如何使用 mvvm 模式从不同的 ViewModel 获取属性和调用命令

Question

I have a ViewModel with all the properties that i will need in every sub ViewModel. It's the first time i try to split commands and viewmodel to multiple files. Last time everything was in the same ViewModel and it was a pain to work with it. Everything shows up as expected but i want to find a way to pass the same data in every viewmodel.

From my GetOrdersCommand, i want to get the HeaderViewModel.SelectedSource property. I didn't find any way to do it without getting a null return or loosing the property data... I would like to call my GetOrdersCommand from HeaderView button too.

Any tips how i can achieve this? Perhaps, my design is not good for what i'm trying to do?

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

Thanks guys. I moved my commands to their owning ViewModel as suggested. I tried MVVVM Light Tools and found about Messenger Class.

I used it to send my SelectedSource (Combobox from HeaderView) from HeaderViewModel to OrderViewModel. Am i suppose to use Messenger class like that? I don't know, but it did the trick!!!

I thought about moving GetOrdersCommand to OrderViewModel, binding my button command to OrderViewModel, binding SelectedSource as CommandParameter but i didn't know how i was suppose to RaiseCanExecuteChanged when HeaderViewModel.SelectedSource changed... Any advice?

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" }
        };
    }
}

Part of HeaderViewModel.cs

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;
}