Question

I’ve created a property “IsLoading” for my main view model. The idea is that a progressbar is displayed whenever this property is set to true. So far so good

The catch is, that I have a command, that calls another viewmodel (the code is there because it’s a functionality from another page, but I want to be able to shortcut it as well from my main viewmodel)

So, I went ahead and modified the main property to something like this :

public const string IsLoadingPropertyName = "IsLoading";

        private bool _isLoading;

        public bool IsLoading
        {
            get
            {
                return _isLoading || ((ViewModelLocator)Application.Current.Resources["Locator"]).SettingsViewModel.IsLoading;
            }
            set
            {
                if (value != _isLoading)
                {
                    _isLoading = value;
                    RaisePropertyChanged(IsLoadingPropertyName);
                }
            }
        }

and the xaml

 <shell:SystemTray.ProgressIndicator>
        <shell:ProgressIndicator IsIndeterminate="true" IsVisible="{Binding Main.IsLoading, Source={StaticResource Locator}}" />
    </shell:SystemTray.ProgressIndicator>

So, I’m saying that main view model is loading when there’s something loading there, or if the settings view model is loading.
The problem is that the binding only works when setting the main view model’s IsLoading property, it doesn’t react when I set it in the inner IsLoading one. Both have the same property name “IsLoading”. Shouldn’t it be detected?

For example, in Main view model (just the execution of the command for simplicity) :

    private void ExecuteRefreshCommand()
    {
        ViewModelLocator viewModelLocator = Application.Current.Resources["Locator"] as ViewModelLocator;
        viewModelLocator.SettingsViewModel.GetCurrentLocationCommand.Execute(null);
    }

and inside the settings view model :

public RelayCommand GetCurrentLocationCommand
        {
            get
            {
                Action getLocation = () =>
                {
                    if (!NetworkInterface.GetIsNetworkAvailable())
                    {
                        return;
                    }

                    var watcher = new GeoCoordinateWatcher(GeoPositionAccuracy.Default);
                    watcher.PositionChanged += WatcherPositionChanged;
                    IsLoading = true; // settings view model "IsLoading" propertychanged raising property
                    watcher.Start();
                };
                return new RelayCommand(getLocation);
            }
        }

Answer 1

You’re looking at the MainViewModel’s isLoading property to determine whether to show the progressbar or not. Silverlight uses the NotifyPropertyChanged event to determine when it should reevaluate a certain property. When setting either the SettingsViewModel’s IsLoading property- or the MainViewModel’s property, you only raise the changedEvent for that ViewModel. You should raise the ChangedEvent for both.

A modified setter example could be (depending on the exposed methods)

set 
{ 
    if (value != _isLoading) 
    { 
        _isLoading = value; 
        RaisePropertyChanged(IsLoadingPropertyName); 
        ((ViewModelLocator)Application.Current.Resources["Locator"]).SettingsViewModel.RaisePropertyChanged(IsLoadingPropertyName);
    } 
} 

Note that many MVVM frameworks offer a functionality called Messaging which is ideal to do cross ViewModel communication without creating the strict dependency you created right now. Alternatively you can use a globally consumed IsLoading property.