Navigate to another ContentPage from async method before the async method returns a value

Question

I need to start Another ContentPage before this async method returns value:

public class GettingCountry : ContentPage
{
    public static List<string> CountriesList = new List<string>();

    MainPage mainPage = new MainPage();
    public async Task<List<RootObject>> FetchAsync(string url)
    {
        string jsonString;
        using (var httpClient = new System.Net.Http.HttpClient())
        {
            var stream = await httpClient.GetStreamAsync(url);
            StreamReader reader = new StreamReader(stream);
            jsonString = reader.ReadToEnd();
        }

        var listOfCountries = new List<RootObject>();

        var responseCountries = JArray.Parse(JObject.Parse(jsonString)["response"]["items"].ToString());

        foreach (var countryInResponse in responseCountries)
        {
            var rootObject = new RootObject((int)countryInResponse["id"], (string)countryInResponse["title"]);

            CountriesList.Add(rootObject.Title);
        }

        //I NEED TO NAVIGATE TO FillingPage() FROM HERE:
        await Navigation.PushAsync(new FillingPage());

        //await Navigation.PushModalAsync(new NavigationPage(new FillingPage()));

        return listOfCountries;
    }

The page that is need to be started is:

[XamlCompilation(XamlCompilationOptions.Compile)]
public partial class FillingPage : ContentPage
{
    public FillingPage ()
    {
        GettingCountry gettingCountry = new GettingCountry();

        Label header = new Label
        {
            Text = "Заполните бланк",
            FontSize = Device.GetNamedSize(NamedSize.Large, typeof(Label)),
            HorizontalOptions = LayoutOptions.Center,
            VerticalOptions = LayoutOptions.CenterAndExpand,
            TextColor = Color.Blue
        };

        Entry nameEntry = new Entry()
        {
            Placeholder = "Имя",
        };

        Entry surnameEntry = new Entry()
        {
            Placeholder = "Фамилия"
        };

        Picker countryPicker = new Picker()
        {
            Title = "Страна",
            VerticalOptions = LayoutOptions.CenterAndExpand
        };

        foreach (string country in GettingCountry.CountriesList)
        {
            countryPicker.Items.Add(country);
        }

        SearchBar townSearchBar = new SearchBar()
        {
            Placeholder = "Город",
            SearchCommand = new Command(() =>
            {
            })
        };

        SearchBar universitySearchBar = new SearchBar()
        {
            Placeholder = "Университет",
            SearchCommand = new Command(() =>
            {
            })
        };
        Button myButton = new Button()
       {
           TextColor = Color.Green,
           Text = "Выполнить",
           FontSize = 22
       };

        // Accomodate iPhone status bar.
        this.Padding = new Thickness(10, Device.OnPlatform(20, 0, 0), 10, 5);

        // Build the page.
        this.Content = new StackLayout
        {
            Children =
            {
                header,
                nameEntry,
                surnameEntry,
                countryPicker,
                townSearchBar,
                universitySearchBar,
                myButton
            }
        };
    }
}

}

But this code await Navigation.PushAsync(new FillingPage()); works well only when I press a button. When I press a button the needed page starts well. But the same code inside a method does not work. I have debagged it. It goes to a FillingPage() but doesn`t launches it when I try to launch it from inside the async method.

Answer 1

This is likely a result of the operation not being performed on the main thread. Try wrapping your code like this:

Device.BeginInvokeOnMainThread(async () =>
{
    await Navigation.PushAsync(new FillingPage());
}

Edit: After a private message, I learned that there was not enough information in the question to know the real issue. The application is calling FetchAsync in Application.OnStart and it's not part of the view hiearchy at all so navigation methods would not work. The following was provided:

protected override void OnStart ()
{
    getCountry();
}

private async void getCountry()
{
    var url = "...";
    GettingCountry gettingCountry = new GettingCountry();
    await gettingCountry.FetchAsync(url);
}

GettingCountry is a ContentPage being used like some kind of data access class and it's not currently part of the UI as MainPage is set to something else. A quick hack would be something more like:

private async void getCountry()
{
    var url = "...";
    GettingCountry gettingCountry = new GettingCountry();
    var data = await gettingCountry.FetchAsync(url);
    await MainPage.Navigation.PushAsync(new FillingPage(data));
 }

I would suggest two further areas to look at improving.

1. Consider refactoring GettingCountry as it does not need to be a ContentPage.
2. Investigate an alternative calling so that async void is not used.