# Ability Development
## When to Use
Ability development in the [stage model](stage-brief.md) is significantly different from that in the FA model. The stage model requires you to declare the application package structure in the **module.json5** and **app.json5** files during application development. For details about the configuration file, see [Application Package Structure Configuration File](../quick-start/application-package-structure-stage.md). To develop an ability based on the stage model, implement the following logic:
- Create an ability that supports screen viewing and human-machine interaction. You must implement the following scenarios: ability lifecycle callbacks, obtaining ability configuration, requesting permissions, and notifying environment changes.
- Start an ability. You need to implement ability startup on the same device, on a remote device, or with a specified UI page.
- Call abilities. For details, see [Call Development](stage-call.md).
- Connect to and disconnect from a Service Extension ability. For details, see [Service Extension Ability Development](stage-serviceextension.md).
- Continue the ability on another device. For details, see [Ability Continuation Development](stage-ability-continuation.md).

### Launch Type
An ability can be launched in the **standard**, **singleton**, or **specified** mode, as configured by **launchType** in the **module.json5** file. Depending on the launch type, the action performed when the ability is started differs, as described below.

| Launch Type    | Description    |Action            |
| ----------- | -------  |---------------- |
| multiton    | Multi-instance mode| A new instance is started each time an ability starts.|
| singleton   | Singleton mode  | The ability has only one instance in the system. If an instance already exists when an ability is started, that instance is reused.|
| specified   | Instance-specific| The internal service of an ability determines whether to create multiple instances during running.|

By default, the singleton mode is used. The following is an example of the **module.json5** file:
```json
{
  "module": {
    "abilities": [
      {
        "launchType": "singleton",
      }
    ]
  }
}
```
## Creating an Ability
### Available APIs
The table below describes the APIs provided by the **AbilityStage** class, which has the **context** attribute. For details about the APIs, see [AbilityStage](../reference/apis/js-apis-app-ability-abilityStage.md).

**Table 1** AbilityStage APIs
|API|Description|
|:------|:------|
|onCreate(): void|Called when an ability stage is created.|
|onAcceptWant(want: Want): string|Called when a specified ability is started.|
|onConfigurationUpdated(config: Configuration): void|Called when the global configuration is updated.|

The table below describes the APIs provided by the **Ability** class. For details about the APIs, see [Ability](../reference/apis/js-apis-application-ability.md).

**Table 2** Ability APIs

|API|Description|
|:------|:------|
|onCreate(want: Want, param: AbilityConstant.LaunchParam): void|Called when an ability is created.|
|onDestroy(): void|Called when the ability is destroyed.|
|onWindowStageCreate(windowStage: window.WindowStage): void|Called when a **WindowStage** is created for the ability. You can use the **window.WindowStage** APIs to implement operations such as page loading.|
|onWindowStageDestroy(): void|Called when the **WindowStage** is destroyed for the ability.|
|onForeground(): void|Called when the ability is switched to the foreground.|
|onBackground(): void|Called when the ability is switched to the background.|
|onNewWant(want: Want, launchParams: AbilityConstant.LaunchParam): void|Called when the ability launch type is set to **singleton**.|
|onConfigurationUpdated(config: Configuration): void|Called when the configuration of the environment where the ability is running is updated.|
### Implementing AbilityStage and Ability Lifecycle Callbacks
To create Page abilities for an application in the stage model, you must implement the **AbilityStage** class and ability lifecycle callbacks, and use the **Window** class to set the pages. The sample code is as follows:
1. Import the **AbilityStage** module.
   ```
   import AbilityStage from "@ohos.application.AbilityStage"
   ```
2. Implement the **AbilityStage** class. The default relative path generated by the APIs is **entry\src\main\ets\Application\AbilityStage.ts**.
   ```ts
   export default class MyAbilityStage extends AbilityStage {
    onCreate() {
        console.log("MyAbilityStage onCreate")
    }
   }
   ```
3. Import the **Ability** module.
   ```js
   import Ability from '@ohos.application.Ability'
   ```
4. Implement the lifecycle callbacks of the **UIAbility** class. The default relative path generated by the APIs is **entry\src\main\ets\entryability\EntryAbility.ts**.

