HarmonyOS 鸿蒙Next 通过首选项记录应用是否第一次启动

拜托各位大神帮我看看吧，写了两天了还是没实现，通过首选项记录App第一次启动时弹窗请求权限，当同意后，下一次启动则不再询问，一直不成功。

@Entry
@Component
struct LauncherPage {
  private context?:common.UIAbilityContext
  pre?: preferences.Preferences;
  @State isAgree:string = ''
  private timerId: number = 0;

  dialogController:CustomDialogController = new CustomDialogController({
    builder:CustomDialogComponent({
      cancel:()=>{
        this.onCancel()
      },
      confirm:()=>{
        this.onConfirm()
      }
    }),
    alignment:DialogAlignment.Bottom,
    offset:{dx:0, dy:-24},
    autoCancel:false
  })

  async aboutToAppear() {
    preferencesUtil.getPreferences()
    this.isAgree = await preferencesUtil.get('isAgree')
    console.log('[LauncherPage] aboutToAppear isAgree:' + this.isAgree)
    if(this.isAgree === 'yes') {
      this.dialogController.close()
    }
    else
      this.dialogController.open()
  }

  onCancel(){
    this.context?.terminateSelf()
  }

  onConfirm(){
    this.isAgree = 'yes'
    this.saveIsAgree(this.isAgree)
    this.jumpToAdvertisingPage()
  }

  jumpToAdvertisingPage() {
    this.timerId = setTimeout(() => {
      router.pushUrl({
        url: 'pages/HomePage'
      }).catch((error: Error) => {
        Logger.error('LauncherPage', 'LauncherPage pushUrl error ' + JSON.stringify(error));
      });
    }, 3000);
  }

  saveIsAgree(value:string){
    preferencesUtil.save('isAgree',value)
  }


   build() {
    Stack() {
      Image($r('app.media.ic_launcher_background'))
        .width('100%')
        .height('100%')
      Column() {
        Image($r('app.media.ic_logo'))
          .width($r('app.float.launcher_logo_size'))
          .height($r('app.float.launcher_logo_size'))
          .margin({ top: '16.2%' })
        Text($r('app.string.healthy_life_text'))
          .width($r('app.float.launcher_life_text_width'))
          .height($r('app.float.launcher_life_text_height'))
          .healthyLifeTextStyle(FontWeight.Bold,
            0.1,
            $r('app.float.launcher_text_title_size'),
            $r('app.color.launcher_text_title_color'))
          .margin({ top: '0.5%' })
        Text($r('app.string.healthy_life_introduce'))
          .height('2.7%')
          .healthyLifeTextStyle(FontWeight.Normal,
            3.4,
            $r('app.float.launcher_text_introduce_size'),
            $r('app.color.launcher_text_introduce_color'))
          .opacity($r('app.float.launcher_text_opacity'))
          .margin({ top: '1.3%' })
      }
      .height('100%')
      .width('100%')
    }
  }
}
// Healthy living text common styles.
@Extend(Text) function healthyLifeTextStyle (fontWeight: number,
  textAttribute: number, fontSize: Resource, fontColor: Resource) {
  .fontWeight(fontWeight)
  .letterSpacing(textAttribute)
  .fontSize(fontSize)
  .fontColor(fontColor)
}

工具类

import { common } from '@kit.AbilityKit'
import { preferences, ValueType } from '@kit.ArkData'

let pre: preferences.Preferences | null = null;

export class PreferencesUtil{
  context:common.UIAbilityContext = getContext(this) as common.UIAbilityContext


  constructor() {
    this.getPreferences()
    console.log('constructor')
  }

  async  getPreferences(){
    pre = await preferences.getPreferences(this.context,'myPre')
  }

  async save(key:string,value:preferences.ValueType){
    // if(pre != undefined){
    //   await pre.put('isAgree', value)
    //   await pre.flush()
    //   console.log('[save]'+value)
    // }
    if(!pre){
      console.info('preferences不存在')
      this.getPreferences
    }
    if(pre?.has(key)){
      console.info('key:'+key+' 存在')
    }
      pre?.put(key,value)
      pre?.flush()

  }

  async get(key:string) {
    let resValue:ValueType = ''
    if(!pre) {
      this.getPreferences()
    }
    resValue = await pre?.get(key,'default') as string

    return resValue
  }
}

export default new PreferencesUtil()

求助一下，我这到底哪里写得不对呢？