Jim Bennett
Xamarin MVP Microsoft MVP

Mobile developer at EROAD, Xamarin MVP and Certified Developer, Microsoft MVP, author of Xamarin In Action, blogger, speaker, father and lover of beer, whisky and Thai food. Opinions are my own.

  Auckland, New Zealand
See me soon at:
NDC Sydney 2017
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As much as I'm loving MVVMLight it does have some limitations. The one I've hit recently is how well it doesn't work when you are using AppCompat (though to be honest I think it's more down to how hacky AppCompat seems to be).

I'm using AppCompatActivity as my base activity class to get an action bar supported on older API versions. The problem is MVVMLight implements it's own ActivityBase which is derived from Activity, and it relies on a set of static methods and properties on this to handle things like navigation and dialogs. If you want to use AppCompatActivity you are stuck - navigation will not work.

Luckily MVVMLight is really well designed with good separation of concerns, so everything is done via interface. This means we can write our own implementation of the navigation service and dialog service that will work with AppCompatActivity and use it in our code. Being open source as well we can take the existing source and tweak it slightly to make it work for us. It's a bit of a hack but it works.

The first thing to do is to define our own base activity class that has the same functionality as the ActivityBase, but derived from AppCompatActivity.

using Android.Support.V7.App;

namespace JimBobBennett.MvvmLight.AppCompat  
{
    public abstract class AppCompatActivityBase : AppCompatActivity
    {
        public static AppCompatActivityBase CurrentActivity { get; private set; }

        internal string ActivityKey { get; private set; }

        internal static string NextPageKey { get; set; }

        public static void GoBack()
        {
            CurrentActivity?.OnBackPressed();
        }

        protected override void OnResume()
        {
            CurrentActivity = this;
            if (string.IsNullOrEmpty(ActivityKey))
            {
                ActivityKey = NextPageKey;
                NextPageKey = null;
            }
            base.OnResume();
        }
    }
}

This is identical to the MVVMLight ActivityBase except the base class is AppCompatActivity.

Now we have our base activity we can use it for the navigation service. This is identical to the MVVMLight Android NavigationService except instead of accessing the statics on AppCompatActivityBase instead of on ActivityBase.

using System;  
using System.Collections.Generic;  
using Android.Content;  
using GalaSoft.MvvmLight.Views;

namespace JimBobBennett.MvvmLight.AppCompat  
{
    public class AppCompatNavigationService : INavigationService
    {
        private readonly Dictionary<string, Type> _pagesByKey = new Dictionary<string, Type>();
        private readonly Dictionary<string, object> _parametersByKey = new Dictionary<string, object>();

        private const string RootPageKey = "-- ROOT --";
        private const string ParameterKeyName = "ParameterKey";

        public string CurrentPageKey => AppCompatActivityBase.CurrentActivity.ActivityKey ?? RootPageKey;

        public void GoBack()
        {
            AppCompatActivityBase.GoBack();
        }

        public void NavigateTo(string pageKey)
        {
            NavigateTo(pageKey, null);
        }

        public void NavigateTo(string pageKey, object parameter)
        {
            AppCompatActivityBase.CurrentActivity.RunOnUiThread(() =>
            {
                if (AppCompatActivityBase.CurrentActivity == null)
                    throw new InvalidOperationException("No CurrentActivity found");

                lock (_pagesByKey)
                {
                    if (!_pagesByKey.ContainsKey(pageKey))
                        throw new ArgumentException($"No such page: {pageKey}. Did you forget to call NavigationService.Configure?", nameof(pageKey));

                    var intent = new Intent(AppCompatActivityBase.CurrentActivity, _pagesByKey[pageKey]);
                    if (parameter != null)
                    {
                        lock (_parametersByKey)
                        {
                            var guid = Guid.NewGuid().ToString();
                            _parametersByKey.Add(guid, parameter);
                            intent.PutExtra(ParameterKeyName, guid);
                        }
                    }

                    AppCompatActivityBase.CurrentActivity.StartActivity(intent);
                    AppCompatActivityBase.NextPageKey = pageKey;
                }
            });
        }

        public void Configure(string key, Type activityType)
        {
            lock (_pagesByKey)
            {
                if (_pagesByKey.ContainsKey(key))
                    _pagesByKey[key] = activityType;
                else
                    _pagesByKey.Add(key, activityType);
            }
        }

        public object GetAndRemoveParameter(Intent intent)
        {
            if (intent == null)
                throw new ArgumentNullException(nameof(intent), "This method must be called with a valid Activity intent");

            var stringExtra = intent.GetStringExtra(ParameterKeyName);
            if (string.IsNullOrEmpty(stringExtra))
                return null;

            lock (_parametersByKey)
                return _parametersByKey.ContainsKey(stringExtra) ? _parametersByKey[stringExtra] : null;
        }

        public T GetAndRemoveParameter<T>(Intent intent)
        {
            return (T)GetAndRemoveParameter(intent);
        }
    }
}