   In the **onWindowStageCreate(windowStage)** API, use **loadContent** to set the application page to be loaded. For details about how to use the **Window** APIs, see [Window Development](../windowmanager/application-window-stage.md).
   ```ts
   export default class MainAbility extends Ability {
    onCreate(want, launchParam) {
        console.log("MainAbility onCreate")
    }
   
    onDestroy() {
        console.log("MainAbility onDestroy")
    }
   
    onWindowStageCreate(windowStage) {
        console.log("MainAbility onWindowStageCreate")
   
        windowStage.loadContent("pages/index").then(() => {
            console.log("MainAbility load content succeed")
        }).catch((error) => {
            console.error("MainAbility load content failed with error: " + JSON.stringify(error))
        })
    }
   
    onWindowStageDestroy() {
        console.log("MainAbility onWindowStageDestroy")
    }
   
    onForeground() {
        console.log("MainAbility onForeground")
    }
   
    onBackground() {
        console.log("MainAbility onBackground")
    }
   }
   ```
### Obtaining AbilityStage and Ability Configurations
Both the **AbilityStage** and **Ability** classes have the **context** attribute. An application can obtain the context of an **Ability** instance through **this.context** to obtain the configuration details.

The following example shows how an application obtains the bundle code directory, HAP file name, ability name, and system language through the **context** attribute in the **AbilityStage** class. The sample code is as follows:

```ts
import AbilityStage from "@ohos.application.AbilityStage"

export default class MyAbilityStage extends AbilityStage {
    onCreate() {
        console.log("MyAbilityStage onCreate")
        let context = this.context
        console.log("MyAbilityStage bundleCodeDir" + context.bundleCodeDir)

        let currentHapModuleInfo = context.currentHapModuleInfo
        console.log("MyAbilityStage hap module name" + currentHapModuleInfo.name)
        console.log("MyAbilityStage hap module mainAbilityName" + currentHapModuleInfo.mainAbilityName)

        let config = this.context.config
        console.log("MyAbilityStage config language" + config.language)
    }
}
```

The following example shows how an application obtains the bundle code directory, HAP file name, ability name, and system language through the **context** attribute in the **Ability** class. The sample code is as follows:
```ts
import Ability from '@ohos.application.Ability'
export default class MainAbility extends Ability {
    onCreate(want, launchParam) {
        console.log("MainAbility onCreate")
        let context = this.context
        console.log("MainAbility bundleCodeDir" + context.bundleCodeDir)

        let abilityInfo = this.context.abilityInfo;
        console.log("MainAbility ability bundleName" + abilityInfo.bundleName)
        console.log("MainAbility ability name" + abilityInfo.name)

        let config = this.context.config
        console.log("MainAbility config language" + config.language)
    }
}
```
### Notifying of Environment Changes
Environment changes include changes of global configurations and ability configurations. Currently, the global configurations include the system language and color mode. The change of global configurations is generally triggered by configuration items in **Settings** or icons in **Control Panel**. The ability configuration is specific to a single **Ability** instance, including the display ID, screen resolution, and screen orientation. The configuration is related to the display where the ability is located, and the change is generally triggered by the window. Before configuring a project, define the project in the [Configuration](../reference/apis/js-apis-application-configuration.md) class.

For an application in the stage model, when the configuration changes, its abilities are not restarted, but the **onConfigurationUpdated(config: Configuration)** callback is triggered. If the application needs to perform processing based on the change, you can override **onConfigurationUpdated**. Note that the **Configuration** object in the callback contains all the configurations of the current ability, not only the changed configurations.

The following example shows the implementation of the **onConfigurationUpdated** callback in the **AbilityStage** class. The callback is triggered when the system language and color mode are changed.  
```ts
import AbilityStage from '@ohos.application.AbilityStage'
import ConfigurationConstant from '@ohos.application.ConfigurationConstant'

export default class MyAbilityStage extends AbilityStage {
    onConfigurationUpdated(config) {
        if (config.colorMode === ConfigurationConstant.ColorMode.COLOR_MODE_DARK) {
            console.log('colorMode changed to dark')
        }
    }
}
```

The following example shows the implementation of the **onConfigurationUpdated** callback in the **Ability** class. The callback is triggered when the system language, color mode, or display parameters (such as the direction and density) change.  
```ts
import Ability from '@ohos.application.Ability'
import ConfigurationConstant from '@ohos.application.ConfigurationConstant'

export default class MainAbility extends Ability {
    direction : number;

    onCreate(want, launchParam) {
        this.direction = this.context.config.direction
    }

    onConfigurationUpdated(config) {
        if (this.direction !== config.direction) {
            console.log(`direction changed to ${config.direction}`)
        }
    }
}
```
## Starting an Ability
### Available APIs
The **Ability** class has the **context** attribute, which belongs to the **AbilityContext** class. The **AbilityContext** class has the **abilityInfo**, **currentHapModuleInfo**, and other attributes as well as the APIs used for starting abilities. For details, see [AbilityContext](../reference/apis/js-apis-inner-application-applicationContext.md).

