This is my first WPF-MVVM application, and this is my structure:
One project with my app.xaml to open the application and override the OnStartup to resolve the MainWindow. (I did that due to the references);
One project for my Views;
One project for my ViewModels;
One project for my Model.
And I have de following problem: I'm on the MainWindowView and I click on a button to show another view. How am I supposed to do to open this another view from my MainWindowViewModel whereas my View Project has reference with the ViewModel Project, and I can't reference the ViewModel Project with the View Project?
By the way, I'm using Unity for the dependency injection.
So, could you help me?
There are several approaches to that.
You can define a dialog/navigation/window service interface, defined in the ViewModels project. You will need to decide how the ViewModels will express which window they want to open. I generally use an IDialogViewModel interface, which some of my ViewModels implement, and pass an instance of the ViewModel to the service, but you can use an enum, string, whatever you want, so your implementation can map to the real window which will be opened.
For example:
public interface IDialogService
{
bool? ShowDialog(object dialogViewModel);
}
ViewModels that want to open new Windows would receive an instance of that service and use it to express the intention of opening a Window. In your Views project, you would define a type which implements your service interface, with the real logic behind opening the Window.
Following the example:
public class DialogService : IDialogService
{
private Stack<Window> windowStack = new Stack<Window>();
public DialogService(Window root)
{
this.windowStack.Push(root);
}
public bool? ShowDialog(object dialogViewModel)
{
Window dialog = MapWindow(dialogViewModel);
dialog.DataContext = dialogViewModel;
dialog.Owner = this.windowStack.Peek();
this.windowStack.Push(dialog);
bool? result;
try
{
result = dialog.ShowDialog();
}
finally
{
this.windowStack.Pop();
}
return result;
}
}
Your main project will be responsible for creating and injecting the dialog service in the ViewModels who need it. In the example, the App would create a new dialog service instance passing the MainWindow to it.
A similar approach to do it would be using some form of the messaging pattern (link1 link2 ).
In addition, if you want something simple you can also make your ViewModels raise events when they want to open Windows and let the Views subscribe to them.
EDIT
The complete solution that I use in my apps a generally a bit more complex, but the idea is basically that. I have a base DialogWindow, which expects a ViewModel which implements an IDialogViewModel interface as DataContext. This interface abstracts some functionalities you expect in dialog, like accept/cancel commands as well as a closed event so you can also close the window from the ViewModel. The DialogWindow consists basically in a ContentPresenter which Content property is bound to the DataContext and hooks the close event when the DataContext is changed (and a few other things).
Each "dialog" consists in an IDialogViewModel and an associated View (UserControl). To map them, I just declare implicit DataTemplates in the resources of the App. In the code I've shown, the only difference would be there wouldn't be a method MapWindow, the window instance would always be a DialogWindow.
I use an additional trick to reuse layout elements between dialogs. On approach is to include them in the DialogWindow (accept/cancel buttons, etc). I like to keep the DialogWindow clean (so I can use it event to "non-dialog" dialogs). I declare a template for a ContentControl with the common interface elements, and when I declare a View-ViewModel mapping template, I wrap the View with a ContentControl with my "dialog template" applied. You can then have as much "Master templates" for your DialogWindow as you want (like a "wizard like" one, for example).
Straight forward approach
If I understand you correctly MainWindowView is resolved through Unity when the app starts, which resolves its dependency on MainWindowViewModel?
If that is the flow you are using I would suggest continuing on the same path and letting the MainWindowView handle the opening of the new view via a simple click handler for the button. In this handler you can then resolve the new view, which would resolve that view's view model and then you're back in MVVM land for the new view as well.
That solution is straight forward and would work perfectly fine for most smaller applications.
Heavier approach for more complex apps
If you don't want that kind of view-first flow I would suggest introducing some kind of controller/presenter that coordinates the views and view models. The presenter is responsible for making decisions about if/when to actually open/close views and so on.
This is a quite heavy abstraction though, which is more suitable for more complex applications, so make sure you actually get enough benefit out of it to justify the added abstraction/complexity.
