Develop Reference

Prism.WPF: Change MainWindow Window.Effect on Navigation

The application is a Prism Application in WPF using C#.
I am attempting to assign a BlurEffect to the Window.Effect property when a button is clicked on the navigation menu.
I have the Window.Effect bound to a property in my viewmodel.
<Window ... other properties ..
Effect = {Binding Fuzzy}>
and the Fuzzy property in the ViewModel.
private Effect _fuzzy;
public Effect Fuzzy { get => _fuzzy; set => SetProperty(ref _fuzzy, value); }
What I am attempting to implement is that when a button is clicked on the navigation menu that the window will blur while a UserControl is loading.
I have tried to implement the change in the Navigate method.
private void Navigate(string viewName)
{
PerformBlur();
_regionManager.RequestNavigate("ContentRegion", viewName);
}
private void PerformBlur()
{
BlurEffect blur = new BlurEffect();
blur.Radius = 4;
var ef = blur;
_fuzzy = ef; //I've tried Fuzzy = ef too
}
But that doesn't work.
I need to make the change to the window effect before it attempts to navigate, and I haven't been able to figure out how to make that happen. I have a feeling that the easiest way to do this would be to use a click event rather than a command, and then call the command in the viewmodel from the codebehind. However, that doesn't seem to be the proper implementation when using MVVM. Any suggestions on how to implement this functionality would be greatly appreciated.
(Bonus points if you can tell me how to animate the blur. lol)

I have a feeling that the easiest way to do this would be to use a click event rather than a command, and then call the command in the viewmodel from the codebehind. However, that doesn't seem to be the proper implementation when using MVVM.
Invoking the command programmatically from the code-behind of the view is not any worse than invoking it from the XAML markup of the very same view as far as MVVM is concerned.
MVVM is not about eliminating code from the views. It's about separation of concerns. You can implement an entire view programmtically in a C# without using XAML at all and still be fully compliant with MVVM.
Trying to do fairly complex stuff in XAML just because you possible can is generally considered as an antipattern. Remember that XAML is a markup language. C# is a much more expressive and concise language so if you can solve your issue by writing some code, then this is most probably what you should do.

Taking a look at the prism source code, I can see iregionmanager is full of abstracted interfaces.
( Wow. I don't know why it still surprises me but prism is very complicated ).
https://github.com/PrismLibrary/Prism/blob/master/src/Wpf/Prism.Wpf/Regions/IRegion.cs
That includes IRegionNavigationService
https://github.com/PrismLibrary/Prism/blob/master/src/Wpf/Prism.Wpf/Regions/IRegionNavigationService.cs
You could therefore override pretty much any functionality you like, if you wanted to.
Notice though, the two events :
/// <summary>
/// Raised when the region is about to be navigated to content.
/// </summary>
event EventHandler<RegionNavigationEventArgs> Navigating;
/// <summary>
/// Raised when the region is navigated to content.
/// </summary>
event EventHandler<RegionNavigationEventArgs> Navigated;
Looks to me like "all" you need is a reference to your region navigation service in the view.
Handle those two events to set blur then remove blur.
You could then do navigation in code behind or viewmodel. Whichever suits.
If you wanted to decouple viewmodel from view, you could use the eventaggregator.
There is another option though.
You don't explain exactly what you have there. So let's imagine and consider a better way to do this.
Say you have a set content of a set control you're always navigating. That's being switched out as you navigate for a new view whose datacontext is a new viewmodel.
You could bind an attached property from the window to the datacontext of that.
In that property you can have a change callback.
In a base viewmodel you could add an IsLoaded bool property which is initialy false.
When your dependency property callback returns null or false then you blur.
You change the viewmodel property to false in the current viewmodel when you start to navigate. The window blurs. The content is switched out and you get a new viewmodel. Once navigation completes you set that ILoaded true. You callback un blurs the window.

WPF - Assign a command to a control's event

I've a Telerik.RadGanttView component that implements some custom behavior, namely being able to adjust the zoom level of the timeline by using the scrollwheel over said timeline. This is done in the code-behind for the control with the following lines:
_gantt = GetTemplateChild("Gantt") as RadGanttView;
_gantt.PreviewMouseWheel += _gantt_PreviewMouseWheel;
_gantt_PreviousMouseWheel is the method that contains the logic to adjust the zoom level. While this works fine, I wanted to move the zooming logic to my view model and, if possible, bind the PreviousMouseWheel event to a command in my ViewModel rather than a method in my code-behind. I've tried using the Interaction.Triggers xaml tags, but while they did register the event, they only registered it when my mouse was NOT hovering over the timeline, which is the opposite of what I'm looking for.
Is there any way to achieve what is done in my code-behind and use it to call a command rather than a method in my code-behind?
Thanks a lot for your help!

