ObservableCollection.CollectionChanged 未触发

我有以下界面：

当一个项目被添加到DataGrid, the Total column将根据（价格*数量）更新，总计TextBox还将添加所有添加的行的总数。

但是，当我更改行的数量时，Total column更新，但总计TextBox才不是。

这是我的代码，提前致谢。

 public partial class pgCheckout : Page {

    ObservableCollection<SaleItem> items = new ObservableCollection<SaleItem>();

    public pgCheckout() {
        InitializeComponent();
        dgItems.ItemsSource = items;
        dgItems.Loaded += SetMinWidths;
        items.CollectionChanged += setTotal;
    }

    public void setTotal(object source, EventArgs e) {
        decimal total = 0;
        foreach(SaleItem i in items) {
            total += i.Total;
        }

        txtTotal.Text = total.ToString();
    }


    public void SetMinWidths(object source, EventArgs e) {
        foreach (var column in dgItems.Columns) {
            if (column.DisplayIndex != 0) {
                column.MinWidth = column.ActualWidth;
                column.Width = new DataGridLength(1, DataGridLengthUnitType.Star);
            }
        }
    }

    private void btnRemove_Click(object sender, RoutedEventArgs e) {
        items.RemoveAt(dgItems.SelectedIndex);
    }

    private void btnAdd_Click(object sender, RoutedEventArgs e) {
        bool exist = false;
        foreach (SaleItem i in items) {
            if (i.ItemID.Equals(txtItemID.Text))
                exist = true;
        }

        if (exist) {
            lblErr.Content = "Item already exist";
            txtItemID.Text = "";
        }

        else {

            using (var db = new PoSEntities()) {
                var query = from i in db.Items
                            where i.ItemID.Equals(txtItemID.Text.Trim())
                            select i;
                var itm = query.FirstOrDefault();
                if (itm == null) {
                    lblErr.Content = "Invalid Item";
                    txtItemID.Text = "";
                }
                else {
                    txtItemID.Text = "";
                    lblErr.Content = "";
                    items.Add(new SaleItem() {
                        Num = items.Count + 1,
                        ItemID = itm.ItemID,
                        Name = itm.Name,
                        Price = decimal.Round(itm.Price, 2, MidpointRounding.AwayFromZero),
                        Quantity = 1,
                    });
                }
            }
        }
    }

    private void txtItemID_KeyUp(object sender, KeyEventArgs e) {
        if (e.Key == System.Windows.Input.Key.Enter) {
            btnAdd_Click(txtItemID, e);
        }
    }
}

class SaleItem : INotifyPropertyChanged {
    public int Num { get; set; }
    public string ItemID { get; set; }
    public string Name { get; set; }

    private decimal price;
    public decimal Price {
        get { return price; }
        set {
            this.price = value;
            OnPropertyChanged("Total");
        }
    }
    public int quantity;
    public int Quantity {
        get { return quantity; }
        set {
            this.quantity = value;
            OnPropertyChanged("Total");
        }
    }

    public decimal Total {
        get { return decimal.Round(Price * Quantity, 2, MidpointRounding.AwayFromZero); }
    }

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

ObservableCollection.CollectionChanged当从集合中添加或删除某些内容时触发，而不是更新单个成员。

...
else 
{
    txtItemID.Text = "";
    lblErr.Content = "";
    SaleItem newItem = new SaleItem() {
                    Num = items.Count + 1,
                    ItemID = itm.ItemID,
                    Name = itm.Name,
                    Price = decimal.Round(itm.Price, 2, MidpointRounding.AwayFromZero),
                    Quantity = 1 };
     newItem.PropertyChanged += 
             new PropertyChangedEventHandler(newSaleItem_PropertyChanged);

    items.Add(newItem);
 }
 ...

这就是newSaleItem_PropertyChanged:

void newSaleItem_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    setTotal(null, null);
}