Develop Reference

Conditional settings in WPF application

I'm making user changeable settings for my media player and I'm struggling to find an elegant solution to the problem.
One of my settings for example - pauses the video at it's last frame, if not checked it will either continue through the playlist or if it's only 1 file, reset it and pause it at the start.
This is how I've implemented it:
private void OnMediaEndedCommand()
{
if (GeneralSettings.PauseOnLastFrame)
{
MediaPlayer.SetMediaState(MediaPlayerStates.Pause);
return;
}
if (PlayListViewModel.FilesCollection.Last().Equals(PlayListViewModel.FilesCollection.Current) && !Repeat)
{
ChangeMediaPlayerSource(PlayListViewModel.ChangeCurrent(() => PlayListViewModel.FilesCollection.MoveNext()));
MediaPlayer.SetMediaState(MediaPlayerStates.Stop);
return;
}
ChangeMediaPlayerSource(PlayListViewModel.ChangeCurrent(() => PlayListViewModel.FilesCollection.MoveNext()));
}
This is contained inside the ViewModel of the main window, where the media element is and GeneralSettings.PauseOnLastFrame is a boolean property.
This command is binded as follows:
<MediaElement ....>
<ia:Interaction.Triggers>
<ia:EventTrigger EventName="MediaEnded">
<ia:InvokeCommandAction Command="{Binding MediaEndedCommand}"/>
</ia:EventTrigger>
</ia:Interaction.Triggers>
</MediaElement>
It works but it's awful, how should I go about implementing such setting system in an elegant way? Some settings might not be boolean, they might have multiple options, some might be applied only on startup and others, as the one illustrated above, event based.

Based on the information and sample code you provided, I would suggest
Approach - 1
A tightly couple ViewModel with System.Configuration.ApplicationSettingsBase and you can mention all you properties in ViewModel and map single of them with a separate application setting property. You can use your settings directly in biding afterwards e.g. : {x:Static Settings.Default.Whatevs}. Othe "Save" button click event or main window close event, you can save all you settings e.g. : Settings.Default.Save();
Approach - 2
A better approach, I would suggest / prefer (if I am developing this app) is to develop a wrapper class (e.g.: SettingProvider) that implement an inheritance (e.g: ISettingProvider) which uncovers all you settings as separate properties and also have a save method which saves all setting values. You can use this wrapper class into your ViewModel to handle all the commands and setting values in better way.
The benefit of this approach is the if you decide to change you setting to database , you need not to make change to you ViewModel as all job is done in SettingProvider class.
I am not sure but based on viewing your code, I assume that you used Approach-1. Please put you comments and any feedback to this answer. I would like to know what you think and may be you have got more simple and interesting way of achieving this.
UPDATE-1
Example
Enum for showing you demo
public enum MediaStatus
{
Playing = 0,
Stopped = 1,
Paused = 2
}
Interface
public interface ISettingProvider
{
double Volumne { get; set; }
string LastMediaUrl { get; set; }
MediaStatus PlayingMediaStatus;
void SaveSettings();
}
Wrapper Class
public class SettingProvider : ISettingProvider
{
private double volumne;
public double Volumne // read-write instance property
{
get
{
return volumne;
}
set
{
volumne = value;
Settings.Default.Volumne = volumne;
}
}
private string lastMediaUrl;
public string LastMediaUrl // read-write instance property
{
get
{
return lastMediaUrl;
}
set
{
lastMediaUrl = value;
Settings.Default.LastMediaUrl = lastMediaUrl;
}
}
private MediaStatus playingMediaStatus;
public MediaStatus PlayingMediaStatus // read-write instance property
{
get
{
return playingMediaStatus;
}
set
{
playingMediaStatus = value;
Settings.Default.PlayingMediaStatus = (int)playingMediaStatus;
}
}
public void SaveSettings()
{
Settings.Default.Save();
}
//Constructor
public SettingProvider()
{
this.Volumne = Settings.Default.Volumne;
this.LastMediaUrl = Settings.Default.LastMediaUrl;
this.PlayingMediaStatus = (MediaStatus)Settings.Default.PlayingMediaStatus;
}
}
ViewModelBase Class
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propName)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(propName));
}
}
}
CommandHandler Class
public class CommandHandler : ICommand
{
public event EventHandler CanExecuteChanged { add { } remove { } }
private Action<object> action;
private bool canExecute;
public CommandHandler(Action<object> action, bool canExecute)
{
this.action = action;
this.canExecute = canExecute;
}
public bool CanExecute(object parameter)
{
return canExecute;
}
public void Execute(object parameter)
{
action(parameter);
}
}
ViewModel
public class SettingsViewModel : ViewModelBase
{
SettingProvider objSettingProvider = new SettingProvider();
public double Volumne
{
get
{
return objSettingProvider.Volumne;
}
set
{
objSettingProvider.Volumne = value;
OnPropertyChanged("Volumne");
}
}
// Implementaion of other properties of SettingProvider with your ViewModel properties;
private ICommand saveSettingButtonCommand;
public ICommand SaveSettingButtonCommand
{
get
{
return saveSettingButtonCommand ?? (saveSettingButtonCommand = new CommandHandler(param => saveSettings(param), true));
}
}
private void saveSettings()
{
objSettingProvider.SaveSettings();
}
}
UPDATE-2
public interface ISettingProvider
{
bool PauseOnLastFrame;
bool IsAutoPlay;
MediaStatus PlayingMediaStatus;
void SaveSettings();
}
public class SettingProvider : ISettingProvider
{
private bool pauseOnLastFrame;
public bool PauseOnLastFrame // read-write instance property
{
get
{
return pauseOnLastFrame;
}
set
{
pauseOnLastFrame = value;
Settings.Default.PauseOnLastFrame = volumne;
}
}
private bool isAutoPlay;
public bool IsAutoPlay // read-write instance property
{
get
{
return isAutoPlay;
}
set
{
isAutoPlay = value;
Settings.Default.IsAutoPlay = volumne;
}
}
}
public class SettingsViewModel : ViewModelBase
{
SettingProvider objSettingProvider = new SettingProvider();
MediaStatus PlayingMediaStatus
{
get
{
return objSettingProvider.PlayingMediaStatus;
}
set
{
if(value == MediaStatus.Paused)
MediaPlayer.Pause();
if(value == MediaStatus.Playing)
MediaPlayer.Play();
if(value == MediaStatus.Stopped)
MediaPlayer.Stop();
objSettingProvider.PlayingMediaStatus = (int)value;
OnPropertyChanged("PlayingMediaStatus");
}
}
private string currentMediaFile;
public string CurrentMediaFile
{
get
{
return currentMediaFile;
}
set
{
currentMediaFile = value;
MediaPlayer.Stop();
MediaPlayer.Current = currentMediaFile;
if(objSettingProvider.IsAutoPlay)
MediaPlayer.Play();
OnPropertyChanged("CurrentMediaFile");
}
}
// Implementaion of other properties of SettingProvider with your ViewModel properties;
private ICommand onMediaEndedCommand;
public ICommand OnMediaEndedCommand
{
get
{
return onMediaEndedCommand ?? (onMediaEndedCommand = new CommandHandler(param => onMediaEnded(param), true));
}
}
private void onMediaEnded()
{
if(objSettingProvider.PauseOnLastFrame)
{
PlayingMediaStatus = MediaStatus.Paused;
}
else if(PlayListViewModel.FilesCollection.Last().Equals(PlayListViewModel.FilesCollection.Current) && !Repeat)
{
PlayingMediaStatus = MediaStatus.Stopped;
}
else
{
CurrentMediaFile = PlayListViewModel.FilesCollection.MoveNext();
}
}
}
NOTE: This is the detailed example I put here and also avoid some syntax error or naming error if I missed somewhere. Please correct it.
I am not aware which media player settings you are using. I took some sample properties. This is just an example of structure you can implement for you application. You may need to alter more code to implement this structure.