We then do the same for the dialog service:

using System;  
using System.Threading.Tasks;  
using Android.App;  
using Android.Content;  
using GalaSoft.MvvmLight.Views;

namespace JimBobBennett.MvvmLight.AppCompat  
{
    public class AppCompatDialogService : IDialogService
    {
        public Task ShowError(string message, string title, string buttonText, Action afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>) (r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback();
                afterHideCallback = null;
            });

            var dialog = CreateDialog(message, title, buttonText, null, afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowError(Exception error, string title, string buttonText, Action afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>)(r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback();
                afterHideCallback = null;
            });

            var dialog = CreateDialog(error.Message, title, buttonText, null, afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowMessage(string message, string title)
        {
            var dialog = CreateDialog(message, title);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowMessage(string message, string title, string buttonText, Action afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>)(r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback();
                afterHideCallback = null;
            });

            var dialog = CreateDialog(message, title, buttonText, null, afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task<bool> ShowMessage(string message, string title, string buttonConfirmText, string buttonCancelText, Action<bool> afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>)(r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback(r);
                afterHideCallback = null;
            });

            var dialog = CreateDialog(message, title, buttonConfirmText, buttonCancelText ?? "Cancel", afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowMessageBox(string message, string title)
        {
            return ShowMessage(message, title);
        }

        private static AlertDialogInfo CreateDialog(string content, string title, string okText = null, string cancelText = null, Action<bool> afterHideCallbackWithResponse = null)
        {
            var tcs = new TaskCompletionSource<bool>();
            var builder = new AlertDialog.Builder(AppCompatActivityBase.CurrentActivity);
            builder.SetMessage(content);
            builder.SetTitle(title);
            var dialog = (AlertDialog)null;
            builder.SetPositiveButton(okText ?? "OK", (d, index) =>
            {
                tcs.TrySetResult(true);
                if (dialog != null)
                {
                    dialog.Dismiss();
                    dialog.Dispose();
                }
                if (afterHideCallbackWithResponse == null)
                    return;
                afterHideCallbackWithResponse(true);
            });

            if (cancelText != null)
            {
                builder.SetNegativeButton(cancelText, (d, index) =>
                {
                    tcs.TrySetResult(false);
                    if (dialog != null)
                    {
                        dialog.Dismiss();
                        dialog.Dispose();
                    }
                    if (afterHideCallbackWithResponse == null)
                        return;
                    afterHideCallbackWithResponse(false);
                });
            }

            builder.SetOnDismissListener(new OnDismissListener(() =>
            {
                tcs.TrySetResult(false);
                if (afterHideCallbackWithResponse == null)
                    return;
                afterHideCallbackWithResponse(false);
            }));

            dialog = builder.Create();

            return new AlertDialogInfo
            {
                Dialog = dialog,
                Tcs = tcs
            };
        }

        private struct AlertDialogInfo
        {
            public AlertDialog Dialog;
            public TaskCompletionSource<bool> Tcs;
        }

        private sealed class OnDismissListener : Java.Lang.Object, IDialogInterfaceOnDismissListener
        {
            private readonly Action _action;

            public OnDismissListener(Action action)
            {
                _action = action;
            }

            public void OnDismiss(IDialogInterface dialog)
            {
                _action();
            }
        }
    }
}

To use these all we need to do is derive all our activities from AppCompatActivityBase and register the AppCompatNavigationService and AppCompatDialogService in our IoC container as implementations of INavigationService and IDialogService.

The code for this is up on GitHub here: https://github.com/jimbobbennett/JimBobBennett.MvvmLight.AppCompat

I've also created a NuGet package available here: https://www.nuget.org/packages/JimBobBennett.MvvmLight.AppCompat/ or searchable from the Package Manager as JimBobBennett.MvvmLight.AppCompat.


Update

Thanks to Samuel Debruyn for adding an update to this package.

MVVMLight has a helper class to aid in running code on the UI thread called DispatcherHelper. This will verify the current activity and use that to run the specified action on the UI thread using the RunOnUIThread method.

Unfortunately for us this relies on the CurrentActivity property on ActivityBase, something we are not using. Sam has added an AppCompatDispatcherHelper which mimics the logic of the DispatcherHelper but using the AppCompatActivityBase.CurrentActivity instead.

The code on GitHub and NuGet package have both been updated with these changes.