Here is a code sample of what this approach might look like:
public partial class App
{
protected override void OnStartup(StartupEventArgs e)
{
var container = new UnityContainer();
container.RegisterType<IMainView, MainWindow>();
container.RegisterType<ISecondView, SecondWindow>();
container.RegisterType<IMainPresenter, MainPresenter>();
container.RegisterType<ISecondPresenter, SecondPresenter>();
var presenter = container.Resolve<IMainPresenter>();
presenter.ShowView();
}
}
public interface IMainPresenter
{
void ShowView();
void OpenSecondView();
}
public interface ISecondPresenter
{
void ShowView();
}
public interface ISecondView
{
void Show();
SecondViewModel ViewModel { get; set; }
}
public interface IMainView
{
void Show();
MainViewModel ViewModel { get; set; }
}
public class MainPresenter : IMainPresenter
{
private readonly IMainView _mainView;
private readonly ISecondPresenter _secondPresenter;
public MainPresenter(IMainView mainView, ISecondPresenter secondPresenter)
{
_mainView = mainView;
_secondPresenter = secondPresenter;
}
public void ShowView()
{
// Could be resolved through Unity just as well
_mainView.ViewModel = new MainViewModel(this);
_mainView.Show();
}
public void OpenSecondView()
{
_secondPresenter.ShowView();
}
}
public class SecondPresenter : ISecondPresenter
{
private readonly ISecondView _secondView;
public SecondPresenter(ISecondView secondView)
{
_secondView = secondView;
}
public void ShowView()
{
// Could be resolved through Unity just as well
_secondView.ViewModel = new SecondViewModel();
_secondView.Show();
}
}
public class MainViewModel
{
public MainViewModel(MainPresenter mainPresenter)
{
OpenSecondViewCommand = new DelegateCommand(mainPresenter.OpenSecondView);
}
public DelegateCommand OpenSecondViewCommand { get; set; }
}
public class SecondViewModel
{
}
<!-- MainWindow.xaml -->
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Height="350" Width="525">
<Grid>
<Button Command="{Binding OpenSecondViewCommand}" Content="Open second view" />
</Grid>
</Window>
<!-- SecondWindow.xaml -->
<Window x:Class="WpfApplication1.SecondWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="SecondWindow" Height="350" Width="525">
<Grid>
<TextBlock>Second view</TextBlock>
</Grid>
</Window>
This article presents a similar solution to what I've used in production before.
To open new window from MainWindowView, you need to pass reference of Frame component or whole window to MainWindowViewModel object(You can do this when binding command to transition button or something, pass them as object), there you can navigate to new page, however, if there is nothing of special that you need to do in ViewModel when transitioning, you can just use classic ButtonClick event or w/e in MainWindowView.cs that will do navigation for you, that is ok for basic transitions.
P.S. I am not sure why you use different projects for ViewModels/Views/Models.
Related
Overview:
I have one MainWindow and two Views. One representing a Login Screen and the other one is representing the Dashboard. The LoginView is shown on StartUp. When the User presses the LoginButton the DashboardView should be shown.
Here's my Problem:
The underlying Command is beeing executed. The Constructor of DashboardViewModel is beeing called. And the View and ViewModel for Dashboard are connected via a DataTemplate. But the View or the InitializeComponent Method are not beeing called.
The LoginCommand looks like this:
LoginMainCommands is a RelayCommand Class derived from ICommand.
public class ButtonViewModel : LoginMainViewModel
{
public ButtonViewModel()
{
_loginCommand = new LoginMainCommands(Login);
}
private LoginMainCommands _loginCommand;
public LoginMainCommands LoginCommand
{
get { return _loginCommand; }
}
public void Login()
{
ViewModelLocator ObjViewModelLocator = new ViewModelLocator();
ObjViewModelLocator.MWInstance.SwitchViewModel();
}
}
The connection between the Views and the ViewModels is in ManWindow.xaml as follows:
<Window.Resources>
<local:ViewModelLocator x:Key="ViewModelLocator"/>
<DataTemplate DataType="{x:Type loginViewModel:LoginMainViewModel}">
<loginView:LoginMainViewUserControl/>
</DataTemplate>
<DataTemplate DataType="{x:Type dashboardViewModel:DashboardMainViewModel}">
<dashboardViews:DashboardMainViewUserControl/>
</DataTemplate>
</Window.Resources>
To switch between the Views I added this Method in MainWindowViewModel:
public void SwitchViewModel()
{
if (!isLoginView)
{
isLoginView = true;
ViewModel = new LoginMainViewModel();
}
else
{
isLoginView = false;
ViewModel = new DashboardMainViewModel();
}
}
What I've tried so far:
I did almost everything this answer suggests. But I can't connect the Views and ViewModels in the App.xaml, cause then I can't use my ResourceDictionaries for Icons and Logos. But they are connected in MainWindow.xaml.