An elegant way to implement this IMHO would be to use a dependency injection container, this will provide great flexibility while allowing you to completely separate concerns (i.e. the settings implementation from your view models and custom controls).
There are many DI frameworks out there, for my example I will use simple injector because it is free (open source), simple and fast but you can apply the same principle to other frameworks (Unity, Ninject, etc..).
Start by creating an interface for your settings service, for example:
public interface ISettingsService
{
double Volumne { get; set; }
string LastMediaUrl { get; set; }
MediaStatus PlayingMediaStatus;
void SaveSettings();
}
Then add your implementation for the service, the beauty of using DI is that you can change the implementation at anytime or completely replace it and your application will continue to work as usual.
Let's say you want to use application settings, here is your service:
public class SettingsServiceFromApplication : ISettingsService
{
public double Volume
{
get
{
return Properties.Settings.Volume;
}
}
[...]
}
Or let's say you want to use a database to store your settings:
public class SettingsServiceFromDb : ISettingsService
{
public double Volume
{
get
{
return MyDb.Volumen;
}
}
[...]
}
Then you can use a DI container to specify which implementation to use:
Start by installing the library using NuGet:
Install-Package SimpleInjector -Version 4.0.12
You need a way to share your container throughout the application, I usually just go with a static class that I initialize when starting the app:
using Container = SimpleInjector.Container;
namespace YourNamespace
{
public class Bootstrapper
{
internal static Container Container;
public static void Setup()
{
//Create container and register services
Container = new Container();
//Let's specify that we want to use SettingsServiceFromApplication
Container.Register<ISettingsService, SettingsServiceFromApplication>();
//You can use your bootstrapper class to initialize other stuff
}
}
You need to call Setup when starting the App, the best place is in the App constructor:
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
Bootstrapper.Setup();
}
}
So now you have an app wide depedency injection container that you can use to request "services" (specific implementations of an interface).
To get the settings implementation in your view models you could simply call the container as follows:
// This will return an instance of SettingsServiceFromApplication
ISettingsService settingsService = Bootstrapper.Container.GetInstance<ISettingsService>();
double volumen = settingsService.Volume;
To make it easier to work with, I usually create a base view model that will allow to get services more easyly, for example:
public abstract BaseViewModel
{
private ISettingsService _settings;
protected ISettingsService GeneralSettings
{
get
{
if (_settings == null)
_settings = Bootstrapper.Container.GetInstance<ISettingsService>();
return _settings;
}
}
}
Every view model inheriting from this class will have access to the settings:
public class YourViewModel : BaseViewModel
{
private void OnMediaEndedCommand()
{
if (GeneralSettings.PauseOnLastFrame)
{
MediaPlayer.SetMediaState(MediaPlayerStates.Pause);
return;
}
if (PlayListViewModel.FilesCollection.Last().Equals(PlayListViewModel.FilesCollection.Current) && !Repeat)
{
ChangeMediaPlayerSource(PlayListViewModel.ChangeCurrent(() => PlayListViewModel.FilesCollection.MoveNext()));
MediaPlayer.SetMediaState(MediaPlayerStates.Stop);
return;
}
ChangeMediaPlayerSource(PlayListViewModel.ChangeCurrent(() => PlayListViewModel.FilesCollection.MoveNext()));
}
}
As you can see the code is the same as your code! But now the settings are coming from your container. Where is the elegance? Well, let's say that one year from now someone decides that you will store your settings in a database, what do you need to change in your code?
Container.Register<ISettingsService, SettingsServiceFromDb>();
A single line. Everything else should work as usual.
As well as view models, you could use this mechanism in your own controls:
public class MyMediaElement : UserControl //Or MediaElement and instead of commands you can override real events in the control code behind, this does not break the MVVM pattern at all, just make sure you use depedency properties if you need to exchange data with your view models
{
private void OnMediaEndedCommand()
{
//Get your settings from your container, do whatever you want to do depending on the settings
[...]
}
}
Then just use your control in your Views / ViewModels:
<local:MyMediaElement />
Yep, that's all you need because you handle everything in your User / Custom control, your view models doesn't need to care about how you handle settings in the control.
There are many options you can use to register containers, I recommend you take a look at the docs:
https://simpleinjector.org/index.html
https://simpleinjector.readthedocs.io/en/latest/index.html