About the Author

Jim Bennett

International C# and Xamarin geek - Microsoft MVP, Xamarin MVP and Certified Developer, blogger, speaker, father and lover of beer, whisky and Thai food

 

As much as I'm loving MVVMLight it does have some limitations. The one I've hit recently is how well it doesn't work when you are using AppCompat (though to be honest I think it's more down to how hacky AppCompat seems to be).

I'm using AppCompatActivity as my base activity class to get an action bar supported on older API versions. The problem is MVVMLight implements it's own ActivityBase which is derived from Activity, and it relies on a set of static methods and properties on this to handle things like navigation and dialogs. If you want to use AppCompatActivity you are stuck - navigation will not work.

Luckily MVVMLight is really well designed with good separation of concerns, so everything is done via interface. This means we can write our own implementation of the navigation service and dialog service that will work with AppCompatActivity and use it in our code. Being open source as well we can take the existing source and tweak it slightly to make it work for us. It's a bit of a hack but it works.

The first thing to do is to define our own base activity class that has the same functionality as the ActivityBase, but derived from AppCompatActivity.

using Android.Support.V7.App;

namespace JimBobBennett.MvvmLight.AppCompat  
{
    public abstract class AppCompatActivityBase : AppCompatActivity
    {
        public static AppCompatActivityBase CurrentActivity { get; private set; }

        internal string ActivityKey { get; private set; }

        internal static string NextPageKey { get; set; }

        public static void GoBack()
        {
            CurrentActivity?.OnBackPressed();
        }

        protected override void OnResume()
        {
            CurrentActivity = this;
            if (string.IsNullOrEmpty(ActivityKey))
            {
                ActivityKey = NextPageKey;
                NextPageKey = null;
            }
            base.OnResume();
        }
    }
}

This is identical to the MVVMLight ActivityBase except the base class is AppCompatActivity.

Now we have our base activity we can use it for the navigation service. This is identical to the MVVMLight Android NavigationService except instead of accessing the statics on AppCompatActivityBase instead of on ActivityBase.

using System;  
using System.Collections.Generic;  
using Android.Content;  
using GalaSoft.MvvmLight.Views;

namespace JimBobBennett.MvvmLight.AppCompat  
{
    public class AppCompatNavigationService : INavigationService
    {
        private readonly Dictionary<string, Type> _pagesByKey = new Dictionary<string, Type>();
        private readonly Dictionary<string, object> _parametersByKey = new Dictionary<string, object>();

        private const string RootPageKey = "-- ROOT --";
        private const string ParameterKeyName = "ParameterKey";

        public string CurrentPageKey => AppCompatActivityBase.CurrentActivity.ActivityKey ?? RootPageKey;

        public void GoBack()
        {
            AppCompatActivityBase.GoBack();
        }

        public void NavigateTo(string pageKey)
        {
            NavigateTo(pageKey, null);
        }

        public void NavigateTo(string pageKey, object parameter)
        {
            AppCompatActivityBase.CurrentActivity.RunOnUiThread(() =>
            {
                if (AppCompatActivityBase.CurrentActivity == null)
                    throw new InvalidOperationException("No CurrentActivity found");

                lock (_pagesByKey)
                {
                    if (!_pagesByKey.ContainsKey(pageKey))
                        throw new ArgumentException($"No such page: {pageKey}. Did you forget to call NavigationService.Configure?", nameof(pageKey));

                    var intent = new Intent(AppCompatActivityBase.CurrentActivity, _pagesByKey[pageKey]);
                    if (parameter != null)
                    {
                        lock (_parametersByKey)
                        {
                            var guid = Guid.NewGuid().ToString();
                            _parametersByKey.Add(guid, parameter);
                            intent.PutExtra(ParameterKeyName, guid);
                        }
                    }

                    AppCompatActivityBase.CurrentActivity.StartActivity(intent);
                    AppCompatActivityBase.NextPageKey = pageKey;
                }
            });
        }

        public void Configure(string key, Type activityType)
        {
            lock (_pagesByKey)
            {
                if (_pagesByKey.ContainsKey(key))
                    _pagesByKey[key] = activityType;
                else
                    _pagesByKey.Add(key, activityType);
            }
        }

        public object GetAndRemoveParameter(Intent intent)
        {
            if (intent == null)
                throw new ArgumentNullException(nameof(intent), "This method must be called with a valid Activity intent");

            var stringExtra = intent.GetStringExtra(ParameterKeyName);
            if (string.IsNullOrEmpty(stringExtra))
                return null;

            lock (_parametersByKey)
                return _parametersByKey.ContainsKey(stringExtra) ? _parametersByKey[stringExtra] : null;
        }

        public T GetAndRemoveParameter<T>(Intent intent)
        {
            return (T)GetAndRemoveParameter(intent);
        }
    }
}

