harmony 鸿蒙Launching a UIAbility in the Background Through the call Event

2023-06-24
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Launching a UIAbility in the Background Through the call Event

There may be cases you want to provide in a widget access to features available in your application running in the foreground, for example, the play, pause, and stop buttons in a music application widget. This is where the call capability of the postCardAction API comes in handy. This capability, when used in a widget, can start the specified UIAbility of the widget provider in the background. It also allows the widget to call the specified method of the application and transfer data so that the application, while in the background, can behave accordingly in response to touching of the buttons on the widget.

NOTE

This topic describes development for dynamic widgets. For static widgets, see FormLink.

Typically, the call event is triggered for touching of buttons. Below is an example.

In this example, two buttons are laid out on the widget page. When one button is clicked, the postCardAction API is called to send a call event to the target UIAbility. Note that the method parameter in the API indicates the method to call in the target UIAbility. It is mandatory and of the string type.

  @Entry
  @Component
  struct WidgetCard {
    build() {
      Column() {
        Button ('Feature A')
          .margin('20%')
          .onClick(() => {
            console.info('call EntryAbility funA');
            postCardAction(this, {
              'action': 'call',
              'abilityName': 'EntryAbility', // Only the UIAbility of the current application is allowed.
              'params': {
                'method': 'funA' // Set the name of the method to call in the EntryAbility.
              }
            });
          })
  
        Button ('Feature B')
          .margin('20%')
          .onClick(() => {
            console.info('call EntryAbility funB');
            postCardAction(this, {
              'action': 'call',
              'abilityName': 'EntryAbility', // Only the UIAbility of the current application is allowed.
              'params': {
                'method': 'funB', // Set the name of the method to call in the EntryAbility.
                'num': 1 // Set other parameters to be passed in.
              }
            });
          })
      }
      .width('100%')
      .height('100%')
    }
  }

The UIAbility receives the call event and obtains the transferred parameters. It then executes the target method specified by the method parameter. Other data can be obtained through the readString method. Listen for the method required by the call event in the onCreate callback of the UIAbility.

  import UIAbility from '@ohos.app.ability.UIAbility';
  import Base from '@ohos.base'
  import rpc from '@ohos.rpc';
  import Want from '@ohos.app.ability.Want';
  import AbilityConstant from '@ohos.app.ability.AbilityConstant';


  class MyParcelable implements rpc.Parcelable {
    num: number;
    str: string;
    constructor(num: number, str: string) {
      this.num = num;
      this.str = str;
    }
    marshalling(messageSequence: rpc.MessageSequence): boolean {
      messageSequence.writeInt(this.num);
      messageSequence.writeString(this.str);
      return true;
    }
    unmarshalling(messageSequence: rpc.MessageSequence): boolean {
      this.num = messageSequence.readInt();
      this.str = messageSequence.readString();
      return true;
    }
  }
  
  export default class CameraAbility extends UIAbility {
    // If the UIAbility is started for the first time, onCreate is triggered after the call event is received.
    onCreate(want: Want, launchParam: AbilityConstant.LaunchParam) {
      try {
        // Listen for the method required by the call event.
        this.callee.on('funA', (data: rpc.MessageSequence) => {
          // Obtain all parameters passed in the call event.
          console.info('FunACall param:' + JSON.stringify(data.readString()));
          return new MyParcelable(1, 'aaa');
        });
        this.callee.on('funB', (data: rpc.MessageSequence) => {
          // Obtain all parameters passed in the call event.
          console.info('FunACall param:' + JSON.stringify(data.readString()));
          return new MyParcelable(2, 'bbb');
        });
      } catch (err) {
        console.error(`Failed to register callee on. Cause: ${JSON.stringify(err as Base.BusinessError)}`);
      }
    }
  
    ...
  
    // Deregister the listener when the process exits.
    onDestroy() {
      try {
        this.callee.off('funA');
        this.callee.off('funB');
      } catch (err) {
        console.error(`Failed to register callee off. Cause: ${JSON.stringify(err as Base.BusinessError)}`);
      }
    }
  };