I think maybe you are looking for an interface approach?
public interface IMediaEndedHandler
{
bool AlternateHandling(MediaPlayer player);
}
public class NullMediaEndedHandler : IMediaEndedHandler
{
public bool AlternateHandling(MediaPlayer player)
{
return false;
}
}
public class PauseOnLastFrameHandler : IMediaEndedHandler
{
public bool AlternateHandling(MediaPlayer player)
{
player.SetMediaState(MediaPlayerStates.Pause);
return true;
}
}
public class GeneralSettings
{
private bool pauseOnLastFrame;
private bool PauseOnLastFrame
{
get
{
return pauseOnLastFrame;
}
set
{
pauseOnLastFrame = value;
MediaEndedHandler = value
? new PauseOnLastFrameHandler()
: new NullMediaEndedHandler();
}
}
public IMediaEndedHandler MediaEndedHandler = new NullMediaEndedHandler();
}
Then:
private void OnMediaEndedCommand()
{
if (GeneralSettings.MediaEndedHandler.AlternateHandling(MediaPlayer))
return;
if (PlayListViewModel.FilesCollection.Last().Equals(PlayListViewModel.FilesCollection.Current) && !Repeat)
{
ChangeMediaPlayerSource(PlayListViewModel.ChangeCurrent(() => PlayListViewModel.FilesCollection.MoveNext()));
MediaPlayer.SetMediaState(MediaPlayerStates.Stop);
return;
}
ChangeMediaPlayerSource(PlayListViewModel.ChangeCurrent(() => PlayListViewModel.FilesCollection.MoveNext()));
}
This way, if your setting is, for example. an enum instead of a bool, you can specify a different implementation of the interface for each possible value.

Set a Property in a ViewModel from Another ViewModel

I am trying to pass a value to a view model from another view model before navigating to the page attached to that view model.
I was previously passing it to the view, then passing it to the view model. This seems like a clumsy way of doing things.
I am not using any kind of framework so that is not an option.
At the moment the property is set as static and this works but im not sure if this is good practice.
The code:
View model 1:
This command opens the new page:
public void OpenRouteDetails()
{
RouteStopPopOverViewModel.RouteName = "TestRoute";
App.Page.Navigation.PushAsync(new RouteStopPopOverView());
}
View model 2: (RouteStopPopOverViewModel)
public static string RouteName { get; set; }
This does work but I would prefer not to use static as a way to achieve this.
Is there some way to set the RouteName property without using static or passing it through view-> view model.
I have seen some answers about this but they don't seem to answer to question clearly.

