I am using a 3rd party WPF control whose MVVM support relies on dependency properties on the VM it is bound to. The sample that comes with the control uses a ViewModelBase class derived from DependencyObject so all is well.
My ViewModelBase implements INotifyPropertyChanged and for various reasons it is unrealistic to change it to DependencyObject.
My question is how do I use my ViewModels with this WPF control? I guess what I need is something like "embedding a dependencyobject" or "plugging dependency properties" in a plain old ViewModel.
By the way my MVVM application is interface based, i.e. everywhere SomeViewModel is ISomeViewModel.
In general, a properly designed control shouldn't require binding to a DependencyProperty, as a DP can bind to any property without issue. As such, I'd revisit whether this is truly a bug in the control implementation first, and correct that.
However, if you must do this, realize you're going to violate MVVM - using DependencyObject within a ViewModel is, by its very nature, injecting view specific framework elements into the VM. Once you decide you're okay with doing this, you can always have your ViewModel expose a DependencyObject as a property, and bind to a DependencyProperty defined on that DependencyObject instead of directly to your VM's property.
Related
The question is related to WPF Data Binding and MVVM pattern.
I am bit confused now distinguishing between the Dependency property defined in the XAML.cs file as well as a CLR property defined in the view model which is bound to some property of a component
For example say, I have a textbox in MyPage.xaml. So I created a dependency property to bind the textbox text property in the MyPage.xaml.cs maybe some String. The next time, I created a viewModel MyPageViewModel.cs which implements the INotifyPropertyChanged interface and
created a CLR property there(String), which emits an event PropertyChanged when it changes or the property is set with a new value. So are these both the same? Is there any difference?
I have 3 questions
Is the Dependency Property same as CLR property which emits a PropertyChanged event when it changes?
Whether Dependency property is written in the view itself(MyPage.xaml.cs) or can it be included in the view
model(MyPageViewModel.cs)?
In MVVM pattern, we use the CLR properties more which emits an event during property change. So can dependency property be replaced
by such kind of CLR properties?
Thanks in advance.
An dependency property is on a DependencyObject from which all WPF UI elements derive from (and only works there), as it's static and saves it's value in a kind of collection assigned to a specific DependencyObject (on which the dependency property is defined). Dependency properties can be defined in a class outside of the actual DependencyObject to extend it's functionality without modifying the original user control class.
When you write a user control and want a ViewModel to allow to bind a value and receive notifications when it's changed, then you create a dependency property.
Imagine it like an USB cable, where you have a male plug and a female receptacle. The CLR property is like the plug and the dependency property is like the receptacle.
A dependency property allows you to store that's associated with a control but isn't part of the instance. As you can see on the MSDN Examples
public static readonly DependencyProperty IsSpinningProperty =
DependencyProperty.Register(
"IsSpinning", typeof(Boolean),
...
);
public bool IsSpinning
{
get { return (bool)GetValue(IsSpinningProperty); }
set { SetValue(IsSpinningProperty, value); }
}
the dependency property is static and GetValue and SetValue are methods of DependencyObject (base class on which all WPF UI elements are based on).
Depencency Properties (and attached properties/attached behavior) can also be used to extend the functionality of a UserControl without inheriting from the actual user control type, i.e. notifying the ViewModel when a certain value changes which is not provided by the original user control.
Is the Dependency Property same as CLR property which emits a PropertyChanged event when it changes?
No, it's not the same. They are both 2 sides of the databinding engine. A DP is defined on the view to allow a view model to bind a INPC Property (Property that rises PropertyChanged event)
Whether Dependency property is written in the view itself(MyPage.xaml.cs) or can it be included in the view model(MyPageViewModel.cs)?
DP are part of the View-Layer as they depend on DependencyObject, which is part of the WPF framework and hence view concern. While technically nothing prevents you from using them in the ViewModel, this causes a tight coupling of your ViewModel towards a certain View technology, so it doesn't fully comply MVVM pattern.
Be aware though that unit testing Dependency Properties may be quite difficult as they don't store the values on the class they are defined on but in some kind of dictionary where the GetValue/SetValue methods warp around.
Last but not least, since DependencyObject is the base class of all UI it is as well as most of the classes that derive from it thread affine, which means you can only access it from the thread you created which may cause you much pain in both unit test (especially if the tests run in parallel like MSTest used to do. Dunno if its still true as of today) and in your code.
In MVVM pattern, we use the CLR properties more which emits an event during property change. So can dependency property be replaced by such kind of CLR properties?
In ViewModels you could and you should use INotifyPropertyChanged. If you are developing a user control, you shouldn't replace DPs with "CLR" properties, because this makes the property not work with databinding in XAML.
If your UI elements should expose a property which can be used with data binding you have to use dependency properties (or attached properties which are pretty similar, but you place attached properties on i.e. the child elements. Grid.Row and Grid.Column are examples of attached properties).
I have a ProductViewModel class which contains different properties.
Then I have a ProductDetailsViewModel class which inherit from ProducViewModel class. The reason I am doing it this way is in order to get correct binding environement and avoid duplication of properties from previous view.
I am allowed to do this or each ViewModel should be clearly isolated?
Through code I can acess the properties of the ProductViewModel class from ProductDetailsViewModel view but when I set the datacontext of my ProductDetailView to ProducDetailsViewModel class and bind properties URI for instance which is define inside the inherited class, binding seems not occurs.
