between different service modules.

It is primarily used for data communication between UIAbility components and UI pages.

Different Context objects have different EventHub objects, and different EventHub objects cannot communicate directly with each other. Event subscription, unsubscription, and triggering all take place on a specific EventHub object.

Since Worker and TaskPool implement multithreaded concurrency through the actor model, where different virtual machine instances have exclusive memory, EventHub objects cannot be used for inter-thread data communication.

NOTE

The initial APIs of this module are supported since API version 9. Newly added APIs will be marked with a superscript to indicate their earliest API version.

The APIs of this module can be used only in the stage model.

Constraints

EventHub cannot be used for data communication between processes.
EventHub cannot be used for data communication between Worker or TaskPool threads. Instead, use Emitter for inter-thread communication.
Data communication between EventHub objects of different Context objects within the same thread is not supported.
A Context object converted by sendableContextManager is considered different from the original Context object, and data communication between their EventHub objects is not supported.

Modules to Import

import { common } from '@kit.AbilityKit';

Usage

You need to obtain an EventHub object through a Context object. The following example demonstrates how to obtain the EventHub object of a UIAbility instance's Context object.

import { common, UIAbility } from '@kit.AbilityKit';

export default class EntryAbility extends UIAbility {
  eventFunc() {
    console.info('eventFunc is called');
  }

  onCreate() {
    // Method 1 (recommended)
    this.context.eventHub.on('myEvent', this.eventFunc);

    // Method 2
    let eventhub = this.context.eventHub as common.EventHub;
    eventhub.on('myEvent', this.eventFunc);
  }
}

EventHub.on

on(event: string, callback: Function): void;

Subscribes to an event.

NOTE

When the callback is triggered by emit, the invoker is the EventHub object. To change the direction of this in callback, use an arrow function.

Atomic service API: This API can be used in atomic services since API version 11.

System capability: SystemCapability.Ability.AbilityRuntime.Core

Parameters

Name	Type	Mandatory	Description
event	string	Yes	Event name.
callback	Function	Yes	Callback invoked when the event is triggered.

Error codes

For details about the error codes, see Universal Error Codes.

ID	Error Message
401	Parameter error. Possible causes: 1. Mandatory parameters are left unspecified; 2. Incorrect parameter types; 3. Parameter verification failed.

Example 1 When the callback is triggered by emit, the invoker is the EventHub object. The EventHub object does not have the value property. Therefore, the result undefined is returned.

import { UIAbility } from '@kit.AbilityKit';
import { BusinessError } from '@kit.BasicServicesKit';

export default class EntryAbility extends UIAbility {
  value: number = 12;

  onCreate() {
    try {
      this.context.eventHub.on('myEvent', this.eventFunc);
    } catch (e) {
      let code: number = (e as BusinessError).code;
      let msg: string = (e as BusinessError).message;
      console.error(`EventHub emit error, code: ${code}, msg: ${msg}`);
    }
  }

  onForeground() {
    try {
      // Result
      // eventFunc is called, value: undefined
      this.context.eventHub.emit('myEvent');
    } catch (e) {
      let code: number = (e as BusinessError).code;
      let msg: string = (e as BusinessError).message;
      console.error(`EventHub emit error, code: ${code}, msg: ${msg}`);
    }
  }

  eventFunc() {
    console.info(`eventFunc is called, value: ${this.value}`);
  }
}

Example 2 When the callback uses an arrow function, the invoker is the EntryAbility object. The EntryAbility object has the value property. Therefore, the result 12 is returned.

