WPF 使用 MVVM 的父事件处理程序

Question

It seems that I would like to the opposite of the question posed here . I have a WPF application that uses the MVVM pattern. The main view determines the view to be displayed with

<!-- Content -->
<ContentControl Grid.Row="1" Content="{Binding CurrentView, Mode=OneWay}" />

Where CurrentView is the "current" view. In this same main view model I also have buttons that are enabled an disabled via a bind to "IsValid" on the view model

<Button Content="{x:Static r:Resources.Next}" Background="LawnGreen"
                Command="{Binding NextCommand,Mode=OneWay}"
                IsEnabled="{Binding Path=IsValid,Mode=TwoWay}"
                Visibility="{Binding Path=IsNextVisible,Converter={StaticResource BoolToVis}}"/>

This has a simple implementation like:

public bool IsValid
{
    get
    {
        return CurrentView.IsValid;
    }
    set
    {
        RaisePropertyChanged();
    }
}

But you will notice that the "IsValid" flag comes from whatever is the CurrentView. Thus each view determines if it is valid or not. In one of these views (UserControls) I have a ComboBox that when something is selected the "IsValid" for that view should go from false to true. This looks like:

private Client selectedItem;
public Client SelectedItem
{
    get { return selectedItem; }
    set
    {
        selectedItem = value;
        IsValid = true;
        ProvisionService.SelectedClient = selectedItem;
        RaisePropertyChanged();
        RaisePropertyChanged("IsValid");
    }
}

private bool _isValid;
public bool IsValid
{
    get
    {
        return _isValid;
    }
    set
    {
        _isValid = value;
        RaisePropertyChanged();
    }
}

The problem that I am seeing is that parent doesn't seem to be seeing the event that has changed from false to true in the child view. How do I notify the parent that this property in the child has changed?

Here is the implementation of CurrentView

private IProvisionView _currentView;
public IProvisionView CurrentView
{
    get { return _currentView; }
    set
    {
        _currentView = value;
        RaisePropertyChanged();
        RaisePropertyChanged(nameof(IsValid));
        RaisePropertyChanged(nameof(IsPrevVisible));
        RaisePropertyChanged(nameof(IsNextVisible));
        RaisePropertyChanged(nameof(IsHomeVisible));
    }
}

Answer 1

You can do the same what WPF does. Assign to the PropertyChanged event. If your CurrentView is set:

// old (if assigned)
_currentView.PropertyChanged -= CurrentView_PropertyChanged;
// new
value.PropertyChanged += CurrentView_PropertyChanged;

and then check if it was the IsValid property

Answer 2

Whenever the CurrentView property is set to a new value in the parent view model, you should hook up an event handler to the child view model's PropertyChanged event and handle it by raising the PropertyChanged event for the IsValid property of the parent view model that the Button is bound to, eg:

private INotifyPropertyChanged _currentView;
public INotifyPropertyChanged CurrentView
{
    get { return _currentView; }
    set
    {
        if (_currentView != null)
            _currentView.PropertyChanged -= OncurrentViewPropertyChanged;
        _currentView = value;
        if (_currentView != null)
            _currentView.PropertyChanged += OncurrentViewPropertyChanged;
    }
}

private void OncurrentViewPropertyChanged(object sender, PropertyChangedEventArgs e)
{
    if (e.PropertyName == "IsValid") // = nameof(ChildViewModel.IsValid)
        RaisePropertyChanged("IsValid"); // = nameof(IsValid)
}