EventSubscription‘s Dispose method is never called in prism

[Value-Lee]

I think, EventSubscription‘s Dispose method,like the following picture, is never called in prism, and should remove it.

But I'm not sure my idea. Maybe as Third Unsubscribe approach like this （i feel the loss outweights the gain）

public class MainPageViewModel
{
    TickerSymbolSelectedEvent _event;
    SubscriptionToken _token;
    public MainPageViewModel(IEventAggregator ea)
    {
        _event = ea.GetEvent<TickerSymbolSelectedEvent>();
        _token = _event.Subscribe(ShowNews); 
    }

    void Unsubscribe()
    {
        _token.Dispose();  // ！！！！！！！！！！！！！！！
    }

    void ShowNews(string companySymbol)
    {
        //implement logic
    }
}

green hand & bad English, Forgive me.