Xamarin Google Calendar Sample

Question

I'm trying to get this sample working with Xamarin (Xamarin Forms)

https://developers.google.com/google-apps/calendar/quickstart/dotnet

I modified the code so that the Xamarin Forms app would load the credentials json from a resource file. I checked that the resource file is getting loaded by stepping to the code. I also modified the DataStore class because Xamarin Forms can't access the path that is specified in the sample.

using Google.Apis.Auth.OAuth2;
using Google.Apis.Calendar.v3;
using Google.Apis.Calendar.v3.Data;
using Google.Apis.Services;
using Google.Apis.Util.Store;
using System;
using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;
using Xamarin.Forms;
using Xamarin.Forms.Xaml;

namespace TestXamarinFormsLibrary.Pages
{
    [XamlCompilation(XamlCompilationOptions.Compile)]
    public partial class CalendarPage : ContentPage
    {
        public CalendarPage()
        {
            InitializeComponent();
            Go();
        }

        static string[] Scopes = { CalendarService.Scope.CalendarReadonly };
        static string ApplicationName = "Google Calendar API .NET Quickstart";

        private async void Go()
        {
            UserCredential credential;

            var assembly = GetType().Assembly;
            using (var stream = assembly.GetManifestResourceStream("TestXamarinFormsLibrary.client_secret.json"))
            {
                credential = GoogleWebAuthorizationBroker.AuthorizeAsync(
                        GoogleClientSecrets.Load(stream).Secrets,
                        Scopes,
                        "user",
                        CancellationToken.None,
                        new DataStore()).Result;
            }

            // Create Google Calendar API service.
            var service = new CalendarService(new BaseClientService.Initializer()
            {
                HttpClientInitializer = credential,
                ApplicationName = ApplicationName,
            });

            // Define parameters of request.
            EventsResource.ListRequest request = service.Events.List("primary");
            request.TimeMin = DateTime.Now;
            request.ShowDeleted = false;
            request.SingleEvents = true;
            request.MaxResults = 10;
            request.OrderBy = EventsResource.ListRequest.OrderByEnum.StartTime;

            // List events.
            Events events = request.Execute();
            if (events.Items != null && events.Items.Count > 0)
            {
                foreach (var eventItem in events.Items)
                {
                    string when = eventItem.Start.DateTime.ToString();
                    if (String.IsNullOrEmpty(when))
                    {
                        when = eventItem.Start.Date;
                    }
                    await DisplayAlert("ok", $"{eventItem.Summary} ({when})", "ok");
                }
            }
        }
    }

    public class DataStore : IDataStore
    {
        private Dictionary<string, object> Data = new Dictionary<string, object>();

        public Task ClearAsync()
        {
            throw new NotImplementedException();
        }

        public Task DeleteAsync<T>(string key)
        {
            throw new NotImplementedException();
        }

        public async Task<T> GetAsync<T>(string key)
        {
            if (Data.ContainsKey(key))
            {
                return (T)Data[key];
            }

            return default(T);
        }

        public async Task StoreAsync<T>(string key, T value)
        {
            Data.Add(key, value);
        }
    }
}

This is the error I get when I run it:

No exception. I get a similar error on Android. My guess is that it is relying on an oAuth library to open up a web browser, to attempt to allow authentication, but Xamarin Forms does not implement this behaviour. I've also tried turning on all the UWP capabilities, but this didn't help.

How can I get the sample running for Xamarin Forms?

Answer 1

The repo for the .NET Client is here: https://github.com/google/google-api-dotnet-client

I think I will be able to figure this out from the source code, but I havenot yet attempted this.

Answer 2

I have the Google .NET Client working in my UWP app. The trick is that you have to put it in a .NET Standard 2.0 Class Library, expose the API services you need, and then reference that library from your UWP app.

Also, you have to handle the getting the auth token yourself. It's not that much work and the Drive APIs and Calendar APIs work just fine (the only ones I've tried). You can see that I pass in a simple class that contains the auth token and other auth details to a method called Initialize.

Here is the single class I used in the .NET Standard 2.0 class library:

namespace GoogleProxy
{
public class GoogleService
{
    public CalendarService calendarService { get; private set; }

    public DriveService driveService { get; private set; }


    public GoogleService()
    {

    }

    public void Initialize(AuthResult authResult)
    {
        var credential = GetCredentialForApi(authResult);
        var baseInitializer = new BaseClientService.Initializer { HttpClientInitializer = credential, ApplicationName = "{your app name here}" };

        calendarService = new Google.Apis.Calendar.v3.CalendarService(baseInitializer);
        driveService = new Google.Apis.Drive.v3.DriveService(baseInitializer);
    }

    private UserCredential GetCredentialForApi(AuthResult authResult)
    {
        var initializer = new GoogleAuthorizationCodeFlow.Initializer
        {
            ClientSecrets = new ClientSecrets
            {
                ClientId = "{your app client id here}",
                ClientSecret = "",
            },
            Scopes = new string[] { "openid", "email", "profile",  "https://www.googleapis.com/auth/calendar.readonly", "https://www.googleapis.com/auth/calendar.events.readonly", "https://www.googleapis.com/auth/drive.readonly" },
        };

        var flow = new GoogleAuthorizationCodeFlow(initializer);

        var token = new TokenResponse()
        {
            AccessToken = authResult.AccessToken,
            RefreshToken = authResult.RefreshToken,
            ExpiresInSeconds = authResult.ExpirationInSeconds,
            IdToken = authResult.IdToken,
            IssuedUtc = authResult.IssueDateTime,
            Scope = "openid email profile https://www.googleapis.com/auth/calendar.readonly https://www.googleapis.com/auth/calendar.events.readonly https://www.googleapis.com/auth/drive.readonly",
            TokenType = "bearer" };

        return new UserCredential(flow, authResult.Id, token);
    }

}
}

Full answer for UWP here: https://stackoverflow.com/a/53971436/1938624