How do I add handlers for exceptions in MvvmCross

1. Using BaseViewModel for Shared Error Handling**

One common pattern is to use a `BaseViewModel` class where you can define shared error handling logic. This includes methods like `GeneralAsyncLoad` which can handle exceptions during asynchronous operations. Here's an example:

csharp
public class BaseViewModel : MvxViewModel
{
    protected void GeneralAsyncLoad(string url, Action processResult)
    {
        try
        {
            // Perform asynchronous operation
            var result = await FetchDataAsync(url);
            processResult(result);
        }
        catch (Exception ex)
        {
            ReportError(ex);
        }
    }

    protected void ReportError(Exception ex)
    {
        // Implement error reporting logic here
        // This could involve logging or displaying an error message
    }
}

2. Implementing an App-Level Error Router**

To handle errors across the application, you can create an app-level error router. This involves using interfaces like `IErrorReporter` and `IErrorSource` to manage error reporting and display. Here’s how you might set it up:

1. Define Interfaces:

csharp
   public interface IErrorReporter
   {
       void ReportError(Exception ex);
   }

   public interface IErrorSource
   {
       event EventHandler ErrorOccurred;
   }

   public class ErrorEventArgs : EventArgs
   {
       public Exception Exception { get; set; }
   }
   

2. Implement the Error Reporter:

csharp
   public class ErrorApplicationObject : IErrorReporter, IErrorSource
   {
       public event EventHandler ErrorOccurred;

       public void ReportError(Exception ex)
       {
           // Log the error
           MvxLog.Instance.Error(ex.ToString());

           // Raise the event to notify UI components
           ErrorOccurred?.Invoke(this, new ErrorEventArgs { Exception = ex });
       }
   }
   

3. Register and Use in App.cs:

csharp
   public class App : MvxApplication
   {
       public override void Initialize()
       {
           base.Initialize();

           // Register the error reporter as a singleton
           Mvx.RegisterSingleton(new ErrorApplicationObject());
       }
   }
   

4. Subscribe to Error Events in UI Projects:
In each platform-specific project, subscribe to the `ErrorOccurred` event to display errors appropriately. For example, on Android, you might show a Toast message, while on iOS, you could display an alert.

3. Handling Async Exceptions with MvxNotifyTask**

MvvmCross provides `MvxNotifyTask` to handle asynchronous operations and exceptions. You can use it to execute tasks and handle exceptions in a centralized manner:

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

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

    public IMvxCommand MyCommand { get; private set; }

    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
        MvxLog.Instance.Error(exception.ToString());
    }
}

4. Logging Exceptions**

MvvmCross provides `MvxLog` for logging messages, including errors. You can extend this to log exceptions to various outputs:

csharp
MvxLog.Instance.Error(exception.ToString());

5. Handling Unhandled Exceptions**

For unhandled exceptions, you can create a platform-specific exception handler. For example, in Xamarin Android, you can override the `UncaughtExceptionHandler` in your `Application` class:

csharp
public class MyApplication : Application
{
    public override void OnCreate()
    {
        base.OnCreate();
        Thread.DefaultUncaughtExceptionHandler = new MyUncaughtExceptionHandler();
    }
}

public class MyUncaughtExceptionHandler : Java.Lang.Object, IUncaughtExceptionHandler
{
    public void UncaughtException(Thread t, Throwable e)
    {
        // Handle the unhandled exception here
        MvxLog.Instance.Error(e.ToString());
    }
}

By implementing these strategies, you can effectively handle exceptions in your MvvmCross applications.