Share a controller class between view models.
The same instance has to be supplied to the constructor in both view models.
So you can set values, and listen for events in both view models.
The controller class becomes the intermediary.
public class SharedController : IControlSomething
{
private string _sharedValue;
public string SharedValue
{
get => _sharedValue;
set
{
if (_sharedValue == value)
return;
_sharedValue = value;
OnSharedValueUpdated();
}
}
public event EventHandler SharedValueUpdated;
protected virtual void OnSharedValueUpdated()
{
SharedValueUpdated?.Invoke(this, EventArgs.Empty);
}
}
public class ViewModel1
{
private readonly IControlSomething _controller;
public ViewModel1(IControlSomething controller)
{
// Save to access controller values in commands
_controller = controller;
_controller.SharedValueUpdated += (sender, args) =>
{
// Handle value update event
};
}
}
public class ViewModel2
{
private readonly IControlSomething _controller;
public ViewModel2(IControlSomething controller)
{
// Save to access controller values in commands
_controller = controller;
_controller.SharedValueUpdated += (sender, args) =>
{
// Handle value update event
};
}
}

here the sample you can achieve your requirement easily with navigation
public class ViewModelFrom : BaseViewModel
{
async Task ExecuteCommand()
{
string routeName="value to trasfer";
Navigation.PushAsync(new View(routeName));
}
}
public partial class View : ContentPage
{
public View(string routeName)
{
InitializeComponent();
BindingContext = new ViewModelTo(routeName);
}
}
public class ViewModelTo : BaseViewModel
{
public string RouteName { get; set; }
public ViewModelTo(string routeName)
{
RouteName=routeName;
}
}

If there is a hierarchy you could express that in a parent to both of them.
public class Route
{
private string Name;
}
public class RouteSelectedArgs : EventArgs
{
public Route Selected { get; set; }
}
public interface IRouteSelection
{
event EventHandler<RouteSelectedArgs> RouteSelected;
}
public interface IRouteDetails { }
public class RouteWizard
{
public UserControl view { get; set; }
private IRouteSelection _selection;
private IRouteDetails _details;
public RouteWizard(IRouteSelection selection, IRouteDetails details)
{
_selection = selection;
_details = details;
_selection.RouteSelected += Selection_RouteSelected;
view = MakeView(_selection);
}
private void Selection_RouteSelected(object sender, RouteSelectedArgs e)
{
_selection.RouteSelected -= Selection_RouteSelected;
view = MakeView(_details, e.Selected);
}
private UserControl MakeView(params object[] args)
{
////magic
throw new NotImplementedException();
}
}

As you are using the MVVM pattern, you can use one of the many MVVM Frameworks to achieve this.
I use FreshMvvm and it allow me to pass parameters between view models like this
await CoreMethods.PushPageModel<SecondPageModel>(myParameter, false);
Then in SecondPageModel I can see access the parameters in the Init method
private MyParamType _myParameter;
public override void Init(object initData)
{
base.Init(initData);
var param = initData as MyParamType;
if (param != null)
{
_myParameter = param;
}
}
You can find more details about FreshMvvm here although most MVVM frameworks have similar functionality.

MVVM pattern: an intermediate View between Command binding and ViewModel execute

Scenario
Some date are loaded into a program (e.g., evaluation of students in a class where each student is a distinct entity with his/her evaluation data) and a summary of them is shown on a datagrid. The user selects selects some of the students, and performs an analysis on their evaluation. The analysis process requires some parameters, therefore before analysis a window pops-up and lets user to specify his preferred parameters; then the analysis process executes.
Implementation summary
The datagrid is defined as following and binded to a ViewModel:
<DataGrid x:Name="CachedSamplesDG" ItemsSource="{Binding cachedDataSummary}">
<DataGrid.Columns>
<DataGridTextColumn Header="name" Binding="{Binding name}"/>
<DataGridTextColumn Header="score" Binding="{Binding score}"/>
</DataGrid.Columns>
</DataGrid>
The button that starts the process is defined as following:
<Button x:Name="AnalysisBT" Content="Analyze" Command="{Binding AnalyzeCommand}" CommandParameter="{Binding ElementName=CachedSamplesDG, Path=SelectedItems}"/>
The ViewModel is pretty basic and summarized as following:
internal class CachedDataSummaryViewModel
{
public CachedDataSummaryViewModel()
{
_cachedDataSummary = new ObservableCollection<CachedDataSummary>();
AnalyzeCommand = new SamplesAnalyzeCommand(this);
}
private ObservableCollection<CachedDataSummary> _cachedDataSummary;
public ObservableCollection<CachedDataSummary> cachedDataSummary { get { return _cachedDataSummary; } }
public ICommand AnalyzeCommand { get; private set; }
}
And here is the definition of analysis command:
internal class SamplesAnalyzeCommand : ICommand
{
public SamplesAnalyzeCommand(CachedDataSummaryViewModel viewModel)
{
_viewModel = viewModel;
}
private CachedDataSummaryViewModel _viewModel;
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public bool CanExecute(object parameter)
{
// canExecute logic
}
public void Execute(object parameter)
{
// process mess ...
// Here I need the selected rows of datagird, which "parameter" delegates them.
// I also need some other parameters for analysis which user can set through another view
}
}
An this is a diagram of my current process and what I would like to do next
Question
When the button is clicked
Apply some UI changes on MainWindow
Pop-up ProcessOptionsWindow
Get set parameters from ProcessOptionsWindow
Pass the selected datagrid rows and user specified parameters to SamplesAnalyzeCommand
What would be the best way to achieve this requirement ?