We then do the same for the dialog service:

using System;  
using System.Threading.Tasks;  
using Android.App;  
using Android.Content;  
using GalaSoft.MvvmLight.Views;

namespace JimBobBennett.MvvmLight.AppCompat  
{
    public class AppCompatDialogService : IDialogService
    {
        public Task ShowError(string message, string title, string buttonText, Action afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>) (r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback();
                afterHideCallback = null;
            });

            var dialog = CreateDialog(message, title, buttonText, null, afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowError(Exception error, string title, string buttonText, Action afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>)(r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback();
                afterHideCallback = null;
            });

            var dialog = CreateDialog(error.Message, title, buttonText, null, afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowMessage(string message, string title)
        {
            var dialog = CreateDialog(message, title);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowMessage(string message, string title, string buttonText, Action afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>)(r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback();
                afterHideCallback = null;
            });

            var dialog = CreateDialog(message, title, buttonText, null, afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task<bool> ShowMessage(string message, string title, string buttonConfirmText, string buttonCancelText, Action<bool> afterHideCallback)
        {
            var afterHideCallbackWithResponse = (Action<bool>)(r =>
            {
                if (afterHideCallback == null)
                    return;
                afterHideCallback(r);
                afterHideCallback = null;
            });

            var dialog = CreateDialog(message, title, buttonConfirmText, buttonCancelText ?? "Cancel", afterHideCallbackWithResponse);
            dialog.Dialog.Show();
            return dialog.Tcs.Task;
        }

        public Task ShowMessageBox(string message, string title)
        {
            return ShowMessage(message, title);
        }

        private static AlertDialogInfo CreateDialog(string content, string title, string okText = null, string cancelText = null, Action<bool> afterHideCallbackWithResponse = null)
        {
            var tcs = new TaskCompletionSource<bool>();
            var builder = new AlertDialog.Builder(AppCompatActivityBase.CurrentActivity);
            builder.SetMessage(content);
            builder.SetTitle(title);
            var dialog = (AlertDialog)null;
            builder.SetPositiveButton(okText ?? "OK", (d, index) =>
            {
                tcs.TrySetResult(true);
                if (dialog != null)
                {
                    dialog.Dismiss();
                    dialog.Dispose();
                }
                if (afterHideCallbackWithResponse == null)
                    return;
                afterHideCallbackWithResponse(true);
            });

            if (cancelText != null)
            {
                builder.SetNegativeButton(cancelText, (d, index) =>
                {
                    tcs.TrySetResult(false);
                    if (dialog != null)
                    {
                        dialog.Dismiss();
                        dialog.Dispose();
                    }
                    if (afterHideCallbackWithResponse == null)
                        return;
                    afterHideCallbackWithResponse(false);
                });
            }

            builder.SetOnDismissListener(new OnDismissListener(() =>
            {
                tcs.TrySetResult(false);
                if (afterHideCallbackWithResponse == null)
                    return;
                afterHideCallbackWithResponse(false);
            }));

            dialog = builder.Create();

            return new AlertDialogInfo
            {
                Dialog = dialog,
                Tcs = tcs
            };
        }

        private struct AlertDialogInfo
        {
            public AlertDialog Dialog;
            public TaskCompletionSource<bool> Tcs;
        }

        private sealed class OnDismissListener : Java.Lang.Object, IDialogInterfaceOnDismissListener
        {
            private readonly Action _action;

            public OnDismissListener(Action action)
            {
                _action = action;
            }

            public void OnDismiss(IDialogInterface dialog)
            {
                _action();
            }
        }
    }
}

To use these all we need to do is derive all our activities from AppCompatActivityBase and register the AppCompatNavigationService and AppCompatDialogService in our IoC container as implementations of INavigationService and IDialogService.

The code for this is up on GitHub here: https://github.com/jimbobbennett/JimBobBennett.MvvmLight.AppCompat

I've also created a NuGet package available here: https://www.nuget.org/packages/JimBobBennett.MvvmLight.AppCompat/ or searchable from the Package Manager as JimBobBennett.MvvmLight.AppCompat.


Update

Thanks to Samuel Debruyn for adding an update to this package.

MVVMLight has a helper class to aid in running code on the UI thread called DispatcherHelper. This will verify the current activity and use that to run the specified action on the UI thread using the RunOnUIThread method.

Unfortunately for us this relies on the CurrentActivity property on ActivityBase, something we are not using. Sam has added an AppCompatDispatcherHelper which mimics the logic of the DispatcherHelper but using the AppCompatActivityBase.CurrentActivity instead.

The code on GitHub and NuGet package have both been updated with these changes.