ShowViewModelAsync and InvokeOnMainThreadAsync for MvvmCross. Also adds OnBack/OnBack2 methods on viewmodels.

BaseViewModel.cs

    public class BaseViewModel : MvxViewModel, IBackViewModel
    {
        #region InvokeOnMainThreadAsync
        protected Task InvokeOnMainThreadAsync(Action action)
        {
            var tcs = new TaskCompletionSource<bool>();
            if (Dispatcher != null)
            {
                Dispatcher.RequestMainThreadAction(() =>
                {
                    action();
                    tcs.TrySetResult(true);
                });
            }
            else
                tcs.TrySetCanceled();

            return tcs.Task;
        }

        protected Task<T> InvokeOnMainThreadAsync<T>(Func<T> action)
        {
            var tcs = new TaskCompletionSource<T>();
            if (Dispatcher != null)
            {
                Dispatcher.RequestMainThreadAction(() =>
                {
                    tcs.TrySetResult(action());
                });
            }
            else
                tcs.TrySetCanceled();

            return tcs.Task;
        }
        #endregion
    
        #region Infrastructure to handle return values from a viewmodel
        protected class ReturnedResult<T>
        {
            public T Result;
            public Guid MessageId;
        }

        public const string ReturnedMessageIdKey = "returnedMessageId";
        private readonly ConcurrentDictionary<Guid, MvxSubscriptionToken> tokens = new ConcurrentDictionary<Guid, MvxSubscriptionToken>();

        /// <summary>
        /// Store viewmodel parameters
        /// </summary>
        private IMvxBundle viewModelParameters;

        protected void SetReturnResult<T>(T result)
        {
            Guid messageId;
            string messageIdstring;
            if (viewModelParameters == null || viewModelParameters.Data == null || !viewModelParameters.Data.TryGetValue(ReturnedMessageIdKey, out messageIdstring) || !Guid.TryParse(messageIdstring, out messageId))
                throw new Exception(String.Format("Can not find {0} in the viewmodel's parameters {1}. Did you override InitFromBundle without calling base ?", ReturnedMessageIdKey, this.GetType().Name));

            SetReturnResult(messageId, result);
        }

        /// <summary>
        /// For unit tests
        /// </summary>
        protected void SetReturnResult<T>(Guid messageId, T result)
        {
            var messenger = Mvx.Resolve<IMvxMessenger>();
            messenger.Publish(new MvxMessage<ReturnedResult<T>>(this, new ReturnedResult<T> { Result = result, MessageId = messageId }));
        }

        protected void Close<T>(T result)
        {
            if(result is IMvxViewModel)
                base.Close((IMvxViewModel)result);
            else
            {
                SetReturnResult(result);
                base.Close(this);
            }
        }

        /// <summary>
        /// TViewModel must call SetReturnResult in any method that closes the viewmodel.
        /// This includes BackCommand (OnBack), BackCommand2 (OnBack2), this.Close() and calls to ShowViewModel which replaces the back stack.
        /// 
        /// NOTE: if the app is tombstoned, the task is lost.
        /// </summary>
        protected Task<TResult> ShowViewModelAsync<TViewModel, TResult>(object parameterValuesObject = null, IMvxBundle presentationBundle=null)
            where TViewModel: IMvxViewModel
        {
            var tcs = new TaskCompletionSource<TResult>();
            var messenger = Mvx.Resolve<IMvxMessenger>();
            var messageId = Guid.NewGuid();

            var token = messenger.Subscribe<MvxMessage<ReturnedResult<TResult>>>(message =>
            {
                if (message.Data.MessageId != messageId)
                    return;

                MvxSubscriptionToken token2;
                if (tokens.TryRemove(messageId, out token2))
                    token2.Dispose();
                else
                    Mvx.Error("Result message from {0} with same id received 2 times. Remove your multiple calls to ReturnResult in {0}.", typeof(TViewModel).Name);

                tcs.TrySetResult(message.Data.Result);
            });
            tokens.TryAdd(messageId, token);

            var parameters = parameterValuesObject.ToSimplePropertyDictionary();
            if(parameters.ContainsKey(ReturnedMessageIdKey))
                throw new ArgumentException(String.Format("parameters can not include a {0} key when using a return value", ReturnedMessageIdKey), "parameterValuesObject");
            parameters.Add(ReturnedMessageIdKey, messageId.ToString());

            ShowViewModel<TViewModel>(parameters, presentationBundle);
            return tcs.Task;
        }

        protected override void InitFromBundle(IMvxBundle parameters)
        {
            viewModelParameters = parameters;
            base.InitFromBundle(parameters);
        }
        #endregion
    
        #region handle "back" events
        /*
        Handling of back events requires the use of a BaseView<TViewModel> class in android and iOS.
        This BaseView class will correctly bind mvvm actions BackCommand and BackCommand2
        */

        protected CancellationTokenSource BackCancel = new CancellationTokenSource();

