Can you provide examples of how to save and restore ViewModel state in MvvmCross

Using MvvmCross's Built-in State Saving

MvvmCross provides a method called `SaveStateToBundle` in the `MvxViewModel` class, which is used to save the state of a ViewModel when the application is tombstoned. This method is called automatically by MvvmCross when necessary, and it allows you to store data in an `IMvxBundle`.

Example of Saving State

To save the state, you override the `SaveStateToBundle` method in your ViewModel:

csharp
public class MyViewModel : MvxViewModel
{
    private string _someData;

    protected override void SaveStateToBundle(IMvxBundle bundle)
    {
        base.SaveStateToBundle(bundle);
        bundle.Data.Add("SomeData", _someData);
    }
}

Example of Restoring State

To restore the state, you can use the `ReloadStateFromBundle` method, which is part of the `IMvxViewModel` interface. However, MvvmCross does not automatically call this method; you need to implement logic to restore the state when the ViewModel is reinitialized.

csharp
public class MyViewModel : MvxViewModel
{
    private string _someData;

    public void ReloadState(SavedState savedState)
    {
        if (savedState != null)
        {
            _someData = savedState.SomeData;
        }
    }

    // Assuming SavedState is a class that holds the saved data
    public class SavedState
    {
        public string SomeData { get; set; }
    }
}

Using JSON Serialization

For more complex data, you can use JSON serialization to store and restore the state. This involves converting your data into a JSON string and storing it in the bundle.

Example with JSON Serialization

csharp
using Newtonsoft.Json;

public class MyViewModel : MvxViewModel
{
    private string _someData;

    protected override void SaveStateToBundle(IMvxBundle bundle)
    {
        base.SaveStateToBundle(bundle);
        var savedState = new SavedState { SomeData = _someData };
        var json = JsonConvert.SerializeObject(savedState);
        bundle.Data.Add("SavedState", json);
    }

    public void ReloadState(string json)
    {
        if (!string.IsNullOrEmpty(json))
        {
            var savedState = JsonConvert.DeserializeObject(json);
            _someData = savedState.SomeData;
        }
    }

    public class SavedState
    {
        public string SomeData { get; set; }
    }
}

Additional Strategies for State Management

While MvvmCross provides these methods for saving and restoring state, you might also consider using platform-specific features for more robust state management, especially in scenarios like process death on Android.

For Android, you can use the `SavedStateHandle` API from the AndroidX library to save and restore state within ViewModels. This approach is particularly useful for handling system-initiated process deaths.

kotlin
// Example in Kotlin for Android
class MyViewModel(private val savedStateHandle: SavedStateHandle) : ViewModel() {
    private val USER_KEY = "userId"

    fun saveCurrentUser(userId: String) {
        savedStateHandle[USER_KEY] = userId
    }

    fun getCurrentUser(): String {
        return savedStateHandle.get(USER_KEY) ?: ""
    }
}

In summary, MvvmCross provides a basic framework for saving and restoring ViewModel state, but you may need to combine this with platform-specific solutions for more robust state management across different lifecycle events.