设置Xamarin C＃中可见的UI元素，然后调用异步函数

Question

Can anybody help me with this code in my Xamarin project. I am trying to set a loading wheel (to signify that an action is happening and to let the user know to wait) when the "Login" button is clicked. For some reason since the function is asynchronous the loading wheel is never set to visible when the API code is run. It just fails to show up when I click login, however, it still does the login function.

    // Defined up above in the file
    var loginButton = new Button
    {
        Text = "Login",
    };
    loginButton.BackgroundColor = Color.Navy;
    loginButton.TextColor = Color.White;
    loginButton.Clicked += OnLoginButtonClicked;

    async void OnLoginButtonClicked(object sender, EventArgs e)
    {
        loadingWheel.IsVisible = true;

        try
        {
            var restUrl = "*******";
            var content = string.Empty;
            using (var client = new HttpClient())
            {

                string body = "{\"UserName\":\"" + usernameEntry.Text + "\", \"Password\":\"" + passwordEntry.Text + "\"}";

                var contentType = new StringContent(body, Encoding.UTF8, "application/json");

                var result = client.PostAsync(restUrl, contentType).Result;
                content = await result.Content.ReadAsStringAsync();
            }

            if (content.ToLower() != "false")
            {
                var menuPage = new MenuPage();
                NavigationPage = new NavigationPage(new HomePage());
                RootPage = new Views.MainPage();
                RootPage.Master = menuPage;
                RootPage.Detail = NavigationPage;
                MainPage = RootPage;

            }
            else
            {
                messageLabel.Text = "Username or password incorrect. Please try again.";
                passwordEntry.Text = string.Empty;
            }

        }
        catch (Exception ex)
        {
            messageLabel.Text = "Please check the internet connection for the connectivity.";
        }
    }

If I comment out the entire try block then the loading wheel does show up. It just does not work with the code in there.

Can anybody help me solve this problem? Thanks.

Answer 1

I think you can try with BeginInvokeOnMainThread

Device.BeginInvokeOnMainThread (() => {
  loadingWheel.IsVisible = true;
});

UPDATE

I have also create this REPO... it works without BeginInvodeOnMainThread

public class MyPage6 : ContentPage
{

    ActivityIndicator _ac = new ActivityIndicator { IsVisible = false, IsRunning = false };

    public MyPage6()
    {
        Button b = new Button {Text = "Press for ActivityIndicator" };
        b.Clicked += B_Clicked;

        Content = new StackLayout
        {
            Children = {
                _ac,
                b,
                new Label { Text = "Hello ContentPage" }
            }
        };
    }

    async void B_Clicked(object sender, EventArgs e)
    {

        _ac.IsRunning = true;
        _ac.IsVisible = true;
        await Task.Delay(2000);
        _ac.IsRunning = false;
        _ac.IsVisible = false;
    }
}