simply use a dialogservice like Good or bad practice for Dialogs in wpf with MVVM?.
then you can do something like this in your ViewModel
var result = this.uiDialogService.ShowDialog("Prozess Options Window", prozessOptionVM);
...
var parameter1 = prozessOptionVM.Parameter1;

You can define another Model and ViewModel for Process Options, and then in the SamplesAnalyzeCommand, display the ProcessOptionsView. When user is done with the ProcessOptionsView, the main ViewModel gets notified (e.g by an event handler) and completes the Process.
Something like this:
internal class SamplesAnalyzeCommand : ICommand {
...
public void Execute(object parameter)
{
this._viewModel.ShowProcessOptions(parameter);
}
}
internal class CachedDataSummaryViewModel {
public string Status {
get {
return this.status;
}
set {
if (!string.Equals(this.status, value)) {
this.status = value;
// Notify property change to UI
}
}
}
...
internal void ShowProcessOptions(object paramter) {
// Model
var processOptions = new ProcessOptionsModel() {
otherInfo = parameter
};
// View-Model
var processOptionsViewModel = new ProcessOptionsViewModel();
processOptionsViewModel.Model = processOptions;
// View
var processOptionsView = new ProcessOptionsView(
processOptionsViewModel
);
// Edit2: Update status
this.Status = "Selecting process options...";
// You can use the event handler or dialog result
processOptionsViewModel.OK += this.PerformProcess;
processOptionsView.ShowDialog();
}
private void PerformProcess(object sender, EventArgs e) {
var processOptionsView = sender as ProcessOptionsView;
var processOptionsModel = processOptionsView.Model;
var processOptions = processOptionsModel.Model;
// Edit2: Update status
this.Status = "Performing process...";
// use processOptions.OtherInfo for initial info
// use processOptions.* for process options info
// and perform the process here
// Edit2: Update status
this.Status = "Process Done.";
}
...
}
class ProcessOptionsModel {
public object OtherInfo {
get;
set;
public int Parameter1 {
get;
set;
}
public IList<ProcessItem> SelectedItems {
get;
set;
}
...
}
class ProcessOptionsViewModel {
public event EventHandler OK;
private SamplesAnalyzeCommand model;
private ICommand okCommand;
public ProcessOptionsViewModel() {
this.okCommand = new OKCommand(this.OnOK);
}
public SamplesAnalyzeCommand Model {
get {
return model;
}
set {
this.model = value;
// Property changed stuff here
}
}
private void OnOK(object parameter) {
if (this.OK != null) {
this.OK = value;
}
}
}
class ProcessOptionsView {
// Interacts with it's view-model and performs OK command if
// user pressed OK or something
}
Hope it helps.
Edit (1):
As blindmeis suggested, you may use some Dialog Service to make the connection between the views.
Edit (2):
Immidiate UI changes after button click can be done in ShowProcessOptions method of the ShowProcessOptions. I don't think you want reflect ui changes of the options window while user works with it, to the main window. UI changes after user closes options window can be done in PerformProcess.
If you want to make an abstraction for options selection (e.g reading from a file) as you mentioned in the comment below, you may define an IOptionsProvider interface, and put ProcessOptionsView and View-Model behind that but still you use the same model.
interface IOptionsProvider {
ProcessOptionsModel GetProcessOptions();
}
class ProcessOptionsView : IOptionsProvider {
public ProcessOptionsModel GetProcessOptions() {
if (this.ShowDialog()) {
return this.ModelView.Model;
}
return null;
}
}
class ProcessOptionsFromFile : IOptionsProvider {
public ProcessOptionsModel GetProcessOptions() {
// Create an instance of ProcessOptionsModel from File
}
}
Note that in this case I removed the OK event since the GetProcessOptions is supposed to block until user closes the main window. If you want a responsive approach in the FromFile case, you may need to work on the async stuff, maybe define GetProcessOptionsAsync instead.
In this case things may get a little bit complicated but I guess it is achievable in this way.

