Sharing data between different ViewModels

Another option is to store such "shared" variables in a SessionContext-class of some kind:

public interface ISessionContext: INotifyPropertyChanged 
{
    int EditorFontSize { get;set; }
}

Then, inject this into your viewmodels (you are using Dependency Injection, right?) and register to the PropertyChanged event:

public class MainWindowViewModel 
{
    public MainWindowViewModel(ISessionContext sessionContext)
    {
        sessionContext.PropertyChanged += OnSessionContextPropertyChanged;        
    }

    private void OnSessionContextPropertyChanged(object sender, PropertyChangedEventArgs e) 
    {
        if (e.PropertyName == "EditorFontSize")
        {
            this.EditorFontSize = sessionContext.EditorFontSize;
        }
    }       
}

There are many ways to communicate between view models and a lot of points what the point is the best. You can see how it is done:

• using MVVMLight

• in Prism

• by Caliburn

In my view, the best approach is using EventAggregator pattern of Prism framework. The Prism simplifies MVVM pattern. However, if you have not used Prism, you can use Rachel Lim's tutorial - simplified version of EventAggregator pattern by Rachel Lim.. I highly recommend you Rachel Lim's approach.

If you use Rachel Lim's tutorial, then you should create a common class:

public static class EventSystem
{...Here Publish and Subscribe methods to event...}

And publish an event into your OptionViewModel:

eventAggregator.GetEvent<ChangeStockEvent>().Publish(
new TickerSymbolSelectedMessage{ StockSymbol = “STOCK0” });

then you subscribe in constructor of another your MainViewModel to an event:

eventAggregator.GetEvent<ChangeStockEvent>().Subscribe(ShowNews);

public void ShowNews(TickerSymbolSelectedMessage msg)
{
   // Handle Event
}

The Rachel Lim's simplified approach is the best approach that I've ever seen. However, if you want to create a big application, then you should read this article by Magnus Montin and at CSharpcorner with an example.

Update: For versions of Prism later than 5 CompositePresentationEvent is depreciated and completely removed in version 6, so you will need to change it to PubSubEvent everything else can stay the same.