Later on I recognized in order for this to work only one Instance of MainWindowViewModel could exist because otherwise ViewModel would be null everytime a new Object is created. That's why I created ViewModelLocator like this answer suggests.
The weird part for me is when I change the if bracket in the SwitchViewModel Method to this:
if (!isLoginView)
{
isLoginView = true;
ViewModel = new DashboardMainViewModel();
}
Now DashboardMainViewModel is the default View to show and it does excatcly that it shows up.
I have no clue why the Dashboard Screen is not beeing shown.
I want to thank everybody in advance for your help and patience!
The problem is this line in the Login method in ButtonViewModel.
ViewModelLocator ObjViewModelLocator = new ViewModelLocator();
In your view model you create a new instance of the ViewModelLocator, which in turn creates a new instance of MainWindowViewModel on which you then call SwitchViewModel. That is the wrong instance.
You could solve this problem in many different ways, e.g.:
Make ViewModelLocator a singleton.
You would set the DataContext like this, without creating an instance in the Window.Resources:
DataContext="{Binding MainWindowViewModel, Source={x:Static local:ViewModelLocator.Instance}}"
Then in ButtonViewModel you can directly call the SwitchViewModel through ViewModelLocator:
ViewModelLocator.Instance.MWInstance.SwitchViewModel();
Pass a reference of the MainWindowViewModel to the other view models
Passing the instance of MainWindowViewModel to the other view models in order to call SwitchViewModel works, but makes them tightly coupled and exposes more of the view model than necessary.
Alternatively, you could create an interface ISwitchViewModels with a method SwitchViewModel that is implemented by MainWindowViewModel and pass it via the interface instead to hide the actual implementation.
Use events to communicate between view models
Many WPF MVVM frameworks like Prism or Caliburn.Micro feature something called an Event Aggregator to communicate via events between view models and enable loose coupling. In this case you would not have to depend on a reference from ButtonViewModel to MainWindowViewModel, instead you would send a message to request changing the current view.
Use dependency injection to register the ViewModelLocator as singleton and pass it to view models
I guess you are not familiar with dependency injection and it might be beyond the scope of this question, so this is just for further reading.
Note that in this case the ViewModelLocator is not implemented as singleton, but only a single instance of it is created by a container and passed to all other types that specify this dependency.
Understanding dependency injection
What is dependency injection?
The weird part for me is when I change the if bracket in the SwitchViewModel Method to this: [...]
Now DashboardMainViewModel is the default View to show and it does excatcly that it shows up.
That is not weird at all, since you directly assign the ViewModel property on the right instance of MainWindowViewModel without even using the ViewModelLocator.
I am trying to learn MVVM and have come across a weird snag. I have a main menu with a drawer control that comes out and shows a menu:
In the main window where this drawer is, I have a ContentControl where I set its content with a Binding.
<ContentControl x:Name="MainWindowContentControl" Content="{Binding Path=WindowContent}"/>
This window's binding is set to a view model.
<Window.DataContext>
<viewmodels:MainWindowViewModel/>
</Window.DataContext>
and here is the ViewModel:
MainWindowViewModel.cs
public class MainWindowViewModel: ViewModelBase
{
private object _content;
public object WindowContent
{
get { return _content; }
set
{
_content = value;
RaisePropertyChanged(nameof(WindowContent));
}
}
public ICommand SetWindowContent { get; set; }
public MainWindowViewModel()
{
SetWindowContent = new ChangeWindowContentCommand(this);
}
}
So far up to this point, everything works fine. So for example, if I click "Recovery Operations", I get this:
RecoveryOperationsView.xaml
In "RecoveryOperationsView.xaml" (which is a UserControl) I also reference the view model from above like so..
