How can I use MvxNotifyTask to handle exceptions in MvvmCross

MvvmCross provides `MvxNotifyTask`, a powerful tool for managing asynchronous operations and handling exceptions within your application. This class acts as a sandbox for async operations, ensuring that if a task fails and raises an exception, your app won't crash. Instead, the exception is available for handling through `MvxNotifyTask.Exception`.

Key Features of MvxNotifyTask

- Exception Handling: `MvxNotifyTask` allows you to catch exceptions using the `onException` parameter. This parameter accepts a callback function that is executed if an exception occurs during the task execution.
- Task State Notifications: It provides property-changed notifications for different task states (e.g., `IsNotCompleted`, `IsCompleted`, `IsSuccessfullyCompleted`, `IsFaulted`), which can be bound to UI elements to display the task's status.
- Sandboxing: By encapsulating async operations within `MvxNotifyTask`, you ensure that exceptions are caught and handled gracefully without crashing the application.

Implementing MvxNotifyTask for Exception Handling

Here's a step-by-step guide on how to use `MvxNotifyTask` to handle exceptions in your MvvmCross application:

1. Define a Command with MvxNotifyTask:
Create a command in your ViewModel that uses `MvxNotifyTask` to execute an asynchronous method. Include an `onException` callback to handle any exceptions that occur.

csharp
   public class MyViewModel : MvxViewModel
   {
       private readonly ISomeService _someService;

       public MyViewModel(ISomeService someService)
       {
           _someService = someService;
           MyCommand = new MvxCommand(() => MyTaskNotifier = MvxNotifyTask.Create(
               async () => await MyMethodAsync(),
               onException: ex => OnException(ex)));
       }

       private MvxNotifyTask _myTaskNotifier;
       public MvxNotifyTask MyTaskNotifier
       {
           get => _myTaskNotifier;
           private set => SetProperty(ref _myTaskNotifier, value);
       }

       private async Task MyMethodAsync()
       {
           await _someService.DoSomethingAsync();
       }

       private void OnException(Exception exception)
       {
           // Handle the exception here, e.g., log it or display an error message.
           Console.WriteLine($"Exception occurred: {exception.Message}");
       }
   }
   

2. Bind Task Properties to UI Elements:
Use the properties exposed by `MvxNotifyTask` (like `IsNotCompleted`, `IsCompleted`, etc.) to bind to UI elements. This allows you to dynamically update your UI based on the task's state.

For example, on Android, you might bind the visibility of a progress indicator to `IsNotCompleted`:

xml
   
       
   
   

On iOS, you can bind the visibility of a control similarly:

csharp
   var set = this.CreateBindingSet();
   set.Bind(_myControl).For("Visibility").To(vm => vm.MyTaskNotifier.IsCompleted);
   set.Apply();
   

3. Customizing Exception Handling:
If you want to further customize how exceptions are handled across your application, you can create a custom wrapper around `MvxNotifyTask`. This allows you to centralize exception handling logic.

csharp
   public static class CustomNotifyTask
   {
       public static MvxNotifyTask Create(Func task)
       {
           return MvxNotifyTask.Create(
               async () =>
               {
                   try
                   {
                       await task.Invoke();
                   }
                   catch (Exception ex)
                   {
                       HandleException(ex);
                       throw ex; // Re-throw to propagate the exception if needed.
                   }
               });
       }

       private static void HandleException(Exception ex)
       {
           // Centralized exception handling logic here.
           Console.WriteLine($"Exception handled: {ex.Message}");
       }
   }
   

By following these steps, you can effectively use `MvxNotifyTask` to handle exceptions in your MvvmCross application, ensuring robust and reliable asynchronous operations.