HarmonyOS如何获取“文件管理”App的 Documents 目录的路径

import fs from '@ohos.file.fs'; 
import { common } from '@kit.AbilityKit'; 
import { picker } from '@kit.CoreFileKit'; 
import { BusinessError } from '@kit.BasicServicesKit'; 
@Entry 
@Component 
struct Index { 
  @State message: string = 'Hello World'; 
  private appContext: common.Context = getContext(this); 
  private fileDir: string = '' 
  createFile(){ 
    let cacheDir = this.appContext.cacheDir; 
    this.fileDir = cacheDir + '/HelloWorld.txt' 
    let file = fs.openSync(this.fileDir, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE) 
    let writeLen = fs.writeSync(file.fd, 'hello world!'); 
    console.info("write data to file succeed and size is:" + writeLen); 
    fs.closeSync(file) 
  } 
  saveFile(){ 
    try { 
      let documentSaveOptions = new picker.DocumentSaveOptions(); 
      documentSaveOptions.newFileNames = ['HelloWorld.txt']; 
      let documentPicker = new picker.DocumentViewPicker(); 
      documentPicker.save(documentSaveOptions).then((documentSaveResult: Array<string>) => { 
        let uri = documentSaveResult[0]; 
        let sanFile = fs.openSync(this.fileDir, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE) 
        let pubFile = fs.openSync(uri, fs.OpenMode.READ_WRITE | fs.OpenMode.CREATE) 
        fs.copyFileSync(sanFile.fd, pubFile.fd) 
        fs.close(sanFile) 
        fs.close(pubFile) 
        console.info('DocumentViewPicker.save successfully, documentSaveResult uri: ' + JSON.stringify(documentSaveResult)); 
      }).catch((err: BusinessError) => { 
        console.error('DocumentViewPicker.save failed with err: ' + JSON.stringify(err)); 
      }); 
    } catch (error) { 
      let err: BusinessError = error as BusinessError; 
      console.error('DocumentViewPicker failed with err: ' + JSON.stringify(err)); 
    } 
  } 
  build() { 
    Row() { 
      Column() { 
        Button(this.message) 
          .fontSize(50) 
          .fontWeight(FontWeight.Bold) 
          .onClick(()=>{ 
            this.createFile(); 
            this.saveFile(); 
          }) 
      } 
      .width('100%') 
    } 
    .height('100%') 
  } 
}