One pattern for error handling in MvvmCross

One pattern for how to handle errors/exceptions during async calls in MvvmCross is available in the BestSellers example: MvvmCross BestSellers Sample

BestSellers uses 2 techniques that I find I've used quite a lot in MvvmCross applications:

• the use of BaseViewModel classes for shared ViewModel code like error handling
• the use of an app level "error router" to get errors from the ViewModels to UI notifications like UIAlertViews, Toasts and/or MessageBoxes.

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At a more detailed level, what BestSellers does is:

Each ViewModel uses a direct call to a webservice for book information. For example the Category List is constructed as:

    public CategoryListViewModel()
    {
        AsyncLoad();
    }

    private void AsyncLoad()
    {
        GeneralAsyncLoad(URL_CATEGORIES, ProcessResult);
    }

where GeneralAsyncLoad is defined in a shared BaseViewModel:

    protected void GeneralAsyncLoad(string url, Action responseStreamHandler)
    {
        try
        {
            IsLoading = true;
            var request = WebRequest.Create(url);
            request.BeginGetResponse((result) => GeneralProcessResponse(request, result, responseStreamHandler), null);
        }
        catch (ThreadAbortException)
        {
            throw;
        }
        // obviously we could do better than catching all `Exception` here!
        catch (Exception exception)
        {
            IsLoading = false;
            ReportError("Sorry - problem seen " + exception.Message);
        }
    }

The ReportError method within the above exception handler uses an injected object - anIErrorReporter.

This injected object is initialised as a singleton during App construction - seeErrorApplicationObject in App.cs

During their construction and setup, the UI projects all subscribe to events from that same singleton - but using an IErrorSource interface instead or IErrorReporter.

This then allows each platform to display it's own error display - e.g.:

• iOS shows an UIAlertView in its ErrorDisplayer.cs
• Android shows a Toast in its ErrorDisplayer.cs
• WP7 shows a MessageBox from code within App.xaml.cs

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Obviously, if you need error handling as well as error displaying - e.g. if you want to retry the asynchronous operation or if you want to load an offline copy of data instead - then you can add this to your error handling within the ViewModel and the BaseViewModel.