Deferred Task (ArkTS)

Overview

Introduction

If an application needs to execute a non-real-time task after switching to the background, for example, if the application wants to obtain emails irregularly when the network is available, the application can request deferred tasks. When the specified trigger conditions (including the network type, charging type, storage status, battery status, and timing status) are met, the system adds the task to the execution queue. Then the system starts the application to execute the deferred task based on the memory, power consumption, device temperature, and user habits.

Working Principle

Figure 1 Working principle of deferred task scheduling WorkScheduler

An application calls the deferred task API to add, delete, and query deferred tasks. The deferred task management module determines the scheduling time based on the task conditions (set by the WorkInfo parameter, including the network type, charging type, and storage status) and system status (including the memory, power consumption, device temperature, and user habits).

When the scheduling conditions are met or the task scheduling ends, the system calls back onWorkStart() or onWorkStop() in WorkSchedulerExtensionAbility. The system also creates an independent process for the WorkSchedulerExtensionAbility and sets a period for the WorkSchedulerExtensionAbility to run. You can implement your own service logic in the callback functions.

Constraints

Quantity limit: An application can request a maximum of 10 deferred tasks during a time segment.

Execution frequency limit: The system controls the execution frequency of deferred tasks based on the application activity group in the device usage statistics. Applications that request the WORK_SCHEDULER resource are placed in the efficiency resource exemption group.

Table 1 Application activity groups

Group	Deferred Task Execution Frequency
Group of active applications	At a minimum interval of 2 hours
Group of frequently used applications	At a minimum interval of 4 hours
Group of commonly used applications	At a minimum interval of 24 hours
Group of rarely used applications	At a minimum interval of 48 hours
Group of restricted applications	Forbidden
Group of applications never used	Forbidden
Exemption group for efficiency resources	No restriction

Timeout: The WorkSchedulerExtensionAbility can run for a maximum of 2 minutes for a single callback. If the application does not cancel the deferred task upon a timeout, the system forcibly terminates the process for the WorkSchedulerExtensionAbility. Privileged system applications can request the WORK_SCHEDULER resource to extend the duration to 20 minutes in the charging state and 10 minutes in the non-charging state.
Scheduling delay: The system schedules deferred tasks in a unified manner based on the memory, power consumption, device temperature, and user habits. For example, when the system memory resources are insufficient or the temperature reaches a certain level, the system delays task scheduling.
Restrictions for WorkSchedulerExtensionAbility: The following APIs cannot be called in the WorkSchedulerExtensionAbility:

@ohos.resourceschedule.backgroundTaskManager (Background Task Management)

@ohos.backgroundTaskManager (Background Task Management)

@ohos.multimedia.camera (Camera Management)

@ohos.multimedia.audio (Audio Management)

@ohos.multimedia.media (Media)

Available APIs

Table 2 Main APIs for deferred tasks

The table below lists the APIs used for developing deferred tasks. For details about more APIs and their usage, see @ohos.resourceschedule.workScheduler (Deferred Task Scheduling).

API	Description
startWork(work: WorkInfo): void	Start a deferred task.
stopWork(work: WorkInfo, needCancel?: boolean): void	Cancel the deferred task.
getWorkStatus(workId: number, callback: AsyncCallback<WorkInfo>): void	Obtains the information about a deferred task. This API uses an asynchronous callback to return the result.
getWorkStatus(workId: number): Promise<WorkInfo>	Obtains the information about a deferred task. This API uses a promise to return the result.
obtainAllWorks(callback: AsyncCallback<Array<WorkInfo>>): void	Obtains all the deferred tasks. This API uses an asynchronous callback to return the result.
obtainAllWorks(): Promise<Array<WorkInfo>>	Obtains all the deferred tasks. This API uses a promise to return the result.
stopAndClearWorks(): void	Stops and clears all the deferred tasks.
isLastWorkTimeOut(workId: number, callback: AsyncCallback<boolean>): void	Checks whether the last execution of a deferred task has timed out. This API uses an asynchronous callback to return the result. It is applicable to repeated tasks.
isLastWorkTimeOut(workId: number): Promise<boolean>	Checks whether the last execution of a deferred task has timed out. This API uses a promise to return the result. It is applicable to repeated tasks.

