c# - 如何在 WPF 中绑定(bind) Observable 集合的总和

Question

我正在学习WPF所以如果它是新手请提出我的问题!我有一个项目集合，我想将该集合绑定(bind)到一个网格并将总和绑定(bind)到一个文本框。在线搜索我发现即使我们更改集合对象的属性，这个类也会引发事件。

ObservableCollection not noticing when Item in it changes (even with INotifyPropertyChanged)

但我似乎无法让它在我的代码中工作。

这是我的代码

促销品

public 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));
        }
    }
}

销售

public class Sale : INotifyPropertyChanged 
{
    private Decimal _total;
    public TrulyObservableCollection<SaleItem> Items { get; set; }

    public Sale()
    {
        Items = new TrulyObservableCollection<SaleItem>();
        Items.Add(new SaleItem { ItemID = "1", Name = "Product 1", Price = 10, Quantity = 1 });
        Items.Add(new SaleItem { ItemID = "2", Name = "Product 2", Price = 10, Quantity = 1 });
        Items.Add(new SaleItem { ItemID = "3", Name = "Product 3", Price = 10, Quantity = 1 });

    }

    public Decimal Total
    {
        get
        { 
            return Items.Sum(x => x.Total);
        }
        set
        {
            _total = value;
            OnPropertyChanged("Total");
        }
    }

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

主窗口

public partial class MainWindow : Window
{
    public Sale Model { get; set; }

    public MainWindow()
    {
        InitializeComponent();
        Model = new Sale();
        this.DataContext = Model;            
    }



    private void btnQuantity_Click(object sender, RoutedEventArgs e)
    {
        Model.Items.Add(new SaleItem { ItemID = "2", Name = "Product 2", Price = 10, Quantity = 1 });

    }
}

XAML

<Window x:Class="WpfApplication1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local ="clr-namespace:WpfApplication1"
    Title="MainWindow" Height="350" Width="525">

<Window.DataContext>
    <local:Sale />
</Window.DataContext>

<Grid>
    <StackPanel>
        <DataGrid x:Name="grdItems" ItemsSource="{Binding Items}"></DataGrid>
        <TextBlock x:Name="txtTotal" Text="{Binding Total}"/>
        <Button x:Name="btnQuantity" Content="Update" Click="btnQuantity_Click"/>
    </StackPanel>
</Grid>

我正在尝试测试在单击按钮时添加项目并更新网格中项目的数量。如果我设置一个断点并看到 Total 的值，那么它是正确的，但不知何故它没有在 UI 上更新。我在这里缺少什么？

Answer 1

你想做什么。是在集合本身发生更改时调用 OnPropertyChanged。重新计算总数。目前，Sale 类不知道集合已更新并且需要更新其 Total 属性。

执行此操作的一种方法是检查 NotifyCollectionChangedEvent，如下所示:

在 Sale() 的构造函数中:

public Sale()
{
 //Instantiate your collection and add items

  Items.CollectionChanged += CollectionChanged;
}

然后为事件添加处理程序:

public void CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
  OnPropertyChanged("Total");
}

如果除了更新 Total 属性之外，当集合发生变化时您永远不会做任何其他事情。您可以通过删除事件处理程序并改用 lambda 表达式来进一步缩短代码:

Items.CollectionChanged += (s,e) => OnPropertyChanged("Total");

除非您计划自己在 Sale 类中显式更新 Total。 setter 可以被删除:

public Decimal Total
{
  get
  {
    return Items.Sum(x => x.Total);
  }
}