caliburn.micro Condutor not working

We are using caliburn.micro for one of our projects and I'm currently having a puzzling problem:
we have the following classes:
ToolViewerViewModel : Conductor<Screen>.Collection.OneActive
DocViewerViewModel : Conductor<DocumentViewModel>
and various document-views, all with this base class:
DocumentViewModel : Screen
The ToolViewerViewModel is to manage multiple dock-able tool views which allow the user to control different aspects of the program.
The DocViewerViewModel is to show the user the data he's working on/with. It is here to present one of the many DocumentViewModel to the user. and is implemented as a special dock-able view which can not be closed or detached from the ToolViewerView. For every aspect of the data a specific DocumentViewModel is generated by the DocViewerViewModel and presented to the user.
The DocumentViewModel is the base class for all presentation aspects of the data. One may present the data as a table an other may present it as a chart, and so on...
We now encounter problems in terms of OnActivate() and OnDeactivate() which are not called when we expect them to be called.
First Problem:
The system is up and running; The DocumentViewModel is displayed in the DocViewerViewModel which is embedded in the ToolViewerViewModel along with one or two other dock-able views. The currently selected dock-able view is the DocViewerViewModel. When the user now selects one of the other dock-able views the OnDeactivate() method from the DocumentViewModel is being called. Which makes absolutely no sense to me. I'd expect the DocViewerViewModel.OnDeactivate() to be called.
Second Problem:
The system is up and running; The DocumentViewModel is displayed in the DocViewerViewModel which is embedded in the ToolViewerViewModel along with one or two other dock-able views. The currently selected dock-able view is the view that enables the user to change the DocumentViewModel presented by the DocViewerViewModel. When the user now selects an other DocumentViewModel the following code is being executed within the DocViewerViewModel:
DocViewerViewModel.DeactivateItem(oldDocumentViewModel, true);
DocViewerViewModel.ActivateItem(new DocumentViewModel());
I'd expect the DocumentViewModel.OnDeactivate() to be called upon the DocViewerViewModel.DeactivateItem(oldDocumentViewModel, true) call. but that never happens.
Conclusion:
The only proper working Conductor is the ToolViewerViewModel which is managing everything. But this behavior is not what we want or expect to happen: We'd like to have the ToolViewerViewModel only Conduct the dock-able views and the DocViewerViewModel to conduct the DocumentViewModel. This is important because there are two different use cases in place: One to manage multiple instances at the same time and the other where only one instance is active and used, the old instance shall be thrown away.
Hopefully anyone here can help me to get the behavior I'm looking for.
I Now have an example code for you:
public class ToolViewerViewModel : Conductor<Screen>.Collection.OneActive
{
private readonly IDockManager _dockManager;
private readonly DocViewerViewModel _docViewerViewModel;
private readonly IList<DockableViewModel> _toolViews = new List<DockableViewModel>();
public ToolViewerViewModel(IViewModelFactory viewModelFactory, DocViewerViewModel docViewerViewModel, IDockManager dockManager)
{
_dockManager = dockManager;
_viewModelFactory = viewModelFactory;
_docViewerViewModel = docViewerViewModel;
}
protected override void OnViewLoaded(object view)
{
_dockManager.Link(this);
_dockManager.CreateSpecialPaneFor(_docViewerViewModel);
ActivateItem(_docViewerViewModel);
ShowToolView<ProjectExplorerViewModel>();
base.OnViewLoaded(view);
}
public void ShowToolView<T>() where T : DockableViewModel
{
if (!IsToolViewOpen<T>())
{
var viewModel = _viewModelFactory.Create<T>();
ActivateItem(viewModel);
RefreshMenu(typeof(T));
}
}
}
Next class:
public class DocViewerViewModel : Conductor<DocumentViewModel>
{
private readonly IViewModelFactory _viewModelFactory;
public DocViewerViewModel(IViewModelFactory viewModelFactory)
{
_viewModelFactory = viewModelFactory;
}
public bool ShowInMainView<T>() where T : DocumentViewModel
{
return ShowInMainView(typeof(T));
}
private bool ShowInMainView(Type viewModelType)
{
var ret = false;
// close the current view
if (ActiveItem != null)
{
DeactivateItem(ActiveItem, true); //The close flag is on true since we want to remove the current instance from the memory
}
// check whether the current viewModel has been closed successfully
if (ActiveItem == null)
{
try
{
var viewModel = _viewModelFactory.Create(viewModelType) as DocumentViewModel;
if (viewModel != null)
{
ActivateItem(viewModel);
ret = true;
}
else
{
ActivateItem(_viewModelFactory.Create<NoDataViewModel>());
}
}
catch (Exception ex)
{
ActivateItem(_viewModelFactory.Create<NoDataViewModel>());
}
}
return ret;
}
}
and the last one:
public abstract class DocumentViewModel : Screen
{
private bool _isDirty;
protected IViewModelFactory ViewModelFactory { get; private set; }
protected IEventAggregator EventAggregator { get; private set; }
public bool IsDirty
{
get
{
return _isDirty;
}
protected set
{
if (value.Equals(_isDirty))
{
return;
}
_isDirty = value;
NotifyOfPropertyChange(() => IsDirty);
}
}
protected DocumentViewModel(IViewModelFactory viewModelFactory, IEventAggregator eventAggregator)
{
ViewModelFactory = viewModelFactory;
EventAggregator = eventAggregator;
}
protected override void OnDeactivate(bool close)
{
if (close)
{
if (EventAggregator != null)
{
EventAggregator.Unsubscribe(this);
}
}
base.OnDeactivate(close);
}
protected override void OnActivate()
{
if (EventAggregator != null)
{
EventAggregator.Subscribe(this);
}
base.OnActivate();
}
public override void CanClose(Action<bool> callback)
{
var ret = true;
if (IsDirty && (ViewModelFactory != null))
{
var saveDialog = ViewModelFactory.Create<SaveDialogViewModel>();
saveDialog.Show();
if (saveDialog.DialogResult == DialogResult.Cancel)
{
ret = false;
}
else
{
if (saveDialog.DialogResult == DialogResult.Yes)
{
Save();
}
else
{
Discard();
}
IsDirty = false;
}
}
callback(ret);
}
public abstract void Save();
public virtual void Discard()
{
}
}
With this code the only time the DocumentViewModel.OnDeactivate() is being called when the user brings an other dock-able view into focus while the DocViewerViewModel was having the focus. This should not happen!
When the user is changing the focus between the dock-able views the DocumentViewModel.OnDeactivate() should not get call. But it must get called when ever the Method DocViewerViewModel.ShowInMainView<SomeDocumentViewModel>() is being called. Which isn't the case currently.

