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 '@ohos.app.ability.common';

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 '@ohos.app.ability.UIAbility';

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

    onForeground() {
        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.

System capability: SystemCapability.Ability.AbilityRuntime.Core

Parameters

Example

import UIAbility from '@ohos.app.ability.UIAbility';

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

    onForeground() {
        this.context.eventHub.on('myEvent', this.eventFunc);
        // Anonymous functions can be used to subscribe to events.
        this.context.eventHub.on('myEvent', () => {
            console.log(`anonymous eventFunc is called, value: ${this.value}`);
        });
        // Result
        // eventFunc is called, value: undefined
        // anonymous eventFunc is called, value: 12
        this.context.eventHub.emit('myEvent');
    }

    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.

System capability: SystemCapability.Ability.AbilityRuntime.Core

Parameters

Example

import UIAbility from '@ohos.app.ability.UIAbility';

export default class EntryAbility extends UIAbility {
    onForeground() {
        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).
    }

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

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

EventHub.emit

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

Triggers an event.

System capability: SystemCapability.Ability.AbilityRuntime.Core

Parameters

Example

import UIAbility from '@ohos.app.ability.UIAbility';

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

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