在WPF MVVM中从一个视图导航到另一个视图

Question

I wrote code which should navigate between user controls in WPF application using MVVM, but I realised that this code doesn't work. From window LoginView I want to change the view to VotingCardView .

Actually, after clicking on the button in the LoginView , the method DisplayVCV gets executed, but the view is not going to change. What am I doing wrong?

MainView.xaml:

<Window x:Class="ElectionCalculator.View.MainView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ElectionCalculator"
        xmlns:v="clr-namespace:ElectionCalculator.View"
        xmlns:vm="clr-namespace:ElectionCalculator.ViewModel"
        mc:Ignorable="d"
        Title="Election calculator" Height="350" Width="525">
    <Window.DataContext>
        <vm:MainViewModel />
    </Window.DataContext>
    <ContentControl Content="{Binding ViewModel}" />
</Window>

LoginView.xaml:

<UserControl x:Class="ElectionCalculator.View.LoginView"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:ElectionCalculator.View"
      xmlns:vm="clr-namespace:ElectionCalculator.ViewModel"

      mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300">

    <Grid>
        <Button Command="{Binding DataContext.DisplayVC, RelativeSource={RelativeSource AncestorType={x:Type Window}}, Mode=OneWay}" Margin="161,147,47,124" />
    </Grid>

</UserControl>

MainViewModel.cs

class MainViewModel : BaseViewModel
{
    public BaseViewModel ViewModel { get; set; }

    public MainViewModel()
    {
        ViewModel = new LoginViewModel();
    }

    public ICommand DisplayVC { get { return new RelayCommand(DisplayVCV); } }

    public void DisplayVCV()
    {
        ViewModel = new VotingCardViewModel();

        MessageBox.Show("DisplayVCCommandExecuted");
    }
}

Answer 1

Your ViewModel property implementation doesn't raise a PropertyChanged event when the value changes. This is usually done via an INotifyPropertyChanged implementation. Because of that, your view doesn't get notified that something has changed.

In your case, this means that you need a backing field for your ViewModel property and implement your ViewModel property similar to this:

private BaseViewModel _viewModel;
public BaseViewModel ViewModel
{
  get { return _viewModel; }
  set
  {
    if(_viewModel != value) 
    {
      _viewModel = value;
      OnPropertyChanged("ViewModel");
    }
  }
}

Since you are already deriving from BaseViewModel I assume that the method OnPropertyChanged (or some method with a similar name) is implemented there. It is also quite common that you don't have to specify the property name ( "ViewModel" ) as an argument, since lots of implementations use the [CallerMemberName] attribute for this purpose.