Question

Im working on a MVVM Windows phone app that displays weather info.

When the app loads up it opens MainPage.xaml. It makes a call the the service to get weather info and binds that data to the UI. Both Fahrenheit and Celcius info are returned but only one is displayed.

On the setting page, the user can select to view the temp in either Fahrenheit or Celcius.
The user can change this setting at any time and its stored in IsolatedStorageSettings.

The issue Im having is this:
when the user navigates to the Settings page and changes their preference for either Fahrenheit or Celcius, this change is not reflected on the main page.

This issue started me thinking about this in a broader context. I can see this being an issue in ANY MVVM app where the display depends on some setting in IsolatedStorage. Any time any setting in the IsoStore is updated, how does the ViewModels know this? When I navigate back in the NavigationStack from the settings page back to MainPage how can I force a rebind of the page?

The data in my model hasnt changed, only the data that I want to display has changed.

Am I missing something simple here?

Thanks in advance.
Alex

Answer 1

ok, so Ive come up with a solution. Before I get to it, let me provide some background. The app that Im working on uses both MVVM Light and WP7Contrib. That being the case, I am using Funq for DI and the MVVMLight Toolkit. After I posted my initial question, I gave the question a bit more thought. I remembered a video that I watched a while back from MIX2011 called Deep Dive MVVM with Laurent Bugnion

http://channel9.msdn.com/Events/MIX/MIX11/OPN03

In it, he talks about just this problem (view models not living at the same time) on Windows Phone. The part in question starts around the 19 minute mark.

Anyway, after I remembered that and realized that the ViewModel locator is exposed in App.xaml, this became a trivial problem to solve. When the user changes the Fahrenheit/Celcius option on the setting page, I simply get a reference to the MainViewModel via the ViewModelLocator and reset the collection that is bound to the UI thus causing the bindings to update.

public bool AddOrUpdateValue(string Key, Object value)
    {
        bool valueChanged = false;

        // If the key exists
        if (settings.Contains(Key))
        {
            // If the value has changed
            if (settings[Key] != value)
            {
                // Store the new value
                settings[Key] = value;
                valueChanged = true;
            }
        }
            // Otherwise create the key.
        else
        {
            settings.Add(Key, value);
            valueChanged = true;
        }
        return valueChanged;
    }


    public bool ImperialSetting
    {
        get
        {
            return GetValueOrDefault<bool>(ImperialSettingKeyName, ImperialSettingDefault);
        }
        set
        {
            if (AddOrUpdateValue(ImperialSettingKeyName, value))
            {
                Save();
                RaisePropertyChanged("ImperialSettingText");
                var vml = new ViewModelLocator();
                vml.MainViewModel.Cities = (App.Current as App).Cities;
            }
        }
    }

It was a mistake on my part not to realize that I could get access to the viewModel via the ViewModelLocator. Hopefully this post saves someone else the time I burned on this issue.