<UserControl.DataContext>
<viewmodels:MainWindowViewModel/>
</UserControl.DataContext>
and have a button to call the command to change the Content property of the ContentControl from the main window..
<Button Grid.Row="2" Content="Restore Database" Width="150" Style="{StaticResource MaterialDesignFlatButton}" Command="{Binding SetWindowContent}" CommandParameter="DatabaseRecovery" >
In my class to process the commands, I change the content based off of the passed parameter using a switch statement like so
ChangeWindowContentCommand.cs
public class ChangeWindowContentCommand : ICommand
{
private MainWindowViewModel viewModel;
public ChangeWindowContentCommand(MainWindowViewModel vm)
{
this.viewModel = vm;
}
public event EventHandler CanExecuteChanged;
public bool CanExecute(object parameter)
{
return true;
}
public void Execute(object parameter)
{
switch (parameter)
{
case "Home":
viewModel.WindowContent = new HomeView();
break;
case "RecoveryOps":
viewModel.WindowContent = new RecoveryOperationsView();
break;
case "DatabaseRecovery":
viewModel.WindowContent = new DatabaseRestoreView();
break;
}
}
}
However, this is where I get lost... If I click something within this new window, say "Restore Database" and inspect it with a breakpoint, I can see the property being changed but the actual ContentControl Content property doesnt change to the new UserControl I made... I can change the content with anything in the drawer, but if I try to click a button in the hosted Content of the ContentControl nothing changes. What am I missing?
It's hard to be 100% sure without having your project to test with, but I am fairly confident that at least one of the issues is that your UserControl and your MainWindow use different instances of the MainWindowViewModel. You do not need to instantiate the VM for the user control, as it will inherit the DataContext from the MainWindow. The way it works in WPF is that if any given UIElement does not have theDataContext assigned explicitly, it will inherit it from the first element up the logical tree that does has one assigned.
So, just delete this code, and it should solve at least that issue.
<UserControl.DataContext>
<viewmodels:MainWindowViewModel/>
</UserControl.DataContext>
And since you're learning WPF, I feel obligated to provide a couple other tips. Even though you're using a ViewModel, you are still mixing UI and logic by creating a very specific implementation of ICommand and assigning a UI element through your ViewModel. This breaks the MVVM pattern. I know MVVM takes a little time to understand, but once you do, it is very easy to use and maintain.
To solve your problem, I would suggest creating View Models for each of your user controls. Please see this answer, where I go into quite a bit of detail on the implementation.
For switching the different views, you have a couple of options. You can either use a TabControl, or if you want to use a command, you can have a single ContentControl bound to a property of MainWindowViewModel that is of type ViewModelBase. Let's call it CurrentViewModel. Then when the command fires, you assign the view model of the desired user control to that bound property. You will also need to utilize implicit data templates. The basic idea is that you create a template for each of the user control VM types, which would just contains an instance of the Views. When you assign the user control VM to the CurrentViewModel property, the binding will find those data templates and render the user control. For example:
<Window.Resources>
<DataTemplate DataType = "{x:Type viewmodels:RecoveryOperationsViewModel}">
<views:RecoveryOperationsView/>
</DataTemplate>
<!-- Now add a template for each of the views-->
</Window.Resources>
<ContentControl x:Name="MainWindowContentControl" Content="{Binding CurrentViewModel}"/>
See how this approach keeps UI and logic at an arm's length?
And lastly, consider creating a very generic implementation of ICommand to use in all your ViewModels rather than many specific implementations. I think most WPF programmers have more or less this exact RelayCommand implementation in their arsenal.
