如何在app.ets中定义全局对象(全局变量、方法)，又如何在其它ets文件中获取并应用它？

onCreate() {
    console.info('Application onCreate')
    globalThis.msg = 'test'
  },
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@Entry
@Component
struct Index {
  @State message: string = ''

  aboutToAppear(){
    this.message = globalThis.msg
  }

  build() {
    Row() {
      Column() {
        Text(this.message)
          .fontSize(50)
          .fontWeight(FontWeight.Bold)
      }
      .width('100%')
    }
    .height('100%')
  }
}
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