在MVVM Windows Phone 8.1中传播PropertyChanged

Question

I have a class:

public class Collection : INotifyPropertyChanged
{
    public Collection()
    {
        this.tracks = new ObservableCollection<Track>();
    }

    public void addTrack(Track track)
    {
        if (this.tracks.Contains(track)) return;

        this.tracks.Add(track);
        RaisePropertyChanged("tracks");
    }

    public ObservableCollection<Track> tracks
    {
        get
        {
            return _tracks;
        }
        set
        {
            if (_tracks != value)
            {
                _tracks = value;
                RaisePropertyChanged("tracks");
            }
        }
    }
    private ObservableCollection<Track> _tracks;

    public event PropertyChangedEventHandler PropertyChanged;
    private void RaisePropertyChanged(string propertyName)
    {
        if (this.PropertyChanged != null)
        {
            this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

And a ViewModel:

public class ScanningPageViewModel : ViewModelBase
{
    private INavigationService navigationService;

    public ScanningPageViewModel(INavigationService navigationService)
    {
        App.Collection.PropertyChanged += Collection_PropertyChanged;

        this.navigationService = navigationService;
        this.LoadData();
    }

    private async void LoadData()
    {
        await CollectionService.PopulateCollection();
        this.navigationService.NavigateTo("MainPage");
    }

    public int CollectionCount
    {
        get
        {
            return App.Collection.tracks.Count;
        }
    }

    public void Collection_PropertyChanged(object sender, PropertyChangedEventArgs e)
    {
        RaisePropertyChanged("CollectionCount");
    }
}

Basically I want to raise a property changed for "CollectionCount" each time my Collection changes.

I would have thought that

App.Collection.PropertyChanged += Collection_PropertyChanged;

would do the trick but the method is not getting called (the RaisePropertyChanged in the Collection class is!)...

I'm new with MVVM so I might miss something obvious here but I'm not seeing it.

Answer 1

You are not getting the PropertyChanged event because it's called too early. It's called when the Collection is constructed. You dont want to know when the whole collection instance has changed but rather when the items in the collections have changed. In this case use this event

Collection.Tracks.CollectionChanged += OnTracksCollectionChanged;