Caliburn Micro content control navigation

问题

I'm using caliburn micro for this project. I have my ShellView with my contentcontrol:

<ContentControl x:Name="ActiveItem"
                        Grid.Row="0" Grid.Column="0" />

In ShellViewModel i got it to show my usercontrol LoginView with:

 public class ShellViewModel : Conductor<object>
    {
        public ShellViewModel()
        {
            ActivateItem(new LoginViewModel());
        }

        public void ShowSignUp()
        {
            ActivateItem(new SignUpViewModel());
        }
    }

However, i can't navigate to SignUpView from LoginView with my button:

<!-- Row 4 -->
<Button x:Name="ShowSignUp"
        Content="Sign Up Now!"
        Grid.Row="3" Grid.Column="1"
        Style="{StaticResource LoginBtnsStyle}" />

LoginViewModel deriving from ShellViewModel:

public class LoginViewModel : ShellViewModel
    {

    }

How do i navigate from LoginView to SignUpView with a button that is on the LoginView? I'm getting no errors, it just isn't changing view. I also tried putting ShowSignUp() on the LoginViewModel but no success.

Update 1 ShellViewModel:

public class ShellViewModel : Conductor<object>, IHandle<ActionInvokedMessage>
    {
        DispatcherTimer dt = new DispatcherTimer();
        private SplashScreenViewModel _splashVM;
        private LoginViewModel _loginVM;
        private SignUpViewModel _signUpVM;
        private IEventAggregator _eventAggregator;

        public ShellViewModel(SplashScreenViewModel splashVM, LoginViewModel loginVM, SignUpViewModel signUpVM)
        {
            _loginVM = loginVM;
            _signUpVM = signUpVM;
            _splashVM = splashVM;
            ActivateItem(_splashVM);

            dt.Tick += new EventHandler(Dt_Tick);
            dt.Interval = new TimeSpan(0, 0, 2);
            dt.Start();
        }

        private void Dt_Tick(object sender, EventArgs e)
        {
            dt.Stop();
            ActivateItem(_loginVM);
        }

        public ShellViewModel(IEventAggregator eventAggregator)
        {
            _eventAggregator = eventAggregator;
            _eventAggregator.Subscribe(this);
            ActivateItem(new LoginViewModel(_eventAggregator));
        }

        public void Handle(ActionInvokedMessage message)
        {
            ActivateItem(message.Page);
        }

        public void ShowSignUp()
        {
            ActivateItem(new SignUpViewModel());
        }



    }

回答1:

You could achieve this using EventAggregator to publish indicative messages from LoginViewModel to ShellViewModel to update the UI.

To begin with, you need to define an message class, which would tells the ShellViewModel which ViewModel needs to be changed. For example,

public class ActionInvokedMessage
    {
        public Screen Page { get; set; }
    }

The Page property would indicate which Screen needs to be loaded. Now, you could change your LoginViewModel as the following.

public class LoginViewModel: Screen
    {
        private IEventAggregator _eventAggregator;
        public LoginViewModel(IEventAggregator eventAggregator)
        {

            _eventAggregator = eventAggregator;
            _eventAggregator.Subscribe(this);
        }

        public void ShowSignUp()
        {
            _eventAggregator.PublishOnUIThread(new ActionInvokedMessage { Page = new SignupViewModel() }); ;
        }
    }

The PublishOnUIThread method would broadcast a message to all the listeners of the Message Type ActionInvokedMessage for the change. Next step would be to ensure the ShellViewModel would be listening to the change.

public class ShellViewModel : Conductor<object>, IHandle<ActionInvokedMessage>
    {
        private IEventAggregator _eventAggregator;
        public ShellViewModel(IEventAggregator eventAggregator)
        {
            _eventAggregator = eventAggregator;
            _eventAggregator.Subscribe(this);
            ActivateItem(new LoginViewModel(_eventAggregator));
        }

        public void Handle(ActionInvokedMessage message)
        {
            ActivateItem(message.Page);
        }

        public void ShowSignUp()
        {
            ActivateItem(new SignupViewModel());
        }


    }

The Implementation of IHandle interface allows us to handle the action that would be required to be executed when the ShellViewModel recieves the ActionInvokedMessage. As seen in the code, this would be an appropriate place to use the ActivateItem method to load the Signup Page.

回答2:

You can create an interface for navigation and use it in view models to navigate around app.

interface INavigation {
    void NavigateTo(System.Type typeId);
}

class ShellViewModel: Conductor<object>,  INavigation  {

    private List<object> pages = new List<Object>();

    public ShellViewModel() {
        pages.Add(new SignupViewModel(this));
        pages.Add(new LoginViewModel(this));
    }

    void NavigateTo(System.Type typeId) {
        var page = pages.Where(x => x.GetType() == typeId).FirstOrDefault()
        ActivateItem(page);
    }
}

class SignupViewModel {
    public SignupViewModel(INavigation navigation) {
        this.ShowLoginCommand= new NavigateCommand<LoginViewModel>(navigation);
    }
}

class LoginViewModel {
    public LoginViewModel (INavigation navigation) {
        this.ShowSignUpCommand = new NavigateCommand<SignupViewModel>(navigation);
    }
}

Navigation command may be implemented like follows:

public class NavigateCommand<T> : ICommand
{
    public event EventHandler CanExecuteChanged;
    private readonly INavigation navigation;

    public NavigateCommand(INavigation navigation)
    {
        this.navigation = navigation;
    }

    public bool CanExecute(object parameter) => true;
    public void Execute(object parameter) => this.navigation.NavigateTo(typeof(T));
}

Here I pass System.Type but you can design type that better describes navigation request so that you may pass additinal paramters.

来源：https://stackoverflow.com/questions/58625046/caliburn-micro-content-control-navigation