As far as I can tell, there is nothing wrong with the way your code is written. Since you are using MVVM, I suggest you design a test case like I've provided here.
And here's a snippet of the test case
// TestHarness.cs
[TestMethod]
public void CheckDeactivation()
{
// We'd like to have the ToolViewerViewModel only Conduct the dock-able views
// and the DocViewerViewModel to conduct the DocumentViewModel.
IViewModelFactory factory = new ViewModelFactory();
DocViewerViewModel docViewer = new DocViewerViewModel(factory);
IDockManager dockManager = null;
var toolViewer = new ToolViewerViewModel(factory, docViewer, dockManager);
var mockToolView = new UserControl();
(toolViewer as IViewAware).AttachView(mockToolView);
DocumentViewModel docView1 = new NoDataViewModel();
DocumentViewModel docView2 = new NoDataViewModel();
docViewer.ActivateItem(docView1);
docViewer.ActivateItem(docView2);
Assert.AreEqual(0, docViewer.CountDeactivated());
}

I have had the exact same problem as you, and ended up using PropertyChangedBase instead of Screen and got the problem to disappear.
Later, after reading the docs on Screens and Conductors here, I realized that I wasn't activating the conductor itself further up in the view hierarchy!
So have a look at wherever you use your ToolViewerViewModel, and make sure you activate that instance!

Thank you very much for your Test. Even thought it is really nice code it tests the wrong code part. Your code simply tests whether the Method ActivateItem() or DeactivateItem() is being called:
public override void ActivateItem(DocumentViewModel item)
{
_countActivated++;
base.ActivateItem(item);
}
public override void DeactivateItem(DocumentViewModel item, bool close)
{
_countDeactivated++;
base.DeactivateItem(item, close);
}
But since these Methods are being called explicitly we don't need to test for that...
The real Problem is that the Conductor is not calling the OnActivate() or OnDeactivate() on the DocumentViewModel class. To enhance your test I used the following code:
public class DummyViewModelFactory : IViewModelFactory
{
private readonly Dictionary<Type, Func<object>> _registredCreators = new Dictionary<Type, Func<object>>();
public T Create<T>() where T : PropertyChangedBase
{
return Create(typeof(T)) as T;
}
public object Create(Type type)
{
if (type == null)
{
return null;
}
if (_registredCreators.ContainsKey(type))
{
return _registredCreators[type]();
}
return null;
}
public void Release(object instance)
{
}
public void RegisterCreatorFor<T>(Func<T> creatorFunction)
{
_registredCreators.Add(typeof(T), () => creatorFunction());
}
}
As concrete DocumentViewModel implementation I made:
public class NoDataViewModel : DocumentViewModel
{
public NoDataViewModel(IEventAggregator eventAggregator,
IViewModelFactory viewModelFactory)
: base(viewModelFactory, eventAggregator, )
{
}
public override void Save()
{
// nothing to do
}
public override void Reload()
{
// nothing to do
}
}
public class NoDataViewModelMock : NoDataViewModel
{
private static int activationCounterForTesting = 0;
private static int deactivationCounterForTesting = 0;
public static int ActivationCounterForTesting
{
get
{
return activationCounterForTesting;
}
}
public static int DeactivationCounterForTesting
{
get
{
return deactivationCounterForTesting;
}
}
public NoDataViewModelMock()
: base(null, null)
{
}
protected override void OnActivate()
{
activationCounterForTesting++;
base.OnActivate();
}
protected override void OnDeactivate(bool close)
{
deactivationCounterForTesting++;
base.OnDeactivate(close);
}
}
And I changed your Testmethod to this:
[TestMethod]
public void CheckDeactivation()
{
var viewModelFactory = new DummyViewModelFactory();
viewModelFactory.RegisterCreatorFor<NoDataViewModel>(() => new NoDataViewModelMock());
var docViewer = new DocViewerViewModel(viewModelFactory);
IDockManager dockManager = null;
var toolViewer = new ToolViewerViewModel(viewModelFactory, docViewer, dockManager);
var mockToolView = new UserControl();
(toolViewer as IViewAware).AttachView(mockToolView);
docViewerViewModel.ShowInMainView<NoDataViewModel>();
docViewerViewModel.ShowInMainView<NoDataViewModel>();
docViewerViewModel.ShowInMainView<NoDataViewModel>();
Assert.AreEqual(3, NoDataViewModelMock.ActivationCounterForTesting);
Assert.AreEqual(2, NoDataViewModelMock.DeactivationCounterForTesting);
}
Then you'll see that the OnActivate() and OnDeactivate() methods are never been called.
With a little more advanced test you'd also see that they are being called but from the ToolViewerViewModel directly. I'd like to know why and how I can change this behavior to fit my needs:
The DocumentViewModel.OnActivate() method should get called when the DocViewerViewModel.ShowInMainView<T>() method gets called.
The DocumentViewModel.OnDeactivate() method should get called on the old DocumentViewModel when a new one is being shown by calling the DocViewerViewModel.ShowInMainView<T>()