Is there any way to achieve what is done in my code-behind and use it to call a command rather than a method in my code-behind?
Just invoke your command from the event handler programmatically:
_gantt = GetTemplateChild("Gantt") as RadGanttView;
_gantt.PreviewMouseWheel += (ss, ee) =>
{
var viewModel = DataContext as YourViewModel;
if (viewModel != null)
viewModel.YourCommand.Execute(null);
}
This is just as good as using an interaction trigger in the very same view to invoke the very same command as far as MVVM is concerned.
This is what I would have liked to do, but due to how the files are structured within the project, I cannot access the DataContext's type without introducing a circular dependency.
Well, then you have obviosuly messed up your structure. A workaround would be to use the dynamic language runtime (DLR) and the dynamic keyword to bypass the static type checking at compile time:
_gantt.PreviewMouseWheel += (ss, ee) =>
{
dynamic viewModel = DataContext;
viewModel.YourCommand.Execute(null);
}
This is not any verse than binding to a potentially unexisiting property using a string identifier in XAML really.

Custom TypeDescriptor and AddValueChange/OnValueChanged

I’m exploring the use of custom TypeDescriptors to intercept when a data-bound property on a WPF control sets a CLR property. Due to reasons I need to figure out which exact control is setting a value.
If I use INotifyPropertyChanged I’ll get all of the different controls that subscribe to any property in my view model, not all that helpful. So I figured I’d go down the custom TypeDescriptor path and try the ProperyDescriptor.AddValueChanged and PropertyDescriptor.OnValueChanged way.
I can see that WPF calls ProperyDescriptor.AddValueChanged as expected, but my call to ProperyDescriptor.OnValueChanged never seems to trigger a call to PropertyDescriptor.GetValue and an update in the UI.
Are there some gotchas to using this stuff that I’m missing? Or are there other ways to figure out who’s subscribing to a CLR property. My alternative as I see it now would be to emit a custom proxy for the whole shebang, but I’d very much like to not have do that.
Edit: On looking at bit closer, I noticed that the delegates I get through PropertyDescriptor.AddValueChanged go to some internal MS helper class, so without some unreliable reflection-fu there's no way to use that to get the source control. I think I'll have to go with a custom proxy with dependency properties.

There's an alternative that may be useful. In a binding, you can set the NotifyOnSourceUpdated to True, which will raise the control's SourceUpdated event when WPF updates the CLR property it's data-bound to. For example, in a TextBox you can write:
<TextBox
Text="{Binding MyText, NotifyOnSourceUpdated=True}"
SourceUpdated="TextBox_OnSourceUpdated"
/>
In the code-behind, you can get the exact control (the sender) and the name of the dependency property that updated the CLR property:
private void TextBox_OnSourceUpdated(object sender, DataTransferEventArgs e)
{
var control = (TextBox)sender;
var propertyName = e.Property.Name;
}
With this method, you need to know that the control was a TextBox, but you can probably update it to test different kinds of controls. It depends on what you need this for, really.

MVVM when to create a viewmodel for a control?

