Subscribing to Common Events in Static Mode (for System Applications Only)

When to Use

A static subscriber is started once it receives a target event published by the system or application. At the same time, the onReceiveEvent callback is triggered, in which you can implement the service logic. For example, if an application needs to execute some initialization tasks during device power-on, the application can subscribe to the power-on event in static mode. After receiving the power-on event, the application is started to execute the initialization tasks. Subscribing to a common event in static mode is achieved by configuring a declaration file and implementing a class that inherits from StaticSubscriberExtensionAbility. Note that this subscribing mode has negative impact on system power consumption. Therefore, exercise caution when using this mode.

How to Develop

Declaring a Static Subscriber

To declare a static subscriber, create an ExtensionAbility, which is derived from the StaticSubscriberExtensionAbility class, in the project. The sample code is as follows:

import StaticSubscriberExtensionAbility from '@ohos.application.StaticSubscriberExtensionAbility'

export default class StaticSubscriber extends StaticSubscriberExtensionAbility {
    onReceiveEvent(event) {
        console.log('onReceiveEvent, event:' + event.event);
    }
}

You can implement service logic in the onReceiveEvent callback.

Project Configuration for a Static Subscriber

After writing the static subscriber code, configure the subscriber in the module.json5 file. The configuration format is as follows:
```
{
  "module": {
    ......
    "extensionAbilities": [
      {
        "name": "StaticSubscriber",
        "srcEntry": "./ets/StaticSubscriber/StaticSubscriber.ts",
        "description": "$string:StaticSubscriber_desc",
        "icon": "$media:icon",
        "label": "$string:StaticSubscriber_label",
        "type": "staticSubscriber",
        "exported": true,
        "metadata": [
          {
            "name": "ohos.extension.staticSubscriber",
            "resource": "$profile:subscribe"
          }
        ]
      }
    ]
    ......
  }
}
```
Pay attention to the following fields in the JSON file:
- srcEntry: entry file path of the ExtensionAbility, that is, the file path of the static subscriber declared in Step 2.
- type: ExtensionAbility type. For a static subscriber, set this field to staticSubscriber.
- metadata: level-2 configuration file information of the ExtensionAbility. The configuration information varies according to the ExtensionAbility type. Therefore, you must use different config files to indicate the specific configuration.
  - name: name of the ExtensionAbility. For a static subscriber, declare the name as ohos.extension.staticSubscriber for successful identification.
  - resource: path that stores the ExtensionAbility configuration, which is customizable. In this example, the path is resources/base/profile/subscribe.json.
  A level-2 configuration file pointed to by metadata must be in the following format:
```
{
  "commonEvents": [
    {
      "name": "xxx",
      "permission": "xxx",
      "events":[
        "xxx"
      ]
    }
  ]
}
```
  If the level-2 configuration file is not declared in this format, the file cannot be identified. The fields are described as follows:
  - name: name of the ExtensionAbility, which must be the same as the name of extensionAbility declared in module.json5.
  - permission: permission required for the publisher. If a publisher without the required permission attempts to publish an event, the event is regarded as invalid and will not be published.
  - events: list of target events to subscribe to.
Device System Configuration

In the device system configuration file /etc/static_subscriber_config.json, add the bundle name of the static subscriber.
```
{
    "xxx",
    "ohos.extension.staticSubscriber",
    "xxx"
}
```