AuthEnforcementHandler.cs

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using System.Threading.Tasks;
using PhotoSharingApp.Universal.Models;
using PhotoSharingApp.Universal.Views;
using Windows.ApplicationModel.Resources;
using Windows.UI.Xaml.Controls;

namespace PhotoSharingApp.Universal.Services
{
    /// <summary>
    /// Handles authentication enforcement.
    /// </summary>
    public class AuthEnforcementHandler : IAuthEnforcementHandler
    {
        private TaskCompletionSource<bool> _contentDialogClosedTaskCompletionSource;
        private readonly ResourceLoader _resourceLoader;
        private TaskCompletionSource<SignInCompletionSource> _signInTaskCompletionSource;

        public AuthEnforcementHandler()
        {
            _resourceLoader = ResourceLoader.GetForCurrentView();

            // We need to be aware of when a sign-in happens.
            AppEnvironment.Instance.CurrentUserChanged += AppEnvironment_CurrentUserChanged;
        }

        private void AppEnvironment_CurrentUserChanged(object sender, User e)
        {
            if (_signInTaskCompletionSource != null)
            {
                _signInTaskCompletionSource.SetResult(SignInCompletionSource.UserChanged);
            }
        }

        /// <summary>
        /// Requires a user to be signed in successfully.
        /// If not signed in already, the user will be prompted to do so.
        /// </summary>
        /// <exception cref="SignInRequiredException">When sign-in was not successful.</exception>
        public async Task CheckUserAuthentication()
        {
            // If there is a user signed in already, we're fine,
            // Otherwise, we need to prompt the user to sign in.
            if (AppEnvironment.Instance.CurrentUser == null)
            {
                // We create a content dialog and host the existing sign-in page
                // control inside of it.
                var contentDialog = new ContentDialog
                {
                    SecondaryButtonText = _resourceLoader.GetString("MessageDialog_Cancel"),
                    Title = _resourceLoader.GetString("SignInRequired_Title")
                };

                // We use the following completion source for making sure the
                // content dialog has closed before proceeding.
                _contentDialogClosedTaskCompletionSource = new TaskCompletionSource<bool>();

                contentDialog.Closed += (s, e) => { _contentDialogClosedTaskCompletionSource.SetResult(true); };

                // Use existing sign-in page as hosted content.
                var signinPage = new SignInPage();
                signinPage.ViewModel.RedirectToProfilePage = false;
                contentDialog.Content = signinPage;

                // We may complete the scenario in two ways:
                // User presses Cancel on the sign-in page, or a successful sign-in has
                // happened. We then need to manually close the existing content dialog as
                // this dialog is independent from the sign-in page.
                _signInTaskCompletionSource = new TaskCompletionSource<SignInCompletionSource>();
                var dialogTask = _signInTaskCompletionSource.Task;

                // Grab the task for showing the dialog.
                var task = contentDialog.ShowAsync();
                task.Completed = (info, status) =>
                {
                    if (_signInTaskCompletionSource != null)
                    {
                        _signInTaskCompletionSource.TrySetResult(SignInCompletionSource.DialogClosed);

                        // We need to reset the completion source after using it, otherwise
                        // sign-out will not be successful.
                        _signInTaskCompletionSource = null;
                    }
                };

                var result = await dialogTask;

                if (result == SignInCompletionSource.UserChanged)
                {
                    var contentDialogClosedTask = _contentDialogClosedTaskCompletionSource.Task;

                    // Close the dialog.
                    task.Cancel();

                    // We need to wait on the content dialog being closed.
                    await contentDialogClosedTask;
                }

                // Sign-in dialog handles failures and cancellations, so we need to check
                // if we have the current user object available.
                if (AppEnvironment.Instance.CurrentUser == null)
                {
                    throw new SignInRequiredException("Sign-in not successful");
                }
            }
        }
    }
}