HarmonyOS 自定义的dialog如何显示在屏幕中间

参考demo:

@CustomDialog
struct CustomDialogExample {
  controller?: CustomDialogController
  private selectDate: Date = new Date()

  build() {
    Column() {
      DatePicker({
        start: new Date('2009-1-1'),
        end: new Date('2100-12-31'),
        selected: this.selectDate
      })
    }
  }
}

@Entry
@Component
struct Index {
  @State textValue: string = ''
  @State inputValue: string = 'click me'
  dialogController: CustomDialogController | null = new CustomDialogController({
    builder: CustomDialogExample(),
    openAnimation: {
      duration: 1200,
      curve: Curve.Friction,
      delay: 500,
      playMode: PlayMode.Alternate,
      onFinish: () => {
        console.info('play end')
      }
    },
    autoCancel: true,
    alignment: DialogAlignment.Center,
    gridCount: 4,
    customStyle: true,
    backgroundColor: 0xd9ffffff,
    cornerRadius: 10,
  })

  // 在自定义组件即将析构销毁时将dialogControlle置空
  aboutToDisappear() {
    this.dialogController = null // 将dialogController置空
  }

  build() {
    Column() {
      Button(this.inputValue)
        .onClick(() => {
          if (this.dialogController != null) {
            this.dialogController.open()
          }
        }).backgroundColor(0x317aff)
    }.width('100%').margin({ top: 5 })
  }
}