Im working on a WPF app and I am trying to use the MainWindowViewModel that holds various views as a type of messenger to pass information between views:
The following is done in a child viewmodel:
public event Action<ModelObject> NameOfEvent= delegate {};
public void Open_Command()
{
ModelObject modelObject= RandomViewModel.ImportModelObject();
NameOfEvent(modelObject); //event is triggered while running the app
}
Then in the constructor of my MainWindowViewModel (the parent of the above view model) I am subscribing to the event. And Its not picking it up
private readonly RandomViewModel _randomViewModel = new RandomViewModel();
public MainWindowViewModel()
{
Random= _randomViewModel; // sets view model to a bindable
//property that lods the view in the main window
_randomViewModel.NameOfEvent+= DoSomething; //subscribes to childs event
}
private void DoSomething(ModelObject obj)
{
//It never reaches here
}
To summarise the issue. When the event is being triggered in the child view model, the parent is not executing DoSomething method, it doesnt seem to work, i cant figure out why
Ok so my issue as seen from the comments above was that I had a double instance of my childViewModel, this was done as I employed two techniques that initialize view models:
1.) Setting Data Context in xaml of my view initializes a viewmodel
<UserControl.DataContext>
<local:MyViewModel>
</UserControl.DataContext>
2.) Using a technique I found to initialize view-models in a MainWindowViewModel:
<Window.Resources>
<DataTemplate DataType="{x:Type viewModel:MyViewModel}">
<view:MyView/>
</Window.Resources>
After you initialize in your MainWindowViewModel you assign it to a Bindable Property in the constructor and add it to xaml in a Content Control
Everything I learned so far for MVVM is to use DataContext but the 2nd method is a new way I found that works very well if you are initialising your viewmodels in a MainWindowViewModel.
I got this technique off of Brian Noyes course "MVVM in depth" on pluralsight.
So after I removed the customary :
<UserControl.DataContext>
<local:MyViewModel>
</UserControl.DataContext>
The solution worked, now my MainWindowViewModel holds all my viewmodels and can act as a messenger service similar to stuff that can be found in MVVM light e.t.c
I'm probably missing the point here, I've been digging around for some time now, looked at the different approaches to bind views and view models and how to navigate between them.
Setup:
1 MainWindow (ViewModel: MainWindowViewModel)
MainWindow contains some visuals and a ContentControl that is binded to a ViewModelBase. So in MainWindowViewModel I can set any other view(model) to display.
Got two user controls, one is a login form, the other one is for now a loading indicator.
Inside App.xaml
<DataTemplate DataType="{x:Type vms:LoginViewModel}">
<Views:LoginView />
</DataTemplate>
<DataTemplate DataType="{x:Type vms:LoadingViewModel}">
<Views:LoadingView />
</DataTemplate>
Goal:
From the ICommand inside LoginViewModel go back to the MainWindowViewModel, a long with the form data. MainWindowViewModel will then switch to LoadingViewModel, do async call to a service. Next time on startup when a refresh token is saved, I will show the LoadingViewModel instead of the login form. On completion a new window will open (or something else, don't know yet).
Problem: Most examples show how to do it when a button is outside of both User Controls, so when the ICommand is inside the MainWindow, then it would be easy, yet the event is called from one of the subviews. Using PropertyChange seems a bit off as well.
Let's dig into some code, shell we?
MainWindow.xaml, only one important line
<ContentControl Content="{Binding CurrentView}" Grid.Column="1" Grid.Row="1" />
MainWindow.xaml.cs, in constructor
this.DataContext = new MainWindowViewModel();
MainWindowViewModel
public class MainWindowViewModel
{
public ViewModelBase CurrentView { get; set; }
public MainWindowViewModel()
{
CurrentView = new LoginViewModel();
}
}
LoginViewModel
public class LoginViewModel : ViewModelBase
{
public DelegateCommand loginCommand { get; set; }
public LoginViewModel()
{
loginCommand = new DelegateCommand(Execute, CanExecute);
}
private bool CanExecute()
{
return true;
}
private void Execute()
{
//I need to go to MainWindowViewModel
throw new NotImplementedException();
}
//more properties below heere.
ViewModelBase inherits from BindableBase (from Prism), so that handles the PropertyChanged events. The properties inside my view models use the correct SetProperty methods. I prefer not to use Prism's region, IEventAggregator or Unitiy.
Solutions
What comes to mind is sending an interface a long with the constructor, and use the interface to do 'callbacks' to the MainWindowViewModel, but I guess this will give errors, since I will change the view, and thus setting MainWindowViewModel.CurrentView to something else, leaving LoginViewModel null. Since the request is coming from that object I can imagine it is not really good.
