The common event service provides two subscription modes: dynamic and static. In dynamic subscription mode, a subscriber calls an API during the running period to subscribe to common events. For details, see [Subscribing to Common Events](common-event-subscription.md). In static subscription mode, no common event subscription API is called. A common event is subscribed by configuring a declaration file and implementing a class that inherits from **StaticSubscriberExtensionAbility**. A static subscriber is started once it receives a target event (for example, a power-on event) published by the system or application. At the same time, the **onReceiveEvent** callback is triggered, in which you can implement the service logic. **The static subscriber APIs are system APIs and can be used only by system applications that have passed the system-level power consumption review.**
## How to Develop
1. Prerequisites
The application must meet the following requirements:
The application is a system application.
The application is developed using the full SDK.
The application's power consumption has passed the system-level power consumption review. If you want to use static subscription in the debugging phase, add the bundle name of your application to the system configuration file **/etc/static_subscriber_config.json**.
2. 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:
Pay attention to the following fields in the JSON file:
**srcEntrance**: 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. The **metadata** field contains two keywords: **name** and **resource**. The **name** field indicates the ExtensionAbility type name. For a static subscriber, declare the name as **ohos.extension.staticSubscriber** for successful identification. The **resource** field indicates the 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:
```ts
{
"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 by 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 subscribed target events
## Samples
For details about how to develop StaticSubscriberExtensionAbility, see [StaticSubscriber (ArkTS, API version 9, Full SDK)](https://gitee.com/openharmony/applications_app_samples/tree/master/ability/StaticSubscriber).