Table 3 Deferred task scheduling callbacks

The table below lists the APIs used for developing deferred task scheduling callbacks. For details about more APIs and their usage, see @ohos.WorkSchedulerExtensionAbility (Deferred Task Scheduling Callbacks).

API	Description
onWorkStart(work: workScheduler.WorkInfo): void	Called when the system starts scheduling the deferred task.
onWorkStop(work: workScheduler.WorkInfo): void	Called when the system stops scheduling the deferred task.

How to Develop

The development of deferred task scheduling consists of two steps: implementing the deferred task scheduling capability and implementing deferred task scheduling.

Implementing the deferred task scheduling capability: Implement the callbacks for starting and stopping the WorkSchedulerExtensionAbility.
Implementing deferred task scheduling: Call the WorkScheduler APIs to start and stop delayed tasks.

Implementing Callbacks for Work Scheduler

Create a project directory.

In the ./entry/src/main/ets directory of the project, create a directory and an ArkTS file. For example, create a directory and name it WorkSchedulerExtension. In the WorkSchedulerExtension directory, create an ArkTS file named WorkSchedulerExtension.ets and implement the callbacks for deferred task scheduling.

Import the module.

import { WorkSchedulerExtensionAbility, workScheduler } from '@kit.BackgroundTasksKit';

Implement the lifecycle callbacks for the WorkSchedulerExtensionAbility.

export default class WorkSchedulerAbility extends WorkSchedulerExtensionAbility {
  // Callback invoked when the system starts scheduling the deferred task.
  onWorkStart(workInfo: workScheduler.WorkInfo) {
    console.info(`onWorkStart, workInfo = ${JSON.stringify(workInfo)}`);
    // Print the parameter, for example, key1, in parameters.
    console.info(`work info parameters: ${JSON.parse(workInfo.parameters?.toString()).key1}`);
  }

  // Callback invoked when the deferred task ends. This callback is triggered when the deferred task times out for 2 minutes or the stopWork API is called to cancel the task.
  onWorkStop(workInfo: workScheduler.WorkInfo) {
    console.info(`onWorkStop, workInfo is ${JSON.stringify(workInfo)}`);
  }
}

WorkSchedulerAbility.ets

Set type to workScheduler.
Set srcEntry to the code path of the WorkSchedulerExtensionAbility component.

{
  "module": {
      "extensionAbilities": [
        {
          "name": "MyWorkSchedulerExtensionAbility",
          "srcEntry": "./ets/WorkSchedulerExtension/WorkSchedulerExtension.ets",
          "type": "workScheduler"
        }
      ]
  }
}

Implementing Deferred Task Scheduling

Import the module.

import { workScheduler } from '@kit.BackgroundTasksKit';
import { BusinessError } from '@kit.BasicServicesKit';

Start a deferred task.

let workInfo: workScheduler.WorkInfo = {
  workId: 1,
  networkType: workScheduler.NetworkType.NETWORK_TYPE_ANY,
  bundleName: 'ohos.samples.workschedulerextensionability',
  abilityName: 'WorkSchedulerAbility',
  // ...
}

try {
  workScheduler.startWork(workInfo);
  console.info(`startWork success`);
}
catch (error) {
  console.error(`startWork failed. code is ${(error as BusinessError).code} message is ${(error as BusinessError).message}`);
}

WorkSchedulerSystem.ets

Cancel the deferred task.

// Create workinfo.
let workInfo: workScheduler.WorkInfo = {
  workId: 1,
  networkType: workScheduler.NetworkType.NETWORK_TYPE_WIFI,
  bundleName: 'ohos.samples.workschedulerextensionability',
  abilityName: 'WorkSchedulerAbility',
}

try {
  workScheduler.stopWork(workInfo);
  console.info(`stopWork success`);
} catch (error) {
  console.error(`stopWork failed. code is ${(error as BusinessError).code} message is ${(error as BusinessError).message}`);
}

WorkSchedulerSystem.ets

Verifying Deferred Task Scheduling

Check whether the onWorkStart and onWorkStop methods of WorkSchedulerExtensionAbility are correctly implemented and can be successfully called.

After the deferred task is requested, its callback can be triggered only when the required conditions are met. To quickly perform the verification, trigger the callback manually using the following hidumper command.

$ hidumper -s 1904 -a '-t com.example.application MyWorkSchedulerExtensionAbility'

-------------------------------[ability]-------------------------------


----------------------------------WorkSchedule----------------------------------