How to trigger intervalDidStart in DeviceActivity API?

I have followed the two videos about Screen time API in wwdc 21 and 22. Now i want to sheild some apps during sleeping, but I encounter some problems.

Problem I faced

1. I write my monitor and override intervalDidStart and intervalDidEnd . Here is a simplified version

class Mymonitor: DeviceActivityMonitor {
  let store = ManagedSettingsStore()

  override func intervalDidStart(for activity: DeviceActivityName) {
      super.intervalDidStart(for: activity)
      store.shield.applications = selection.applicationToken
   }

1. then I use deviceActivityCenter to start monitor

func StartSleep (_ startTime : DateComponents)
{
  let schedule = DeviceActivitySchedule(intervalStart: startTime,
                     intervalEnd: DateComponents(hour: 4, minute: 0),
                     repeats: true
                     )
  let center = DeviceActivityCenter()
  center.stopMonitoring([.daily])
  try! center.startMonitoring(.daily, during: schedule)
}

However, when I use StartSleep. The applications is not shielded. I have noticed that I haven't use Mymonitor in any place. In the demo Code, it seems i just need to finish two parts (DeviceActivityMonitor and DeviceActivityCenter).

But in the DeviceActivityCenter i do not use Mymonitor. Then how can i triggger intervalDidStart that i overwrited?

Do i miss other details. Where can i use Mymonitor?

Troubleshooting

• I have successfully shield apps using store.shield.applications = selection,applicationToken directly.

Thanks for your reading and help!