Question

我有两个类，一个用于 ViewModel ，另一个用于产品。 Product类具有名为行总计的属性，ViewModel类具有名为总金额的属性。 Product类绑定到DataGrid和用户随后插入数量并自动更新行总计。

以下是 ViewModel 类：

public class ViewModel : INotifyPropertyChanged
{

    public ObservableCollection<Product> products { get; set; }// the children

    private decimal _TotalAmount; 
    public decimal TotalAmount // <=== has to hold sum of [products.LineTotal]
    {
        get
        {
            return totalAmount;
        }
        set
        {
            if (value != _TotalAmount)
            {
                _TotalAmount = value;
                onPropertyChanged(this, "TotalAmount");
            }
        }
    }

以下是产品类，它是一个孩子：

public class Product : INotifyPropertyChanged
    {
        private decimal _LineTotal;
        public decimal LineTotal
        {
            get
            {
                return _LineTotal;
            }
            set
            {
                if (value != _LineTotal)
                {
                    _LineTotal = value;
                    onPropertyChanged(this, "LineTotal");
                }

            }

        }
}

我的问题是： TotalAmount 如何计算所有产品的总和 [行总数] ？儿童产品如何通知家长 ViewModel 更新 TotalAmount ？

类似的东西：

foreach(var product in Products)
{
     TotalAmount += product.LineTotal;
}

Answer 1

实现此目的的一种方法是，每次用户编辑行总计时以及每次从ObservableCollection添加或删除产品时重新计算总金额。

由于Product实现INotifyPropertyChanged并在设置新行总计时引发PropertyChanged事件，ViewModel可以处理该事件并重新计算总金额。

ObservableCollection在添加或删除项目时会引发CollectionChanged事件，因此ViewModel也可以处理该事件并重新计算。（如果产品只能更改，而不是由用户添加/删除等，则此部分不是必需的。）

你可以试试这个小程序，看看它是如何完成的：

<强>代码隐藏

public partial class MainWindow : Window
{
    ViewModel vm = new ViewModel();

    public MainWindow()
    {
        InitializeComponent();

        vm.Products = new ObservableCollection<Product>
        {
            new Product { Name = "Product1", LineTotal = 10 },
            new Product { Name = "Product2", LineTotal = 20 },
            new Product { Name = "Product3", LineTotal = 15 }
        };

        this.DataContext = vm;
    }

    private void AddItem(object sender, RoutedEventArgs e)
    {
        vm.Products.Add(new Product { Name = "Added product", LineTotal = 50 });
    }

    private void RemoveItem(object sender, RoutedEventArgs e)
    {
        vm.Products.RemoveAt(0);
    }
}

public class ViewModel : INotifyPropertyChanged
{
    private ObservableCollection<Product> _products;
    public ObservableCollection<Product> Products
    {
        get { return _products; }
        set
        {
            _products = value;

            // We need to know when the ObservableCollection has changed.
            // On added products: hook up eventhandlers to their PropertyChanged events.
            // On removed products: recalculate the total.
            _products.CollectionChanged += (sender, e) =>
            {
                if (e.NewItems != null)
                    AttachProductChangedEventHandler(e.NewItems.Cast<Product>());
                else if (e.OldItems != null)
                    CalculateTotalAmount();
            };

            AttachProductChangedEventHandler(_products);
        }
    }

    private void AttachProductChangedEventHandler(IEnumerable<Product> products)
    {
        // Attach eventhandler for each products PropertyChanged event.
        // When the LineTotal property has changed, recalculate the total.
        foreach (var p in products)
        {
            p.PropertyChanged += (sender, e) =>
            {
                if (e.PropertyName == "LineTotal")
                    CalculateTotalAmount();
            };
        }

        CalculateTotalAmount();
    }

    public void CalculateTotalAmount()
    {
        // Set TotalAmount property to the sum of all line totals.
        TotalAmount = Products.Sum(p => p.LineTotal);
    }

    private decimal _TotalAmount;
    public decimal TotalAmount
    {
        get { return _TotalAmount; }
        set
        {
            if (value != _TotalAmount)
            {
                _TotalAmount = value;

