swift - 获取位置点和标题并在 mapkit 视图上显示

Question

我使用 longtapgesture 识别器添加地点，虽然它显示在我的地图上，但当我离开视图然后返回时它消失了。

所以我使用firebase来添加位置来获取位置，我希望它显示在我的地图视图上。我正在获取 snapshot.value 的值，我正试图在我的 mapview 上展示，请帮助我。这是我似乎无法弄清楚的最后一步。

 func fetchlocations() {
    let ref = Database.database().reference()
    ref.child("Map").observe(.value) { (snapshot) in
        print(snapshot.value)

        let activity = (snapshot.value as AnyObject!)!["activity"] as? String?
        let lat = (snapshot.value as AnyObject!)!["lat"] as? String?
        let long = (snapshot.value as AnyObject!)!["long"] as? String?

        let annotation = MKPointAnnotation()
        annotation.coordinate = CLLocationCoordinate2D(latitude: (Double(lat!))!, longitude: (Double(long!))!)
        annotation.title = activity
        self.mapView?.addAnnotation(annotation)
    }
}