How to register service with platform code and use it?

Question

I need to show material alert dialog. I have created implementation and want to use it in any place of my code.
I know that in MAUI we can register class: builder.Services.AddSingleton(typeof(IDialogService), typeof(DialogService));
How to use this instead of injecting in constructor? Because I prefer viewModels with empty constructors.
For example like in Xamarin Forms: DependencyService.Get<IDialogService>();
In blazor we can write something like @inject IDialogService DialogService

I remember in Xamarin Forms creating singleton class is popular solution:

public class MenuService : IMenuService
    {
        private static Lazy<IMenuService> _instance = new Lazy<IMenuService>(() => new MenuService(), System.Threading.LazyThreadSafetyMode.PublicationOnly);

        public static IMenuService Instance => _instance.Value;

        public Task<string> ShowMenu(View view, string[] items) =>
            DependencyService.Get<IMenuService>().ShowMenu(view, items);
    }

I want to create something similar to this approach for my maui service. How to do this properly?
My implementation:

public interface IDialogService
    {
        Task<string> ShowDialog(string title, string message, string accept, string cancel, string neutral);
        Task<bool> ShowDialog(string title, string message, string accept, string cancel);
    }

    public partial class DialogService : IDialogService
    {
        public partial Task<string> ShowDialog(string title, string message, string accept, string cancel, string neutral);
        public partial Task<bool> ShowDialog(string title, string message, string accept, string cancel);
    }

Android implementation:

public partial class DialogService : IDialogService
    {
        static Context context;
        internal static Context Context
        {
            get
            {
                var page = Application.Current.MainPage;
                var renderer = page.GetRenderer();

                if (renderer?.View.Context != null)
                    context = renderer.View.Context;

                return renderer?.View.Context ?? context ?? throw new NullReferenceException($"{nameof(Context)} cannot be null");
            }
        }

        private ISpanned FromHtml(string message)
        {
            if (Build.VERSION.SdkInt >= BuildVersionCodes.N)
                return Html.FromHtml(message, FromHtmlOptions.ModeLegacy);
            return Html.FromHtml(message);
        }

        public partial Task<string> ShowDialog(string title, string message, string accept, string cancel, string neutral)
        {
            TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>();
            MaterialAlertDialogBuilder alertBuilder = new MaterialAlertDialogBuilder(Context);
            alertBuilder
                .SetTitle(title)
                .SetMessage(FromHtml(message))
                .SetPositiveButton(accept, (sender, args) => taskCompletionSource.SetResult(accept));
            if (cancel != null)
                alertBuilder.SetNegativeButton(cancel, (sender, args) => taskCompletionSource.SetResult(cancel));
            if (neutral != null)
                alertBuilder.SetNeutralButton(neutral, (sender, args) => taskCompletionSource.SetResult(neutral));
            AndroidX.AppCompat.App.AlertDialog dialog = alertBuilder.Create();
            dialog.CancelEvent += (object sender, EventArgs e) => taskCompletionSource?.SetResult(null);
            dialog.Show();
            return taskCompletionSource.Task;
        }

        public partial Task<bool> ShowDialog(string title, string message, string accept, string cancel)
        {
            TaskCompletionSource<bool> taskCompletionSource = new TaskCompletionSource<bool>();
            MaterialAlertDialogBuilder alertBuilder = new MaterialAlertDialogBuilder(Context);
            alertBuilder
                .SetTitle(title)
                .SetMessage(FromHtml(message))
                .SetPositiveButton(accept, (sender, args) => taskCompletionSource.SetResult(true))
                .SetNegativeButton(cancel, (sender, args) => taskCompletionSource.SetResult(false));
            AndroidX.AppCompat.App.AlertDialog dialog = alertBuilder.Create();
            dialog.CancelEvent += (object sender, EventArgs e) => taskCompletionSource?.SetResult(false);
            dialog.Show();
            return taskCompletionSource.Task;
        }
    }

Answer 1

Hello,​

When you want to use this services, It can be invoked from cross-platform code by creating an object instance and invoking its operation like following code.

   internal class MyViewModel  
   {  
       public MyViewModel()  
       {  
        }  
     
       private void PopAlert()  
       {  
           IDialogService dialogService = new DialogService();  
           dialogService.ShowDialog("info", "this is a test dialog", "OK", "NO");  
     }  
   }  

Here is article about :Invoke the cross-platform API.

I test your code in my side, I find you need to set Material style, and your code get the empty context, so I change this initialize MaterialAlertDialogBuilder code.

   MaterialAlertDialogBuilder alertBuilder = new MaterialAlertDialogBuilder(MainActivity.InStance, Resource.Style.MaterialComponentsAlert);  

Add a style in styles.xml.

   <resources>  
   <style name="MaterialComponentsAlert" parent="Theme.MaterialComponents.Dialog.Alert">  
   </style>  
   </resources>  

Create Instance in the as context in the MainActivity.cs.

   ...  
   public class MainActivity : MauiAppCompatActivity  
   {  
   //add this property  
       public static MainActivity InStance { get; set; }  
       protected override void OnCreate(Bundle savedInstanceState){  
   //set it  
           InStance = this;  
   ....  
     
   }  

Best Regards,

Leon Lu

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