1# Launching a UIAbility in the Background Through the call Event 2 3 4There 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](../reference/apis-arkui/js-apis-postCardAction.md#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. 5 6> **NOTE** 7> 8> This topic describes development for dynamic widgets. For static widgets, see [FormLink](../reference/apis-arkui/arkui-ts/ts-container-formlink.md). 9 10## Constraints 11 12This action type requires that the widget provider should have the [ohos.permission.KEEP_BACKGROUND_RUNNING](../security/AccessToken/permissions-for-all.md#ohospermissionkeep_background_running) permission. 13 14## How to Develop 15 16Typically, the call event is triggered for touching of buttons. Below is an example. 17 18 19- 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. 20 21 ```ts 22 @Entry 23 @Component 24 struct WidgetEventCallCard { 25 @LocalStorageProp('formId') formId: string = '12400633174999288'; 26 27 build() { 28 Column() { 29 //... 30 Row() { 31 Column() { 32 Button() { 33 //... 34 } 35 //... 36 .onClick(() => { 37 postCardAction(this, { 38 action: 'call', 39 abilityName: 'WidgetEventCallEntryAbility', // Only the UIAbility of the current application is allowed. The ability name must be the same as that defined in module.json5. 40 params: { 41 formId: this.formId, 42 method: 'funA' // Set the name of the method to call in the EntryAbility. 43 } 44 }); 45 }) 46 47 Button() { 48 //... 49 } 50 //... 51 .onClick(() => { 52 postCardAction(this, { 53 action: 'call', 54 abilityName: 'WidgetEventCallEntryAbility', // Only the UIAbility of the current application is allowed. The ability name must be the same as that defined in module.json5. 55 params: { 56 formId: this.formId, 57 method: 'funB', // Set the name of the method to call in the EntryAbility. 58 num: 1 // Set other parameters to be passed in. 59 } 60 }); 61 }) 62 } 63 }.width('100%').height('80%') 64 .justifyContent(FlexAlign.Center) 65 } 66 .width('100%') 67 .height('100%') 68 .alignItems(HorizontalAlign.Center) 69 } 70 } 71 ``` 72 73- 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](../reference/apis-ipc-kit/js-apis-rpc.md#readstring) method. Listen for the method required by the call event in the **onCreate** callback of the UIAbility. 74 75 ```ts 76 import { AbilityConstant, UIAbility, Want } from '@kit.AbilityKit'; 77 import { promptAction } from '@kit.ArkUI'; 78 import { BusinessError } from '@kit.BasicServicesKit'; 79 import { rpc } from '@kit.IPCKit'; 80 import { hilog } from '@kit.PerformanceAnalysisKit'; 81 82 const TAG: string = 'WidgetEventCallEntryAbility'; 83 const DOMAIN_NUMBER: number = 0xFF00; 84 const CONST_NUMBER_1: number = 1; 85 const CONST_NUMBER_2: number = 2; 86 87 class MyParcelable implements rpc.Parcelable { 88 num: number; 89 str: string; 90 91 constructor(num: number, str: string) { 92 this.num = num; 93 this.str = str; 94 } 95 96 marshalling(messageSequence: rpc.MessageSequence): boolean { 97 messageSequence.writeInt(this.num); 98 messageSequence.writeString(this.str); 99 return true; 100 } 101 102 unmarshalling(messageSequence: rpc.MessageSequence): boolean { 103 this.num = messageSequence.readInt(); 104 this.str = messageSequence.readString(); 105 return true; 106 } 107 } 108 109 export default class WidgetEventCallEntryAbility extends UIAbility { 110 // If the UIAbility is started for the first time, onCreate is triggered after the call event is received. 111 onCreate(want: Want, launchParam: AbilityConstant.LaunchParam): void { 112 try { 113 // Listen for the method required by the call event. 114 this.callee.on('funA', (data: rpc.MessageSequence) => { 115 // Obtain all parameters passed in the call event. 116 hilog.info(DOMAIN_NUMBER, TAG, `FunACall param: ${JSON.stringify(data.readString())}`); 117 promptAction.showToast({ 118 message: 'FunACall param:' + JSON.stringify(data.readString()) 119 }); 120 return new MyParcelable(CONST_NUMBER_1, 'aaa'); 121 }); 122 this.callee.on('funB', (data: rpc.MessageSequence) => { 123 // Obtain all parameters passed in the call event. 124 hilog.info(DOMAIN_NUMBER, TAG, `FunBCall param: ${JSON.stringify(data.readString())}`); 125 promptAction.showToast({ 126 message: 'FunBCall param:' + JSON.stringify(data.readString()) 127 }); 128 return new MyParcelable(CONST_NUMBER_2, 'bbb'); 129 }); 130 } catch (err) { 131 hilog.error(DOMAIN_NUMBER, TAG, `Failed to register callee on. Cause: ${JSON.stringify(err as BusinessError)}`); 132 } 133 } 134 135 // Deregister the listener when the process exits. 136 onDestroy(): void | Promise<void> { 137 try { 138 this.callee.off('funA'); 139 this.callee.off('funB'); 140 } catch (err) { 141 hilog.error(DOMAIN_NUMBER, TAG, `Failed to register callee off. Cause: ${JSON.stringify(err as BusinessError)}`); 142 } 143 } 144 } 145 ``` 146