如何使用 mvvm 模式从不同的 ViewModel 获取属性和调用命令

Question

我有一个 ViewModel，其中包含我在每个子 ViewModel 中需要的所有属性。 这是我第一次尝试将命令和视图模型拆分为多个文件。上次一切都在同一个 ViewModel 中，使用它很痛苦。一切都按预期显示，但我想找到一种方法在每个视图模型中传递相同的数据。

从我的 GetOrdersCommand 中，我想获取 HeaderViewModel.SelectedSource 属性。如果没有返回空值或丢失属性数据，我没有找到任何方法...... 我也想从 HeaderView 按钮调用我的 GetOrdersCommand。

任何提示我可以如何做到这一点？也许，我的设计不适合我想做的事情？

MainWindow.xaml

        <views:HeaderView Grid.Row="0" Grid.Column="1" Grid.ColumnSpan="2" DataContext="{Binding HeaderViewModel}" LoadHeaderViewCommand="{Binding LoadHeaderViewCommand}"/>
        <TabControl TabStripPlacement="Bottom" Grid.Row="1" Grid.Column="1" Grid.RowSpan="2" Grid.ColumnSpan="2">
            <TabItem Header="General">
            </TabItem>
            <TabItem Header="Orders">
                <views:OrderView DataContext="{Binding OrderViewModel}" GetOrdersCommand="{Binding GetOrdersCommand}"/>
            </TabItem>
        </TabControl>

HeaderView.xaml

        <DockPanel>
            <ComboBox DockPanel.Dock="Left" Width="120" Margin="4" VerticalContentAlignment="Center" ItemsSource="{Binding SourceList}" SelectedItem="{Binding SelectedSource}" DisplayMemberPath="SourceName"/>
            <Button x:Name="btnTest" HorizontalAlignment="Left" DockPanel.Dock="Left" Margin="4" Content="Test"/>
        </DockPanel>

HeaderView.xaml.cs

    public partial class OrderView : UserControl
    {
        public ICommand GetOrdersCommand
        {
            get { return (ICommand)GetValue(GetOrdersCommandProperty); }
            set { SetValue(GetOrdersCommandProperty, value); }
        }
        public static readonly DependencyProperty GetOrdersCommandProperty =
            DependencyProperty.Register("GetOrdersCommand", typeof(ICommand), typeof(OrderView), new PropertyMetadata(null));

        public OrderView()
        {
            InitializeComponent();
        }

        private void UserControl_Loaded(object sender, RoutedEventArgs e)
        {
            if (GetOrdersCommand != null)
            {
                GetOrdersCommand.Execute(this);
            }
        }
    }

MainViewModel.cs

        private OrderViewModel orderViewModel;
        public OrderViewModel OrderViewModel { get; set; } // Getter, setter with OnPropertyChanged

        private HeaderViewModel headerViewModel;
        public HeaderViewModel HeaderViewModel { get; set; } //  Getter, setter with OnPropertyChanged

        public MainViewModel()
        {
            HeaderViewModel = new HeaderViewModel();
            OrderViewModel = new OrderViewModel();
        }

HeaderViewModel.cs

public ICommand LoadHeaderViewCommand { get; set; }
public HeaderViewModel()
{
   LoadHeaderViewCommand = new LoadHeaderViewCommand(this);
}

GetOrdersCommand.cs

    public class GetOrdersCommand : ICommand
    {
        public event EventHandler CanExecuteChanged;
        private readonly OrderViewModel _orderViewModel;

        public GetOrdersCommand(OrderViewModel orderViewModel)
        {
            _orderViewModel = orderViewModel;
        }

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public void Execute(object parameter)
        {
            /* Build Order List according to HeaderViewModel.SelectedSource */
            _orderViewModel.Orders = new ObservableCollection<Order>()
            {
                new Order { ID = 1, IsReleased = false, Name = "Test1"},
                new Order { ID = 2, IsReleased = true, Name = "Test2"},
            };
        }
    }

Answer 1

谢谢大家！我按照建议将我的命令移到了他们拥有的 ViewModel 中。 我尝试了 MVVVM Light Tools，发现关于 Messenger Class。

我使用它将我的 SelectedSource（来自 HeaderView 的组合框）从 HeaderViewModel 发送到 OrderViewModel。我想像那样使用 Messenger 类吗？我不知道，但它成功了！！！

我考虑过将 GetOrdersCommand 移动到 OrderViewModel，将我的按钮命令绑定到 OrderViewModel，将 SelectedSource 绑定为 CommandParameter，但我不知道当 HeaderViewModel.SelectedSource 更改时我应该如何 RaiseCanExecuteChanged...有什么建议吗？

MainWindow.xaml

<views:HeaderView DataContext="{Binding Source={StaticResource Locator}, Path=HeaderVM}" Grid.Row="0" Grid.Column="1" Grid.ColumnSpan="2"/>
<TabControl TabStripPlacement="Bottom" Grid.Row="1" Grid.Column="1" Grid.RowSpan="2" Grid.ColumnSpan="2">
    <TabItem Header="General">
    </TabItem>
    <TabItem Header="Orders">
        <views:OrderView DataContext="{Binding Source={StaticResource Locator}, Path=OrderVM}"/>
    </TabItem>
</TabControl>

OrderViewModel.cs

private ObservableCollection<Order> _orders;
public ObservableCollection<Order> Orders
{
    get { return _orders; }
    set
    {
        if (_orders != value)
        {
            _orders = value;
            RaisePropertyChanged(nameof(Orders));
        }
    }
}
public OrderViewModel()
{
    Messenger.Default.Register<Source>(this, source => GetOrders(source));
}

private void GetOrders(Source source)
{
    if (source.SourceName == "Production")
    {
        Orders = new ObservableCollection<Order>(){
            new Order { ID = 1, IsReleased = false, Name = "Production 1" }
        };
    }
    else
    {
        Orders = new ObservableCollection<Order>(){
            new Order { ID = 2, IsReleased = true, Name = "Test 1" }
        };
    }
}

HeaderViewModel.cs 的一部分

private Source _SelectedSource;
public Source SelectedSource
{
    get { return _SelectedSource; }
    set
    {
        if (_SelectedSource != value)
        {
            _SelectedSource = value;
            RaisePropertyChanged(nameof(SelectedSource));
            GetOrdersCommand.RaiseCanExecuteChanged();
        }
    }
}

private RelayCommand _GetOrdersCommand;
public RelayCommand GetOrdersCommand
{
    get
    {
        if (_GetOrdersCommand == null)
        {
            _GetOrdersCommand = new RelayCommand(GetOrders_Execute, GetOrders_CanExecute);
        }
        return _GetOrdersCommand;
    }
}

private void GetOrders_Execute()
{
        Messenger.Default.Send(SelectedSource);
}

private bool GetOrders_CanExecute()
{
    return SelectedSource != null ? true : false;
}