MVVM-从ViewModel访问View中的xaml元素

Question

Models:

• TabModel.cs

• DisplayTabViewModel.cs - contains few overwritten properties from TabModel like Name, Title etc.

ViewModels:

• MainViewModel.cs

Views:

• MainWindow.xaml - Window

• DisplayTabView.xaml - Usercontrol

---

This is pure MVVM pattern, without code-behind. TabModel invokes EventHandler - RefreshRequested after a Word Document is closed. MainViewModel contains the subscriber. Method Refresh() in MainViewModel works fine. The method is triggered after I close the document. Now I would like to refresh the WebBrowser in DisplayTabView.xaml from MainViewModel . I'm struggling with this too many hours already. Could you please point me in the right direction. Thank you.

---

TabModel.cs

public abstract class TabModel : ITabModel, INotifyPropertyChanged
{
    public async void HiddenFileExists()
    {
        (...)

        OnRefreshRequested();
    }

    public delegate void RefreshRequestedEventHandler(object source, EventArgs args);

    public event RefreshRequestedEventHandler RefreshRequested;

    public virtual void OnRefreshRequested()
    {
        RefreshRequested?.Invoke(this, EventArgs.Empty);
    }
}

MainViewModel.cs

public class MainViewModel : INotifyPropertyChanged
{
    private readonly ObservableCollection<ITabModel> tabs;

    private void DisplayFileTab(object parameter)
    {
        (...)

        tabs.ElementAt(TabIndex).RefreshRequested += Refresh;
    }

    public void Refresh(object source, EventArgs args)
    {
        MessageBox.Show("Refresh");
    }

DisplayTabView.xaml

<Grid>
    <WebBrowser h:WebBrowserExtensions.BindableSource="{Binding FileUrl}" Tag="{Binding AcceptedKeywordsArray}" h:WebBrowserExtensions.BindableLoaded="{Binding}" />
</Grid>

MainWindow.xaml

<Grid>
    <DockPanel>
        <TabControl Name="tabControl" ItemsSource="{Binding Tabs}" SelectedIndex="{Binding TabIndex}">
            <TabControl.Resources>
                <DataTemplate DataType="{x:Type m:DisplayTabViewModel}">
                    <v:DisplayTabView x:Name="DisplayTab"/>
                </DataTemplate>
            </TabControl.Resources>
            <TabControl.ItemTemplate>
                <DataTemplate DataType="{x:Type m:ITabModel}">
                    <Grid>
                        (...)
                    </Grid>
                </DataTemplate>
            </TabControl.ItemTemplate>
        </TabControl>
    </DockPanel>
</Grid>

Answer 1

The MainViewModel may use an event aggreator or a messenger to send an event/message that any other component, like for example another view model or a view, in your application can subscribe to.

By introducing an event aggregator or a messenger in between the publishers and subscribers, you remove the tight coupling between the publisher and the consumer of the event. Both the publisher and the consumer know only about an event aggregator but they don't know anything about nor have any references to each other. Please refer to the following links for more information about the concept.

Using the event aggregator pattern to communicate between view models: https://blog.magnusmontin.net/2014/02/28/using-the-event-aggregator-pattern-to-communicate-between-view-models/

MVVM - Messenger and View Services in MVVM: https://msdn.microsoft.com/en-us/magazine/jj694937.aspx

Most MVVM libraries out there have their own implementation of this kind of thing. In Prism it's called an EventAggreator . And in MvvmLight it is called a Messenger .

Here are some code samples on how to use them:

https://github.com/PrismLibrary/Prism-Samples-Wpf/tree/master/14-UsingEventAggregator

https://marcominerva.wordpress.com/2014/06/25/how-to-send-string-and-content-messages-with-mvvm-light-messenger/

You could of course also implement your own custom event aggregator if you want to.

Answer 2

Pretty much all MVVM frameworks implement a Message Bus you can use to send messages between View Models, in your case from MainViewModel to DisplayTabViewModel.

If you are for some reason against using an MVVM framework you'll have to implement the Message Bus of your own which wouldn't make much sense since there are so many great MVVM frameworks out there...