Our Solution for that Problem is to remove the use Screen as BaseClass for DocViewerViewModel an implement the Conductor Logic our self.

In WPF, how can a Command's CanExecute method gain visibility of other UI elements?

I've been using WPF for a while but I'm new to Commands, but would like to start using them properly for once. Following a code example, I've established a separate static Commands class to hold all of my commands, and it looks like this.
public static class Commands
{
public static RoutedUICommand OpenDocument { get; set; }
static Commands()
{
OpenDocument = new RoutedUICommand("Open Document", "OpenDocument", typeof(Commands));
}
public static void BindCommands(Window window)
{
window.CommandBindings.Add(new CommandBinding(OpenDocument, OpenDocument_Executed, OpenDocument_CanExecute));
}
private static void OpenDocument_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
// Should be set to true if an item is selected in the datagrid.
}
private static void OpenDocument_Executed(object sender, ExecutedRoutedEventArgs e)
{
}
}
My problem is that although the command is going to be bound to a Button control in MainWindow.xaml, the OpenDocument_CanExecute method needs to look at a DataGrid in MainWindow.xaml to see if an item is selected.
How can I wire things up such that the method can see the DataGrid?
SOLUTION
Inspired by Ken's reply (thanks again!), I put the following in place, which works perfectly.
MainWindow.xaml.cs
public partial class MainWindow
{
public MainWindow()
{
InitializeComponent();
Loaded += delegate
{
DataContext = ViewModel.Current;
Commands.BindCommands(this);
};
}
}
ViewModel.cs
public class ViewModel
{
private static ViewModel _current;
public static ViewModel Current
{
get { return _current ?? (_current = new ViewModel()); }
set { _current = value; }
}
public object SelectedItem { get; set; }
}
Commands.cs
public static class Commands
{
public static RoutedUICommand OpenDocument { get; set; }
static Commands()
{
OpenDocument = new RoutedUICommand("Open Document", "OpenDocument", typeof(Commands));
}
public static void BindCommands(Window window)
{
window.CommandBindings.Add(new CommandBinding(OpenDocument, OpenDocument_Executed, OpenDocument_CanExecute));
}
private static void OpenDocument_CanExecute(object sender, CanExecuteRoutedEventArgs e)
{
e.CanExecute = ViewModel.Current.SelectedItem != null;
}
private static void OpenDocument_Executed(object sender, ExecutedRoutedEventArgs e)
{
}
}

ICommand implementations work best in the MVVM pattern:
class ViewModel : INotifyPropertyChanged {
class OpenDocumentCommand : ICommand {
public bool CanExecute(object parameter) {
return ViewModel.ItemIsSelected;
}
public OpenDocumentCommand(ViewModel viewModel) {
viewModel.PropertyChanged += (s, e) => {
if ("ItemIsSelected" == e.PropertyName) {
RaiseCanExecuteChanged();
}
};
}
}
private bool _ItemIsSelected;
public bool ItemIsSelected {
get { return _ItemIsSelected; }
set {
if (value == _ItemIsSelected) return;
_ItemIsSelected = value;
RaisePropertyChanged("ItemIsSelected");
}
}
public ICommand OpenDocument {
get { return new OpenDocumentCommand(this); }
}
}
Obviously, I left out a whole bunch of stuff. But this pattern has worked well for me in the past.

why even implement a command if you are tightly coupling it to UI implementation? Just respond to datagrid.SelectionChanged and code in what supposed to happen.
Otherwise, put it in the ViewModel. Have the ViewModel monitor it's state and evaluate when CanExe is true.
Edit
On the other hand, you can pass a parameter to your command, as well as Exe() & CanExe() methods
//where T is the type you want to operate on
public static RoutedUICommand<T> OpenDocument { get; set; }

If you are doing an MVVM solution, this would be the perfect time to implement a publish / subscribe aggregator that allows controls to "talk" to each other. The gist behind it is that the datagrid would publish an event, 'Open Document'. Subsequent controls could subscribe to the event and react to the call to 'Open Document'. The publish / subscribe pattern prevents tightly coupling the datagrid and the control. Do some searches for event aggregators and I think you'll be on your way.