**Table 3** AbilityContext APIs
|API|Description|
|:------|:------|
|startAbility(want: Want, callback: AsyncCallback\<void>): void|Starts an ability.|
|startAbility(want: Want, options?: StartOptions): Promise\<void>|Starts an ability.|
|startAbilityWithAccount(want: Want, accountId: number, callback: AsyncCallback\<void>): void|Starts an ability with the account ID.|
|startAbilityWithAccount(want: Want, accountId: number, options?: StartOptions): Promise\<void>|Starts an ability with the account ID.|
|startAbilityForResult(want: Want, callback: AsyncCallback\<AbilityResult>): void|Starts an ability with the returned result.|
|startAbilityForResult(want: Want, options?: StartOptions): Promise\<AbilityResult>|Starts an ability with the returned result.|
|startAbilityForResultWithAccount(want: Want, accountId: number, callback: AsyncCallback\<AbilityResult>): void|Starts an ability with the execution result and account ID.|
|startAbilityForResultWithAccount(want: Want, accountId: number, options?: StartOptions): Promise\<AbilityResult>|Starts an ability with the execution result and account ID.|
### Starting an Ability on the Same Device
An application can obtain the context of an **Ability** instance through **this.context** and then use the **startAbility** API in the **AbilityContext** class to start the ability. The ability can be started by specifying **Want**, **StartOptions**, and **accountId**, and the operation result can be returned using a callback or **Promise** instance. The sample code is as follows:
```ts
let context = this.context
var want = {
    "deviceId": "",
    "bundleName": "com.example.MyApplication",
    "abilityName": "MainAbility"
};
context.startAbility(want).then(() => {
    console.log("Succeed to start ability")
}).catch((error) => {
    console.error("Failed to start ability with error: "+ JSON.stringify(error))
})
```

### Starting an Ability on a Remote Device
>This feature applies only to system applications, since the **getTrustedDeviceListSync** API of the **DeviceManager** class is open only to system applications.
In the cross-device scenario, you must specify the ID of the remote device. The sample code is as follows:
```ts
let context = this.context
var want = {
    "deviceId": getRemoteDeviceId(),
    "bundleName": "com.example.MyApplication",
    "abilityName": "MainAbility"
};
context.startAbility(want).then(() => {
    console.log("Succeed to start remote ability")
}).catch((error) => {
    console.error("Failed to start remote ability with error: " + JSON.stringify(error))
})
```
Obtain the ID of a specified device from **DeviceManager**. The sample code is as follows:
```ts
import deviceManager from '@ohos.distributedHardware.deviceManager';
function getRemoteDeviceId() {
    if (typeof dmClass === 'object' && dmClass != null) {
        var list = dmClass.getTrustedDeviceListSync();
        if (typeof (list) == 'undefined' || typeof (list.length) == 'undefined') {
            console.log("MainAbility onButtonClick getRemoteDeviceId err: list is null");
            return;
        }
        console.log("MainAbility onButtonClick getRemoteDeviceId success:" + list[0].deviceId);
        return list[0].deviceId;
    } else {
        console.log("MainAbility onButtonClick getRemoteDeviceId err: dmClass is null");
    }
}
```
Request the permission **ohos.permission.DISTRIBUTED_DATASYNC** from consumers. This permission is used for data synchronization. For details about the sample code for requesting permissions, see [abilityAccessCtrl.requestPermissionsFromUse](../reference/apis/js-apis-abilityAccessCtrl.md#requestpermissionsfromuser9).
### Starting an Ability with the Specified Page
If the launch type of an ability is set to **singleton** and the ability has been started, the **onNewWant** callback is triggered when the ability is started again. You can pass start options through the **want**. For example, to start an ability with the specified page, use the **uri** or **parameters** parameter in the **want** to pass the page information. Currently, the ability in the stage model cannot directly use the **router** capability. You must pass the start options to the custom component and invoke the **router** method to display the specified page during the custom component lifecycle management. The sample code is as follows:

When using **startAbility** to start an ability again, use the **uri** parameter in the **want** to pass the page information.
```ts
async function reStartAbility() {
  try {
    await this.context.startAbility({
      bundleName: "com.sample.MyApplication",
      abilityName: "MainAbility",
      uri: "pages/second"
    })
    console.log('start ability succeed')
  } catch (error) {
    console.error(`start ability failed with ${error.code}`)
  }
}
```

Obtain the **want** parameter that contains the page information from the **onNewWant** callback of the ability.
```ts
import Ability from '@ohos.application.Ability'

export default class MainAbility extends Ability {
  onNewWant(want, launchParams) {
    globalThis.newWant = want
  }
}
```

Obtain the **want** parameter that contains the page information from the custom component and process the route based on the URI.
```ts
import router from '@ohos.router'

@Entry
@Component
struct Index {
  newWant = undefined

  onPageShow() {
    console.info('Index onPageShow')
    let newWant = globalThis.newWant
    if (newWant.hasOwnProperty("uri")) {
      router.push({ url: newWant.uri });
      globalThis.newWant = undefined
    }
  }
}
```