Display some progress dialog during the time data are being fetched

I am building an app in Xamarin.Forms that display news from different sources. I fetch the data from the internet using JSON Format. I am using the code below. What I want is to display some progress dialog during the time data are being fetched but I cannot place them inside the populate method.

How can it be possible. I am mainly interested for iOS project.

public partial class SearchNews : ContentPage
    {


        public static ObservableCollection<NewsClusterItem> newsItems { get; set; }


        public static SearchResponseObject respObj;

        public string base_url = "http://time.ikub.al/services/servicesNew/SearchNewsNew.ashx?";
        public string news_cluster_per_page = "newsClustersPerPage=15&";
        public string news_page_number = "pageNumber=";
        public string query_string = "&query=";
        public int page_number = 1;
        public static string search_url;

        public SearchNews (String text)
        {
            InitializeComponent ();

            NavigationPage.SetBackButtonTitle (this, "");
            this.Title = "Rezultatet për: " + text;

            //forming the url
            search_url = base_url + news_cluster_per_page + news_page_number + page_number + query_string + text;

            newsItems = new ObservableCollection<NewsClusterItem> ();
            listView.ItemsSource = newsItems;

            populate ();
        }

        async static void populate(){

            try
                {
                string contents;
                string Url = String.Format(search_url);
                HttpClient hc = new HttpClient();
                contents = await hc.GetStringAsync(Url);

                respObj = JsonConvert.DeserializeObject<SearchResponseObject>(contents);

                if(respObj.moreNews){

                    for(int i =0; i<respObj.news.Count;i++){

                        newsItems.Add(respObj.news[i]);
                    }
                }
            }
            catch (System.Exception sysExc)
            {
                // Do something to log this error.
                throw;
            }
        }