Or should I only create viewmodels for the domain data being represented? While reading on MVVM, I came across this:
"The ViewModel is responsible for these tasks. The term means "Model of a View", and can be thought of as abstraction of the view, but it also provides a specialization of the Model that the View can use for data-binding. In this latter role the ViewModel contains data-transformers that convert Model types into View types, and it contains Commands the View can use to interact with the Model. "
http://blogs.msdn.com/b/johngossman/archive/2005/10/08/478683.aspx
If the viewmodel is a model of the view, then doesn't it make sense to put properties of the view in the viewmodel rather than on the code behind of the view itself?
I guess in making a custom control I just have a hard time deciding when I should just add a property to the control's code behind and when it is worthwhile to make a viewmodel for the control to represent it. Honestly I kind of feel that moving all of the control's view related properties to the viewmodel would clean up the code behind of the control leaving only the control logic.
However, if I were to change things like this, then at times when an item needs properties from the control itself I can no longer use {Binding ElementName = control, Path=property} and have to instead get the data context of the parent (because the current datacontext would be on the individual subitem of the observable collection.
Basically I was considering whether I should move properties from Class GraphViewer into a GraphViewerViewModel and then just bind to it.
Code is worth a million words so:
public class GraphViewerViewModel :DependencyObject
{
private const int DEFAULT_PEN_WIDTH = 2;
private const int DEFAULT_GRAPH_HEIGHT = 25;
public SignalDataViewModel _SignalDataViewModel
{
get;
set;
}
public PreferencesViewModel _PreferencesViewModel
{
get;
set;
}
}
Meanwhile
public class SignalDataViewModel : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
ObservableCollection<SignalViewModel> _signals;
public ObservableCollection<SignalViewModel> Signals
{
get
{
return _signals;
}
private set
{
_signals = value;
}
}
ObservableCollection<SignalViewModel> _AddedSignals;
public ObservableCollection<SignalViewModel> AddedSignals
{
get
{
return _AddedSignals;
}
private set
{
_AddedSignals = value;
}
}
it is a pain to type:
PenWidth="{Binding RelativeSource = {RelativeSource AncestorType={x:Type DaedalusGraphViewer:GraphViewer}},
Path = _GraphViewerViewModel._SignalDataViewModel._AxisDivisionUnit.GraphPenWidth, Mode=OneWay}"
and I'm wondering if it is worthwhile to make the change or whether I'm misunderstanding what a view model should be used for in mvvm.

I guess in making a custom control I just have a hard time deciding when I should just add a property to the control's code behind and when it is worthwhile to make a viewmodel for the control to represent it. Honestly I kind of feel that moving all of the control's view related properties to the viewmodel would clean up the code behind of the control leaving only the control logic.
In general, a custom control is 100% View layer code. As such, it really falls outside of MVVM entirely.
The main goal when making a custom control to be used within an application being designed with MVVM is to make sure that you design and build the custom control in a way that it is fully compatible with data binding. This will allow it to be used within your View layer of your application exactly like other controls.
As such, this pretty much guarantees that you'll have code behind, since implementing Dependency Properties really requires code behind. You also don't want to set the DataContext of a custom control within the control (since you want to inherit the data context of the user control or window using the control).
Basically I was considering whether I should move properties from Class GraphViewer into a GraphViewerViewModel and then just bind to it.
If the types are specific to your domain, then this is really typically more of a UserControl being used by your application. In that case, creating a ViewModel and just binding is likely good.
If this is, on the other hand, a true custom control that's made to be completely general purpose (ie: usable by anybody in any application), then keeping it as a "pure view" custom control typically means that you 1) won't take a dependency on any ViewModels or domain specific objects, and 2) not set the data context (which means no view model).

Having trouble deciding how to wire up a UserControl with MVVM

I've been doing the best I can to try to stay true to the separation recommended by the MVVM pattern. One thing I haven't figure out how to do correctly has to do with initializing my UserControls.
My most recent example of this has to do with a library that I wrote to talk to some low-level hardware. That assembly happens to have a UserControl that I can simply drop into any GUI that uses this hardware. All that is necessary for it to work is to set a reference to the object that has access to the low level methods.
However, that's where my problem lies -- currently, the UserControl is added to the GUI via XAML, where I define the namespace and then add the UserControl to my window. Of course, I have no control over its creation at this point, so the default constructor gets called. The only way to set the necessary reference for hardware control involves calling a method in the UC to do so. The ViewModel could feasibly call a method in the Model, e.g. GetController(), and then call the method in the UserControl to set the reference accordingly. The GUI can pass a reference to the UserControl to the ViewModel when said GUI creates the ViewModel, but this violates MVVM because the ViewModel shouldn't know anything about this control.
Another way I could deal with this is to not create the UserControl in XAML, but instead do it all from code-behind. After the ViewModel gets initialized and retrieves an initialized UserControl (i.e. one that has the low-level object reference set), it can set the Content of my Window to the UserControl. However, this also violates MVVM -- is there a way to databind the Content of a Window, TabControl, or any other element to a UserControl?
I'd like to hear if anyone has had to deal with this before, and if they approached it the first or second way I have outlined here, or if they took a completely different approach. If what I have asked here is unclear, please let me know and I'll do my best to update it with more information, diagrams, etc.
UPDATE
Thanks for the responses, guys, but I must not have explained the problem very well. I already use RelayCommands within the UserControl's ViewModel to handle all of the calls to the hardware layer (Model) when the user clicks in the control in the UserControl itself. My problem is related to initially passing a reference to the UserControl so it can talk to the hardware layer.
If I create the UserControl directly in XAML, then I can't pass it this reference via a constructor because I can only use the default constructor. The solution I have in place right now does not look MVVM-compliant -- I had to name the UserControl in XAML, and then in the code-behind (i.e. for the View), I have to call a method that I had added to be able to set this reference. For example, I have a GUI UserControl that contains the diagnostics UserControl for my hardware:
partial class GUI : UserControl
{
private MainViewModel ViewModel { get; set; }
public GUI( Model.MainModel model)
{
InitializeComponent();
ViewModel = new MainViewModel( model, this.Dispatcher);
ViewModel.Initialize();
this.DataContext = ViewModel;
diagnostics_toolbar.SetViewModel( ViewModel);
user_control_in_xaml.SetHardwareConnection( model.Connection);
}
}
where the outer class is the main GUI UserControl, and user_control_in_xaml is the UserControl I had to name in the GUI's XAML.
Looking at this again, I realize that it's probably okay to go with the naming approach because it's all used within the View itself. I'm not sure about passing the model information to user_control_in_xaml, because this means that a designer would have to know to call this method if he is to redo the GUI -- I thought the idea was to hide model details from the View layer, but I'm not sure how else to do this.
You will also notice that the main GUI is passed the Model in the constructor, which I assume is equally bad. Perhaps I need to revisit the design to see if it's possible to have the ViewModel create the Model, which is what I usually do, but in this case I can't remember why I had to create it outside of the GUI.

