EventHub

The EventHub module provides APIs to subscribe to, unsubscribe from, and trigger events.

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.

Modules to Import

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

Usage

Before using any APIs in the EventHub, you must obtain an EventHub instance through the member variable context of the UIAbility instance.

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

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

  onCreate() {
    this.context.eventHub.on('myEvent', this.eventFunc);
  }
}

EventHub is not a global event center. Different context objects have different EventHub objects. Event subscription, unsubscription, and triggering are performed on a specific EventHub object. Therefore, EventHub cannot be used for event transmission between VMs or processes.

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

Error codes

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

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.log(`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.log(`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.log(`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

Error codes

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

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.log('eventFunc1 is called');
  }

  eventFunc2() {
    console.log('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

Error codes

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

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.log(`eventFunc is called, ${argOne}, ${argTwo}`);
  }
}

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