在MVVM Windows Phone 8.1中传播PropertyChanged

Question

我有一堂课：

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));
        }
    }
}

还有一个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");
    }
}

基本上，我想每次更改我的Collection时都为“ CollectionCount”更改一个属性。

我本以为

App.Collection.PropertyChanged += Collection_PropertyChanged;

可以解决问题，但是方法没有被调用（Collection类中的RaisePropertyChanged是！）...

我是MVVM的新手，所以我在这里可能会错过一些显而易见的事情，但我没有看到。

Answer 1

您没有收到PropertyChanged事件，因为该事件为时过早。 在构造Collection时调用它。 您不想知道整个集合实例何时更改，而是想知道集合中的项目何时更改。 在这种情况下，请使用此事件

Collection.Tracks.CollectionChanged + = OnTracksCollectionChanged;