                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("TotalAmount"));
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

public class Product : INotifyPropertyChanged
{
    public string Name { get; set; }

    private decimal _LineTotal;
    public decimal LineTotal
    {
        get { return _LineTotal; }
        set
        {
            if (value != _LineTotal)
            {
                _LineTotal = value;

                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs("LineTotal"));
            }
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}

<强> XAML：

<Window x:Class="WpfApplication3.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <DataGrid ItemsSource="{Binding Products}" AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTextColumn Binding="{Binding Name}" />
                <DataGridTextColumn Binding="{Binding LineTotal}" />
            </DataGrid.Columns>
        </DataGrid>

        <Button Click="AddItem">Add item</Button>
        <Button Click="RemoveItem">Remove item</Button>

        <TextBlock>
            <Run>Total amount:</Run>
            <Run Text="{Binding TotalAmount}" />
        </TextBlock>
    </StackPanel>
</Window>

Answer 2

如果ParentViewModel关注ChildModel上的某个属性何时更新，则应该订阅其PropertyChanged事件。

但是，由于您拥有ChildModels的集合，因此应在PropertyChanged事件中添加/删除挂接CollectionChanged事件的处理程序。

// Hook up CollectionChanged event in Constructor
public MyViewModel()
{
    Products = new ObservableCollection<Product>();
    MyItemsSource.CollectionChanged += Products_CollectionChanged;
}

// Add/Remove PropertyChanged event to Product item when the collection changes
void Products_CollectionChanged(object sender, CollectionChangedEventArgs e)
{
    if (e.NewItems != null)
        foreach(Product item in e.NewItems)
            item.PropertyChanged += Product_PropertyChanged;

    if (e.OldItems != null)
        foreach(Product item in e.OldItems)
            item.PropertyChanged -= Product_PropertyChanged;
}

// When LineTotal property of Product changes, re-calculate Total
void Product_PropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "LineTotal")
    {
        TotalAmount = products.Sum(p => p.LineTotal);

        // Or if calculation is in the get method of the TotalAmount property
        //onPropertyChanged(this, "TotalAmount");
    }
}

Answer 3

我认为只有在设置TotalAmount以便触发NotifyPropertyChanged事件时，才会更新UI中TotalAmount的值。为此，您必须监听所有产品的PropertyChangedEvent，当产品集合更改或产品的LineTotal发生更改时，您必须将TotalAmount设置为与_TotalAmount不同的某个值。

但是这段代码真的很难理解：不清楚为什么要存储一个值，每次在变量（_TotalAmount）中读取它（TotalAmount）时计算一个值。由于_TotalAmount未设置为零，因此不是正确的值。

Answer 4

我相信bernd_rausch的回答是朝着正确的方向发展的。基本问题是为什么要将TotalAmount存储在ViewModel中？唯一的原因可能是您有这么多产品会影响性能。但即使在这种情况下，您也必须小心保持价值一致。

最安全的方法是编写一个TotalAmount属性，即可动态计算TotalAmount。然后链接Changed事件。

public class ViewModel : INotifyPropertyChanged
{
  ViewModel()
  {
    Products = new ObservableCollection<Product>();
    Products.CollectionChanged += OnProductsChanged;
  }
  public ObservableCollection<Product> Products { get; private set; }// the children
  public decimal TotalAmount { get { return Products.Select(p => p.LineTotal).Sum(); } }

  private void OnProductChanged(object sender, PropertyChangedEventArgs eventArgs)
  {
     if("LineTotal" != eventArgs.PropertyName)
           return;
     onPropertyChanged(this, "TotalAmount");
  }

  private void OnProductsChanged(object sender, NotifyCollectionChangeEventArgs eventArgs)
  {
     // This ignores a collection Reset...
     // Process old items first, for move cases...
     if (eventArgs.OldItems != null)
       foreach(Product item in eventArgs.OldItems)
         item.PropertyChanged -= OnProductChanged;

     if (eventArgs.NewItems != null)
       foreach(Product item in eventArgs.NewItems)
        item.PropertyChanged += OnProductChanged;


     onPropertyChanged(this, "TotalAmount");
  }

}

我忽略了重置案例。但我认为这应该给你正确的方向。如果要缓存计算结果，我仍然会使用此方法，通过在其中一个更改处理程序中重置的内部惰性值进行缓存。

通知/绑定父属性以计算Children属性的总和

4 个答案: