在MVVM Windows Phone 8.1中傳播PropertyChanged

Question

我有一堂課：

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));
        }
    }
}

還有一個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時都為“ CollectionCount”更改一個屬性。

我本以為

App.Collection.PropertyChanged += Collection_PropertyChanged;

可以解決問題，但是方法沒有被調用（Collection類中的RaisePropertyChanged是！）...

我是MVVM的新手，所以我在這里可能會錯過一些顯而易見的事情，但我沒有看到。

Answer 1

您沒有收到PropertyChanged事件，因為該事件為時過早。 在構造Collection時調用它。 您不想知道整個集合實例何時更改，而是想知道集合中的項目何時更改。 在這種情況下，請使用此事件

Collection.Tracks.CollectionChanged + = OnTracksCollectionChanged;