Saving and Restoring app state

Application state is very simple to save by serializing the model into app.Properties. For example, you can store as JSON as follows using Json.NET:

type Application() =
    let modelId = "model"
    override __.OnSleep() =

        let json = Newtonsoft.Json.JsonConvert.SerializeObject(runner.CurrentModel)
        Debug.WriteLine("OnSleep: saving model into app.Properties, json = {0}", json)

        app.Properties.[modelId] <- json

    override __.OnResume() =
        Debug.WriteLine "OnResume: checking for model in app.Properties"
            match app.Properties.TryGetValue modelId with
            | true, (:? string as json) ->

                Debug.WriteLine("OnResume: restoring model from app.Properties, json = {0}", json)
                let model = Newtonsoft.Json.JsonConvert.DeserializeObject<App.Model>(json)

                Debug.WriteLine("OnResume: restoring model from app.Properties, model = {0}", (sprintf "%0A" model))
                runner.SetCurrentModel (model, Cmd.none)

            | _ -> ()
        with ex ->
            App.program.onError("Error while restoring model found in app.Properties", ex)

    override this.OnStart() =
        Debug.WriteLine "OnStart: using same logic as OnResume()"

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