c# - 计算 Property Changed MVVM 上两个值的结果

Question

我正在尝试通过 MVVM 中的以下简单公式计算“NetAmount”

GrossAmount + Carriage - 折扣 = NetAmount

我正在使用 MVVM Light Toolkit 并声明属性如下

public const string DiscountPropertyName = "Discount";
private double _discount;
public double Discount
{
    get
    {
        return _discount;
    }

    set
    {
        if (_discount == value)
        {
            return;
        }
        _discount = value;
        // Update bindings, no broadcast
        RaisePropertyChanged(DiscountPropertyName);
    }
}

public const string CarriagePropertyName = "Carriage";
private double _carriage;

public double Carriage
{
    get
    {
        return _carriage;
    }
    set
    {
        if (_carriage == value)
        {
            return;
        }
        _carriage = value;
        RaisePropertyChanged(CarriagePropertyName);
    }
}

public const string NetAmountPropertyName = "NetAmount";
private double _netAmount;

public double NetAmount
{
    get
    {
        _netAmount = Carriage + Discount;
        return _netAmount;
    }
    set
    {
        if (_netAmount == value)
        {
            return;
        }
        _netAmount = value;
        RaisePropertyChanged(NetAmountPropertyName);
    }
}

public const string GrossAmountPropertyName = "GrossAmount";
private double _grossAmount;

public double GrossAmount
{
    get
    {
        return _grossAmount;
    }

    set
    {
        if (_grossAmount == value)
        {
            return;
        }
        _grossAmount = value;
        RaisePropertyChanged(GrossAmountPropertyName);
    }
}

我在 XAML 中将这些属性与如下文本框绑定：

<TextBox Text="{Binding GrossAmount, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"  DataContext="{Binding Mode=OneWay}"/>

<TextBox Text="{Binding Carriage, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"  DataContext="{Binding Mode=OneWay}"/>

<TextBox Text="{Binding Discount, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"  DataContext="{Binding Mode=OneWay}"/>

我将一个文本块与NetAmount属性绑定，如下所示：

<TextBlock Text="{Binding NetAmount}" />

视图模型是SalesOrderViewModel.

我不知道将上述公式放在哪里，以便在更改任何文本框的值时，它会导致更改NetAmount属性。

我对 C# 并不陌生，但对 MVVM 和PropertyChanged事件很陌生，我知道我做错了一些非常小的愚蠢的事情，但我无法理解它。

任何帮助将不胜感激。

Answer 1

由于NetAmount是计算，因此将其建模为视图模型中的只读属性是有意义的。访问该属性实际上执行了计算。RaisePropertyChanged(NetAmountProperty)最后一个技巧是在任何影响NetAmount变化的因素时调用

public const string GrossAmountPropertyName = "GrossAmount";
private double _grossAmount;
public double GrossAmount
{
    get { return _grossAmount; }
    set 
    {
           if (_grossAmount == value)
                return;

           RaisePropertyChanged(GrossAmountPropertyName);
           RaisePropertyChanged(NetAmountPropertyName);
    }
}

public double Discount{} ... //Implement same as above
public double Carriage {} ... //Implement same as above

public const string NetAmountPropertyName = "NetAmount";
public double NetAmount
{
    get { return GrossAmount + Carriage - Discount; }
}

---

编辑：如果您不想添加对RaisePropertyChanged影响 NetAmount 的每个属性的调用，那么您可以进行修改RaisePropertyChanged，以便它也引发该属性的PropertyChanged事件。NetAmount它会引发一些不必要PropertyChanged的事件，但更易于维护

private void RaisePropertyChanged(string propertyName)
{
     var handler = PropertyChanged;
     if (handler != null)
     {
          handler(this, new PropertyChangedEventArgs(propertyName);
          handler(this, new PropertyChangedEventArgs(NetAmountProperty);
     }
}