# 后台任务开发指导

## 场景介绍

应用或业务模块处于后台（无可见界面）时，如果有需要继续执行或者后续执行的业务，可基于业务类型，申请短时任务延迟挂起（Suspend）或者长时任务避免进入挂起状态。


## 接口说明

```js
import backgroundTaskManager from '@ohos.backgroundTaskManager';
```

## 短时任务

**表1** backgroundTaskManager主要接口

| 接口名 | 描述 |
| -------- | -------- |
| function&nbsp;requestSuspendDelay(reason:&nbsp;string,&nbsp;callback:&nbsp;Callback&lt;void&gt;):&nbsp;**DelaySuspendInfo**; | 后台应用申请延迟挂起。<br/>延迟挂起时间一般情况下默认值为180000，低电量（依据系统低电量广播）时默认值为60000。 |
| function&nbsp;getRemainingDelayTime(requestId:&nbsp;number,&nbsp;callback:&nbsp;AsyncCallback&lt;number&gt;):&nbsp;void;<br/>function&nbsp;getRemainingDelayTime(requestId:&nbsp;number):&nbsp;Promise&lt;number&gt;; | 获取应用程序进入挂起状态前的剩余时间。(requestId从requestSuspendDelay的返回值获取)<br/>提供两种异步方法，使用Callback形式其任务执行结果以参数形式提供给回调函数，Promise形式则返回一个Promise对象。 |
| function&nbsp;cancelSuspendDelay(requestId:&nbsp;number):&nbsp;void; | 取消延迟挂起。(requestId从requestSuspendDelay的返回值获取) |

**表2** DelaySuspendInfo包含参数

| 参数名 | 类型 | 是否必选 | 描述 |
| -------- | -------- | -------- | -------- |
| requestId | number | 是 | 延迟挂起的请求ID。 |
| actualDelayTime | number | 是 | 应用的实际挂起延迟时间，以毫秒为单位。 |


## 开发步骤


1. 申请延迟挂起
```js
import backgroundTaskManager from '@ohos.backgroundTaskManager';

let myReason = 'test requestSuspendDelay';
let delayInfo = backgroundTaskManager.requestSuspendDelay(myReason, () => {
    console.info("Request suspend delay will time out.");
});

var id = delayInfo.requestId;console.info("requestId is: " + id);
```


2. 获取进入挂起前的剩余时间
```js
backgroundTaskManager.getRemainingDelayTime(id).then( res => {
    console.log('promise => Operation succeeded. Data: ' + JSON.stringify(res));
}).catch( err => {
    console.log('promise => Operation failed. Cause: ' + err.data);
});
   ```


3. 取消延迟挂起
```js
backgroundTaskManager.cancelSuspendDelay(id);
```


## 开发实例

```js
import backgroundTaskManager from '@ohos.backgroundTaskManager';
let myReason = 'test requestSuspendDelay';

// 申请延迟挂起
let delayInfo = backgroundTaskManager.requestSuspendDelay(myReason, () => {
    console.info("Request suspend delay will time out.");
});

// 打印延迟挂起信息
var id = delayInfo.requestId;
var time = delayInfo.actualDelayTime;
console.info("The requestId is: " + id);
console.info("The actualDelayTime is: " + time);

// 获取应用程序进入挂起状态前的剩余时间
backgroundTaskManager.getRemainingDelayTime(id).then( res => {
    console.log('promise => Operation succeeded. Data: ' + JSON.stringify(res));
}).catch( err => {
    console.log('promise => Operation failed. Cause: ' + err.data);
});

// 取消延迟挂起
backgroundTaskManager.cancelSuspendDelay(id);
```

## 长时任务

### 权限

ohos.permission.KEEP_BACKGROUND_RUNNING

**表3** 长时任务主要接口

| 接口名 | 描述 |
| -------- | -------- |
| function startBackgroundRunning(context: Context, bgMode: BackgroundMode, wantAgent: WantAgent, callback: AsyncCallback&lt;void&gt;): void;<br/>function startBackgroundRunning(context: Context, bgMode: BackgroundMode, wantAgent: WantAgent): Promise&lt;void&gt;; | 服务启动后，向系统申请长时任务，使服务一直保持后台运行 |
| function stopBackgroundRunning(context: Context, callback: AsyncCallback&lt;void&gt;): void;<br/>function stopBackgroundRunning(context: Context): Promise&lt;void&gt;; | 停止后台长时任务的运行 |

**表4** 长时任务BackgroundMode参数
| 参数名 | id值 | 描述 |
| -------- | -------- | -------- |
| DATA_TRANSFER           | 1 | 数据传输 |
| AUDIO_PLAYBACK          | 2 | 音频播放 |
| AUDIO_RECORDING         | 3 | 录音 |
| LOCATION                | 4 | 定位导航 |
| BLUETOOTH_INTERACTION   | 5 | 蓝牙相关 |
| MULTI_DEVICE_CONNECTION | 6 | 多设备互联 |
| WIFI_INTERACTION        | 7 | WLAN相关（系统保留） |
| VOIP                    | 8 | 音视频通话（系统保留） |
| TASK_KEEPING            | 9 | 计算任务（仅供PC使用） |


## 开发步骤

1. 申请长时任务

```js
import backgroundTaskManager from '@ohos.backgroundTaskManager';
import featureAbility from '@ohos.ability.featureAbility';
import wantAgent from '@ohos.wantAgent';

let wantAgentInfo = {
    wants: [
        {
            bundleName: "com.example.myapplication",
            abilityName: "com.example.myapplication.MainAbility"
        }
    ],
    operationType: wantAgent.OperationType.START_ABILITY,
    requestCode: 0,
    wantAgentFlags: [wantAgent.WantAgentFlags.UPDATE_PRESET_FLAG]
};

wantAgent.getWantAgent(wantAgentInfo).then((wantAgentObj) => {
    backgroundTaskManager.startBackgroundRunning(featureAbility.getContext(),
        backgroundTaskManager.BackgroundMode.DATA_TRANSFER, wantAgentObj).then(() => {
        console.info("Operation succeeded");
    }).catch((err) => {
        console.error("Operation failed Cause: " + err);
    });
});

```

2. 停止长时任务

```js
import backgroundTaskManager from '@ohos.backgroundTaskManager';
import featureAbility from '@ohos.ability.featureAbility';

backgroundTaskManager.stopBackgroundRunning(featureAbility.getContext()).then(() => {
    console.info("Operation succeeded");
}).catch((err) => {
    console.error("Operation failed Cause: " + err);
});

```

## 开发实例

当服务启动后，在serviceAbility的onStart回调方法中，调用长时任务的申请接口，声明此服务需要在后台长时运行。在onStop回调方法里，调用长时任务取消接口，声明取消长时任务。
在service.js文件中：
```js
import backgroundTaskManager from '@ohos.backgroundTaskManager';
import featureAbility from '@ohos.ability.featureAbility';
import wantAgent from '@ohos.wantAgent';

function startBackgroundRunning() {
    let wantAgentInfo = {
        wants: [
            {
                bundleName: "com.example.myapplication",
                abilityName: "com.example.myapplication.MainAbility"
            }
        ],
        operationType: wantAgent.OperationType.START_ABILITY,
        requestCode: 0,
        wantAgentFlags: [wantAgent.WantAgentFlags.UPDATE_PRESET_FLAG]
    };

    // 通过getWantAgent方法获取WantAgent对象
    wantAgent.getWantAgent(wantAgentInfo).then((wantAgentObj) => {
        backgroundTaskManager.startBackgroundRunning(featureAbility.getContext(),
            backgroundTaskManager.BackgroundMode.DATA_TRANSFER, wantAgentObj).then(() => {
            console.info("Operation succeeded");
        }).catch((err) => {
            console.error("Operation failed Cause: " + err);
        });
    });
}

function stopBackgroundRunning() {
    backgroundTaskManager.stopBackgroundRunning(featureAbility.getContext()).then(() => {
        console.info("Operation succeeded");
    }).catch((err) => {
        console.error("Operation failed Cause: " + err);
    });
}

export default {
    onStart(want) {
        console.info('ServiceAbility onStart');
        startBackgroundRunning();
    },
    onStop() {
        console.info('ServiceAbility onStop');
        stopBackgroundRunning();
    },
    onConnect(want) {
        console.info('ServiceAbility onConnect');
        return {};
    },
    onReconnect(want) {
        console.info('ServiceAbility onReconnect');
    },
    onDisconnect() {
        console.info('ServiceAbility onDisconnect');
    },
    onCommand(want, restart, startId) {
        console.info('ServiceAbility onCommand');
    }
};
```