        /// <summary>
        /// A cancellable back command (Android, and some ios screens where back is replaced)
        /// </summary>
        public virtual ICommand BackCommand2
        {
            get
            {
                return new MvxCommand(async () =>
                {
                    Mvx.Trace("BackCommand2 detected on {0}", GetType().Name);
                    if (await OnBack2().ConfigureAwait(false))
                    {
                        BackCancel.Cancel();
                        Close(this);
                    }
                    else
                        Mvx.Trace("BackCommand2 NOT cancelled on {0}", GetType().Name);
                });
            }
        }

        /// <summary>
        /// A non cancellable back command (ios back navigation button)
        /// </summary>
        public virtual ICommand BackCommand
        {
            get
            {
                return new MvxCommand(async () =>
                {
                    Mvx.Trace("BackCommand detected on {0}", GetType().Name);
                    if (await OnBack().ConfigureAwait(false))
                    {
                        BackCancel.Cancel();
                    }
                    else
                        Mvx.Trace("BackCommand NOT cancelled on {0}", GetType().Name);
                });
            }
        }

        /// <summary>
        /// A cancellable back command (Android, and some ios screens where back is replaced). Return false to prevent the back command.
        /// </summary>
        /// <returns></returns>
        public virtual Task<bool> OnBack2()
        {
            return Task.FromResult(true);
        }

        /// <summary>
        /// A non cancellable back command (ios back navigation button). Return false to prevent the "BackCancel" cancellationtoken from being cancelled.
        /// </summary>
        /// <returns></returns>
        public virtual Task<bool> OnBack()
        {
            return Task.FromResult(true);
        }
        #endregion
    }

CustomBaseActivity.cs

    [Preserve(AllMembers = true)]
    [Activity(ScreenOrientation = ScreenOrientation.Portrait)]
    public class CustomBaseActivity<T> : BaseActivity where T : class, IBackViewModel
    {
        private CancellationTokenSource backPressedSource = new CancellationTokenSource();

        protected new T ViewModel => base.ViewModel as T;
        protected CancellationToken BackPressed { get { return backPressedSource.Token; } }

        public ICommand BackPressedCommand { get; set; }

        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            BindBackPressed();

            backPressedSource.Token.Register(() =>
            {
                var backPressedCommand = BackPressedCommand;
                backPressedCommand?.Execute(null);
            });
        }

        public override bool OnOptionsItemSelected(IMenuItem item)
        {
            switch (item.ItemId)
            {
                //Action bar: home button will back
                case Android.Resource.Id.Home:
                    OnBackPressed();
                    return true;
            }
            return base.OnOptionsItemSelected(item);
        }

        public override void OnBackPressed()
        {
            backPressedSource.Cancel();
            backPressedSource = new CancellationTokenSource();
            backPressedSource.Token.Register(() =>
            {
                var backPressedCommand = BackPressedCommand;
                if (backPressedCommand != null)
                    backPressedCommand.Execute(null);
            });
        }

        /// <summary>
        /// Required to put that here for correct linking in release mode
        /// </summary>
        protected void BindBackPressed()
        {
            var set = this.CreateBindingSet<CustomBaseActivity<T>, IBackViewModel>();
            set.Bind(this).For(s => s.BackPressedCommand).To(vm => vm.BackCommand2);
            set.Apply();
        }
    }
    

CustomBaseView.cs

    public class CustomBaseView<T> : MvxViewController where T: MvxViewModel, IBackViewModel
    {
        private readonly CancellationTokenSource backPressedSource = new CancellationTokenSource();
        protected CancellationToken BackPressed => backPressedSource.Token;
        public ICommand BackPressedCommand { get; set; }

        protected new T ViewModel
        {
            get { return base.ViewModel as T; }
			      set { base.ViewModel = value; }
        }

        public override void ViewDidLoad()
        {
            base.ViewDidLoad();

            backPressedSource.Token.Register(() =>
            {
                var backPressedCommand = BackPressedCommand;
                backPressedCommand?.Execute(null);
            });

            var set = this.CreateBindingSet<MvxSignupBaseView<T>, T>();
            set.Bind(this).For(t => t.BackPressedCommand).To(vm => vm.BackCommand);
            set.Apply();
        }

        /// <summary>
        /// Not called if vc is popped up
        /// </summary>
        public override void WillMoveToParentViewController(UIViewController parent)
        {
            if (parent == null)
            {
#if DEBUG
                Mvx.Trace("{0}: back detected on WillMoveToParentViewController", GetType().Name);
#endif
                backPressedSource.Cancel();
            }
            base.WillMoveToParentViewController(parent);
        }
          
        public override void DidMoveToParentViewController(UIViewController parent)
        {
            if (parent == null)
            {
#if DEBUG
                Mvx.Trace("{0}: back detected on DidMoveToParentViewController", GetType().Name);
#endif
                backPressedSource.Cancel();
            }
            base.DidMoveToParentViewController(parent);
        }
    }

IBackViewModel.cs

    public interface IBackViewModel : IMvxViewModel
    {
        ICommand BackCommand { get; }

        ICommand BackCommand2 { get; }
    }
    

The 2 suffix reminds me a little of the Win32 API.

👍 yeah i'm not proud of it. There should be an easy way to merge OnBack and OnBack2.

anyway good job ! I like these extensions 👍