Am new to MVVM myself but here's a possible solution:
Create a property in your VM that is of the object type (that controls the hardware) and bind it to an attached property on your UserControl. Then you could set the property in your VM using dependency injection, so it would be set when the VM is created. The way I see it, the class that talks to the hardware (hardware controller) is a service. The service can be injected to your view model and bound to your UserControl. Am not sure if this is the best way to do it and if it is strict enough to all the MVVM principles but it seems like a possible solution.

if your question is: How do i show my viewmodel in the view? then my solution is always using viewmodelfirst approach and datatemplates.
so all you have to do is wire up your viewmodel via binding to a contentcontrol.content in xaml. wpf + datatemplates will do the work and instantiate your usercontrol for your viewmodel.

You are right, the ViewModel shouldn't know about anything in the View - or even that there is such a thing as a View, hence why MVVM rocks for unit testing too as the VM couldn't care less if it is exposing itself to a View or a test framework.
As far as I can see you might have to refactor things a little if you can. To stick to the MVVM pattern you could expose an ICommand, the ICommand calls an internal VM method that goes and gets the data (or whatever) from the Model, this method then updates an ObservableCollection property of the data objects for the View to bind to. So for example, in your VM you could have
private ICommand _getDataCommand;
public ICommand GetDataCommand
{
get
{
if (this._getDataCommand == null)
{
this._getDataCommand = new RelayCommand(param => this.GetMyData(), param => true);
}
return this._getDataCommand;
}
}
private void GetMyData{
//go and get data from Model and add to the MyControls collection
}
private ObservableCollection<MyUserControls> _uc;
public ObservableCollection<MyUserControls> MyControls
{
get
{
if (this._uc == null)
{
this._uc = new ObservableCollection<MyUserControls>();
}
return this._uc;
}
}
For the RelayCommand check out Josh Smiths MSDN article.
In the View you could either call the ICommand in the static constructor of your UC - I am guessing youwould need to add an event in your class for this - or call the ICommand from some sort of click event on your UC - maybe just have a 'load' button on the WPF window. And set the databinding of your UC to be the exposed observable collection of the VM.
If you can't change your UC at all then you could derive a new class from it and override certain behaviour.
Hope that helps a bit at least, like I say, have a look at Josh Smiths MVVM article as he covers the binding and ICommand stuff in there brilliantly.

If you set the DataContext of the Window or UserControl containing thisUserControl to the main view model, the user control can call SetHardwareConnection() on itself in its Loaded event (or DataContextChanged event handler).
If that's not possible because you're saying the UserControl is 'fixed', you should derive from it or wrap it up in another UserControl, which would serve as a MVVM 'adapter'.
(In order to bind the window: you could make the MainViewModel a singleton with a static Instance property and use DataContext="{x:Static MyClass.Instance}". A nice way to get things going quickly)
Note; this is based on my understanding that MVVM works because of Bindings.. I always bind the control to a ViewModel, not pass a ViewModel as a parameter.
Hope that helps!