import { UIAbility } from '@kit.AbilityKit';
import { BusinessError } from '@kit.BasicServicesKit';

export default class EntryAbility extends UIAbility {
  value: number = 12;

  onCreate() {
    try {
      // Anonymous functions can be used to subscribe to events.
      this.context.eventHub.on('myEvent', () => {
        console.info(`anonymous eventFunc is called, value: ${this.value}`);
      });
    } catch (e) {
      let code: number = (e as BusinessError).code;
      let msg: string = (e as BusinessError).message;
      console.error(`EventHub emit error, code: ${code}, msg: ${msg}`);
    }
  }

  onForeground() {
    try {
      // Result
      // anonymous eventFunc is called, value: 12
      this.context.eventHub.emit('myEvent');
    } catch (e) {
      let code: number = (e as BusinessError).code;
      let msg: string = (e as BusinessError).message;
      console.error(`EventHub emit error, code: ${code}, msg: ${msg}`);
    }
  }

  eventFunc() {
    console.info(`eventFunc is called, value: ${this.value}`);
  }
}

EventHub.off

off(event: string, callback?: Function): void;

Unsubscribes from an event.

If callback is specified, this API unsubscribes from the given event with the specified callback.
If callback is not specified, this API unsubscribes from the given event with all callbacks.

Atomic service API: This API can be used in atomic services since API version 11.

System capability: SystemCapability.Ability.AbilityRuntime.Core

Parameters

Name	Type	Mandatory	Description
event	string	Yes	Event name.
callback	Function	No	Callback for the event. If callback is unspecified, the given event with all callbacks is unsubscribed.

Error codes

For details about the error codes, see Universal Error Codes.

ID	Error Message
401	Parameter error. Possible causes: 1. Mandatory parameters are left unspecified; 2. Incorrect parameter types; 3. Parameter verification failed.

Example

import { UIAbility } from '@kit.AbilityKit';
import { BusinessError } from '@kit.BasicServicesKit';

export default class EntryAbility extends UIAbility {
  onCreate() {
    try {
      this.context.eventHub.on('myEvent', this.eventFunc1);
      this.context.eventHub.off('myEvent', this.eventFunc1); // Unsubscribe from the myEvent event with the callback eventFunc1.
      this.context.eventHub.on('myEvent', this.eventFunc1);
      this.context.eventHub.on('myEvent', this.eventFunc2);
      this.context.eventHub.off('myEvent'); // Unsubscribe from the myEvent event with all the callbacks (eventFunc1 and eventFunc2).
    } catch (e) {
      let code: number = (e as BusinessError).code;
      let msg: string = (e as BusinessError).message;
      console.error(`EventHub emit error, code: ${code}, msg: ${msg}`);
    }
  }

  eventFunc1() {
    console.info('eventFunc1 is called');
  }

  eventFunc2() {
    console.info('eventFunc2 is called');
  }
}

EventHub.emit

emit(event: string, ...args: Object[]): void;

Triggers an event.

Atomic service API: This API can be used in atomic services since API version 11.

System capability: SystemCapability.Ability.AbilityRuntime.Core

Parameters

Name	Type	Mandatory	Description
event	string	Yes	Event name.
...args	Object[]	No	Variable parameters, which are passed to the callback when the event is triggered.

Error codes

For details about the error codes, see Universal Error Codes.

ID	Error Message
401	Parameter error. Possible causes: 1. Mandatory parameters are left unspecified; 2. Incorrect parameter types; 3. Parameter verification failed.

Example

import { UIAbility } from '@kit.AbilityKit';
import { BusinessError } from '@kit.BasicServicesKit';

export default class EntryAbility extends UIAbility {
  onCreate() {
    this.context.eventHub.on('myEvent', this.eventFunc);
  }

  onDestroy() {
    try {
      // Result
      // eventFunc is called,undefined,undefined
      this.context.eventHub.emit('myEvent');
      // Result
      // eventFunc is called,1,undefined
      this.context.eventHub.emit('myEvent', 1);
      // Result
      // eventFunc is called,1,2
      this.context.eventHub.emit('myEvent', 1, 2);
    } catch (e) {
      let code: number = (e as BusinessError).code;
      let msg: string = (e as BusinessError).message;
      console.error(`EventHub emit error, code: ${code}, msg: ${msg}`);
    }
  }

  eventFunc(argOne: number, argTwo: number) {
    console.info(`eventFunc is called, ${argOne}, ${argTwo}`);
  }
}