How to correctly saving the viewmodel of page to handle tombstoning

Question

I'm building a WP7 app, and I'm now at the point of handling the tombstoning part of it.

What I am doing is saving the viewmodel of the page in the Page.State bag when the NavigatedFrom event occurs, and reading it back in the NavigatedTo (with some check to detect whether I should read from the bag or read from the real live data of the application).

First my VM was just a wrapper to the domain model

public string Nome
    {
        get
        {
            return _dm.Nome;
        }
        set
        {
            if (value != _dm.Nome)
            {
                _dm.Nome= value;
                NotifyPropertyChanged("Nome");
            }
        }
    }

But this didn't always work because when saving to the bag and then reading back, the domain model was not deserialized correctly.

Then I changed my VM implementation to be just a copy of the properties I needed from the DM:

public string Nome
    {
        get
        {
            return _nome;
        }
        set
        {
            if (value !=nome)
            {
                _nome= value;
                NotifyPropertyChanged("Nome");
            }
        }
    }

and with the constructor that does:

_nome = dm.Nome;

And now it works, but I was not sure if this is the right approach.

Thx Simone

Answer 1

Any transient state information should be persisted in the Application.Deactivated event and then restored in the Application.Activated event for tombstoning support.

If you need to store anything between application sessions then you could use the Application.Closing event, but depending on what you need to store, you could just store it whenever it changes. Again, depending on what you need to store, you can either restore it in the Application.Launching event, or just read it when you need it.

The approach that you take depends entirely on your application's requirements and the method and location that you store your data is also up to you (binary serialization to isolated storage is generally accepted is being the fastest).

I don't know the details of your application, but saving and restoring data in NavigatedFrom/NavigatedTo is unlikely to be the right place to do it if you are looking to implement support for tombstoning.

Answer 2

I'd recommend against making a copy of part of the model as when tombstoning you'd (probably) need to persist both the full (app level) model and the page level copy when handling tombstoning.

Again the most appropriate solution will depend on the complexity of your application and the models it uses.

Answer 3

Application.Activated/Deactivated is a good place to handle tombstoning.

See why OnNavigatedTo/From may not be appropriate for your needs here.

How to correctly handle application deactivation and reactivation - Peter Torr's Blog

Execution Model Overview for Windows Phone