A good way to communicate is the Messanger (MVVM Light) or EventAggregator (Prism) concept.
It's basically an in memory pub/sub system.
Here is an example from an article on MSDN
MVVM - Messenger and View Services in MVVM
Using the event aggregator pattern to communicate between view models
I am not sure which framework/library are you using but most MVVM framework have a similar concept for lousily coupled communication.
This is a very powerful concept for handling communication. But with great power comes responsibility =)...
HTH
I'd like to reuse a view for 2 different viewmodels, in my example MyEntityEditViewModel and MyEntityCreateViewModel. The view is basically just a form with a Save button, so pretty common layout.
I created both view models along with a parent view / view model (MyEntitySummaryViewModel) and now I'd like to define the form view using a ContentControl.
Summary view:
<ContentControl x:Name="ActiveItem" cal:View.Model="{Binding ActiveItem}" cal:View.Context="MyEntityDetailView" />
MyEntitySummaryViewModel:
public MyEntity SelectedEntity {
get { return _selectedEntity; }
set {
_selectedEntity = value;
NotifyOfPropertyChange();
ActivateItem(new MyEntityEditViewModel(_selectedEntitity));
}
}
public void Create() {
ActivateItem(new MyEntityCreateViewModel(new MyEntity()));
}
My problem is now that Caliburn tries to locate a 'MyEntityEditView' due to it's view locating conventions, even if I strictly defined the context of the ContentControl as a custom view. Is there a way around this? Or am I doing something completely wrong here?
If my understanding is right, You want 2 type of ViewModel to point on the same view. If so juste create a base classe for your Entity (EntityBaseViewModel) and Create a View (EntityBaseView).
To Bind a ContentControl set his x:Name so the name match a Property of your ViewModel.
Example:
View (ShellView):
<ContentControl x:Name="SelectedEntity"/>
ViewModel (ShellViewModel):
public EntityBaseViewModel SelectedEntity
{
get
{
return this._selectedEntity;
}
set
{
this._selectedEntity = value;
this.NotifyOfPropertyChange(() => SelectedEntity);
}
}
And Caliburn will find the View for the ViewModel and bind the DataContext if you did create your ViewModel / View along the naming convention like you said.
A little late to the party, but perhaps this will help someone. This video helped a lot for me - (Tim Corey, WPF and Caliburn with MVVM)
Setting up the ShellView with a control that points to ActiveItem as you mentioned, allows that control to display whatever view you tell it from the ShellViewModel code. I was also using Fody with this project, so that took care of the change notifications so you won't see those listed in code.
ShellView -
<Button x:Name="LoadMainPage" />
<Button x:Name="LoadSecondPage" />
<ContentControl x:Name="ActiveItem"/>
The ShellViewModel -
public class ShellViewModel : Conductor<object>.Collection.OneActive
{
public MainPageViewModel MainPageVM = new MainPageViewModel();
public SecondPageViewModel SecondPageVM = new SecondPageViewModel();
public ShellViewModel()
{
LoadMainPage(); // auto load main page on startup
}
public void LoadMainPage()
{
ActivateItem(MainPageVM);
}
public void LoadSecondPage()
{
ActivateItem(SecondPageVM);
}
}
Instead of creating a new instance of a ViewModel when using ActivateItem, you're just re-using the initial ones created. Or, if you DO prefer to create another instance each time that particular view is launched, then simply use the ActivateItem as you already have.
In your SecondPageViewModel for the view, which will occupy the space in the ContentControl for ActiveItem -
public class SecondPageViewModel : Screen
SecondPageView.xaml added as a User Control (and any other sub/child views you want to create) -
<UserControl x:Class="MyNamespace.Views.SecondPageView"
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:MyNamespace.Views"
mc:Ignorable="d"
d:DesignHeight="450" d:DesignWidth="800">
<Grid>
</Grid>
</UserControl>
This will allow you to flip back and forth between whatever views you want from a base view/viewmodel, and display the contents of the child views (however many you want) that you choose within the ContentControl box.