在 NET MAUI 中的 MVVM 命令中更改双参数时，显示中的双参数不会更新

Question

我正在为产品商店制作客户端应用程序。购物车位于服务器上，数据通过对服务器的请求传输到那里。产品篮子是

ObservableCollection

。该视图还显示

ViewModel

构造函数中生成的总价格。我实现了从 observablecollection 中删除项目的命令，并且还输入了从总金额参数中减去已删除项目的成本的命令。删除的项目将从视图的列表中消失，但显示的数量保持不变。 XAML 代码：

<StackLayout>
    
    <CollectionView ItemsSource="{Binding ProductsInBasket}" VerticalOptions="FillAndExpand" x:Name="clv_basket">
        <CollectionView.ItemTemplate>
            <DataTemplate  x:DataType="dataModel:DTOProductAndQuantity">
                <Frame Padding="2" Margin="25,10,25,0" HasShadow="False" BorderColor="LightGray" CornerRadius="15">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="*"/>
                            <ColumnDefinition Width="*"/>
                        </Grid.ColumnDefinitions>
                        <Frame Grid.Column="0" HasShadow="False" CornerRadius="15" WidthRequest="200" HeightRequest="200" HorizontalOptions="Center">
                            <Image Grid.Column="0" HeightRequest="200" WidthRequest="200" Margin="0,0,0,0" Source="{Binding product.Photo, Mode=OneWay, Converter={StaticResource ByteToImage}}"/>
                        </Frame>
                        <Grid Grid.Column="1">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="*"/>
                                <RowDefinition Height="*"/>
                                <RowDefinition Height="*"/>
                            </Grid.RowDefinitions>
                            <Label Text="{Binding product.NameProduct}" FontSize="19" TextColor="#006064" VerticalOptions="Center" Grid.Row="0"/>
                            <Label Text="{Binding product.Category}" FontSize="18" TextColor="#006064" VerticalOptions="Center" Grid.Row="1"/>
                            <Label Text="{Binding product.Cost, StringFormat='cost per piece: {0} rub.'}" FontSize="18" TextColor="#006064" VerticalOptions="Center" Grid.Row="2"/>
                        </Grid>
                        <Grid Grid.Column="2">
                            <StackLayout VerticalOptions="End">
                                <Label Text="{Binding quantityInBusket}" FontSize="17" HorizontalOptions="Center" TextColor="BlueViolet"/>
                                <Stepper Maximum="10"  HorizontalOptions="Center" Increment="1" Value="{Binding quantityInBusket}"/>
                            </StackLayout>
                            <Button VerticalOptions="Start" HorizontalOptions="End" Command="{Binding Path=BindingContext.DeleteProductCommand, Source={x:Reference clv_basket}}" CommandParameter="{Binding}"/>
                        </Grid>
                    </Grid>
                </Frame>
            </DataTemplate>
        </CollectionView.ItemTemplate>
    </CollectionView>
    <Frame CornerRadius="2">
        <StackLayout>
            <Label Text="{Binding Sum, StringFormat='Sum: {0} rub.'}" HorizontalOptions="Center" FontSize="20" TextColor="Black"/>

        </StackLayout>
    </Frame>
</StackLayout>

视图模型代码：

class BusketViewModel
{
    #region Fields
    private double sum;
    private ObservableCollection<DTOProductAndQuantity> productsInBasket;
    private ObservableCollection<DTOProductAndQuantity> productsFromHttp;
    HttpClient httpClient = new HttpClient();

    private Command<object> deleteProductCommand;

    public event PropertyChangedEventHandler PropertyChanged;
    #endregion

    #region Constructor
    public BusketViewModel()
    {
        productsFromHttp = httpClient.GetFromJsonAsync<ObservableCollection<DTOProductAndQuantity>>("http://10.0.2.2:5125/api/Busket").Result;
        deleteProductCommand = new Command<object>(DeleteProduct);


        ProductsInBasket = new ObservableCollection<DTOProductAndQuantity>(productsFromHttp);
        foreach (var i in ProductsInBasket)            
            Sum += i.product.Cost * i.quantityInBusket;


    }
    
    private void DeleteProduct(object obj)
    {            
        Sum -= (obj as DTOProductAndQuantity).product.Cost * (obj as DTOProductAndQuantity).quantityInBusket;
        ProductsInBasket.Remove(obj as  DTOProductAndQuantity);
        
    }
    #endregion

    #region Properties
    public ObservableCollection<DTOProductAndQuantity> ProductsInBasket
    {
        get { return productsInBasket; }
        set
        {
            if (productsInBasket != value)
            {
                productsInBasket = value;
                NotifyPropertyChanged();
            }
        }
    }
    public Command<object> DeleteProductCommand
    {
        get { return deleteProductCommand; }
        set { deleteProductCommand = value; }
    }

    public double Sum
    {
        get { return sum; }
        set
        {
            if (sum != value)
            {
                sum = value;
                NotifyPropertyChanged();
            }
        }
    }
    #endregion


    protected virtual void NotifyPropertyChanged([CallerMemberName] string propertyName = "")
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

Answer 1

重新计算总和最安全

private void DeleteProduct(object obj)
{   
    var selectedItem_DTOProductAndQuantity = (obj as DTOProductAndQuantity);
    if(selectedItem_DTOProductAndQuantity is null) 
    {
        //You can display error message
        return;
    }

    // Sum -= (obj as DTOProductAndQuantity).product.Cost * (obj as DTOProductAndQuantity).quantityInBusket;
    
    ProductsInBasket.Remove(selectedItem_DTOProductAndQuantity);

    // Recalculate Sum
    Sum = 0;
    foreach (var i in ProductsInBasket)            
        Sum += i.product.Cost * i.quantityInBusket;
        
}