inabox/InABox.Mobile/InABox.Mobile.Shared/TaskExtensions.cs

using System;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
using Xamarin.Forms;

namespace InABox.Mobile.Shared
{
    
    public struct SynchronizationContextAwaiter : INotifyCompletion
    {
        private static readonly SendOrPostCallback _postCallback = state => ((Action)state)();

        private readonly SynchronizationContext _context;
        public SynchronizationContextAwaiter(SynchronizationContext context)
        {
            _context = context;
        }

        public bool IsCompleted => _context == SynchronizationContext.Current;

        public void OnCompleted(Action continuation) => _context.Post(_postCallback, continuation);

        public void GetResult() { }
    }
    
    public static class TaskExtensions
    {
        public static void BeginInvokeOnMainThread(this Task task, Action<Task> action)
        {
            task.ContinueWith((task) =>
            {
                Device.BeginInvokeOnMainThread(() => action(task));
            });
        }
        
        public static SynchronizationContextAwaiter GetAwaiter(this SynchronizationContext context)
        {
            return new SynchronizationContextAwaiter(context);
        }
    }
}