Xamarin What happens to Lazy initialized singleton after App goes to background

Issue

I use Lazy initialization for app singleton in Xamarin.Forms (app runs on iOS):

public sealed class DataSingleton
{
    private static readonly Lazy<DataSingleton> lazy = new Lazy<DataSingleton>(() => new DataSingleton(), LazyThreadSafetyMode.PublicationOnly); // tried withou 2nd parameter as well

    public static DataSingleton Instance
    {
        get { return lazy.Value; }
    }
    ...
}

And i call it in webserver which runs to provide data for Front-end which is in Angular (used web view to show Angular code)

var server = new WebServer(o => o
                .WithUrlPrefix($"http://localhost:{appSettings.ApiPort}")
                .WithMode(HttpListenerMode.EmbedIO))
            .WithCors()
            .WithLocalSessionManager()
            .WithWebApi("/api", m => m
                .WithController<TestController>()
                .WithController<SettingsController>()
                .WithController<ReportController>()
                .WithController<SystemController>())
            .WithModule(new ActionModule("/", HttpVerbs.Any,
                ctx => ctx.SendDataAsync(new { Message = "Error" })))
            .RunAsync();

In controllers is called DataSingleton to get/set data, but after app returns from background, DataSingleton.Instance is null.

What should I do to don't lose data of singleton, while app is in background for short time (approximately 5 minutes)

Update - I've figured out that this problem is only in Controllers, cause when app gets back to front I can see all the data in AppDelegate WillEnterForeground event..

Solution

Given that it is the webserver that is having the problem, stop it when app goes into background. Start it again when app returns (or lazy-start as needed).

Code might be something like this:

App.xaml.cs:

public static Webserver MyWebServer
{
    get
    {
        if (_server == null)
        {
            _server = new Webserver(...);
        }

        return _server;
    }
}
public static void StopWebServer()
{
    if (_server != null)
    {
        _server.Dispose();
        // So will be created again, on next reference to MyWebServer.
        _server = null;
    }
}
private static Webserver _server;

...

protected override void OnSleep()
{
    StopWebServer();
}

Usage elsewhere:

... App.MyWebServer ...

---

If you don't want to make static variable (though IMHO that is okay for App, because there is only one, and its lifetime is that of the app itself), then remove the "static"s above, usage elsewhere becomes:

... (Xamarin.Forms.Application.Current as App).MyWebServer ...

Answered By - ToolmakerSteve