Any idea ?
You can do this too, but i think maybe better would be to separate them and use Dependency Injection.
You create and interface for your ProductViewModel and implement it and then you inject this into your ProductDetailsViewModel.
MVVM + WPF + DI
MSDN DI
Yes this is fine, and I do this all the time in my WPF projects so it should just work. Some suggestions:
Can you check your output window when debugging the application. Are there any binding errors suggesting a mis-typed xaml binding?
Are you using any DataTemplates in xaml which bind to a specific type, e.g. ProductViewModel not ProductDetailsViewModel?
Does the base type (ProductViewModel) implement INotifyPropertyChanged?
Are all the properties in ProductViewModel and ProductSetailsViewModel raising the PropertyChanged event with directly typed string property name?
Best regards,
I am currently migrating a number of attached behaviours I have created to Blend Behaviours so that they support drag and drop within Expression Blend. I have noticed that authors of Blend behaviours tend to define the behaviour properties as dependency properties.
I have created a behaviour, TiltBehaviour, which exposes a public dependency property, TiltFactor, of type double. Within Expression Blend I can set the value of this property, however, the option to add a "Data Binding ..." is grayed out:
I have also noticed that Behaviors extend DependencyObject, therefore they do not have a DataContext and therefore cannot inherit the DataContext of the element to which they are attached. This feels like a real weakness to me!
So, the bottom-line is, if I cannot set a binding to my behaviors dependency property in Blend, and it does not inherit a DataContext, why bother using dependency properties at all? I could just use CLR properties instead.
Blend behaviors would be almost useless unless they supported binding! I recreated your tilt behavior and it supports binding in Blend 4 with no problems so I don't know exactly where you went wrong. Perhaps you can reproduce my simple example and then infer what's wrong with your setup.
Here's the (non-functional) tilt behavior with dependency property:
public class TiltBehavior : Behavior<FrameworkElement>
{
public double TiltFactor
{
get { return (double)GetValue(TiltFactorProperty); }
set { SetValue(TiltFactorProperty, value); }
}
public static readonly DependencyProperty TiltFactorProperty =
DependencyProperty.Register("TiltFactor", typeof(double), typeof(TiltBehavior), new UIPropertyMetadata(0.0));
}
Then just create a new window and drop the behavior onto the grid and Blend creates this:
<Grid>
<i:Interaction.Behaviors>
<local:TiltBehavior/>
</i:Interaction.Behaviors>
</Grid>
and the Blend "Data Binding..." option is available in the properties tab.
I tested this with both WPF and Silverlight projects. The built-in behaviors, triggers and actions all support binding by virtue of using being dependency properties and all the Blend samples use binding heavily and so this has to work.
In fact you can just drop a built-in behavior like FluidMoveBehavior onto your grid and check that Duration, which is a dependency property, supports binding. If that doesn't work, I have no idea what's going on!
Let's consider then how binding works for these strange beasts called behaviors.
As WPF or Silverlight programmers we are very familiar with binding for things like FrameworkElement. It has a property called DataContext that we can manipulate to control the default binding source and that property is inherited by nested elements when we don't override it.
But behaviors (and triggers and actions) are not of type FrameworkElement. They are ultimately derived from DependencyObject, as we might expect. But while we can using binding on any class derived from DependencyObject, our familiar DataContext is missing at this low-level and so the binding has to supply the source. That's not very convenient.
So behaviors are derived (on WPF anyway) from Animatable and Animatable is derived from Freezable. The Freezable class is where the simplicity of dependency objects intersects with the complexity of framework elements. The Freezable class is also the base class for more familiar things like brushes and image sources. These classes don't need the full complexity of a framework element, but they want to participate, in a limited way with the elements that they are associated with.
Through a complicated magical process, Freezable instances acquire an inheritance context: the framework element they are most closely associated with, and when a default binding is used (one without a source), the Freezable uses the DataContext of it's associated element instead.
In fact as you learn about behaviors the AssociatedObject is a central concept; for a behavior it is the thing that the behavior is attached to. But the important point is that all Freezable objects can use the DataContext of their AssociatedObject by proxy.
All this magic is what Josh Smith calls the:
Hillberg Freezable Trick
And so all this leads up to saying that due to the Hillberg Freezable Trick, Blend behaviors support binding using the data context of their associated element as the default source. As a result, bindings for behaviors seem to "just work" without any effort on our part. Behaviors are a thousand times more useful because of this.
Edit: dain is correct you can still bind to the DataContext which is created artificially, how often have you seen people bind to a SolidColorBrush.Color? It also works even though SolidColorBrush inherits from DependencyObject and hence has no DataContext.
See this blog post on the inheritance context.
The thing is that since the behaviours are attached, they do not appear in the logical tree and hence would not inherit a DataContext anyway.
When implementing the ViewModel in a Model-View-ViewModel architecture WPF application there seem to be two major choices how to make it databindable. I have seen implementations that use DependencyProperty for properties the View is going to bind against and I have seen the ViewModel implementing INotifyPropertyChanged instead.
My question is when should I prefer one over the other? Are there any performance differences? Is it really a good idea to give the ViewModel dependencies to WPF? What else do I need to consider when make the design decision?
Kent wrote an interesting blog about this topic: View Models: POCOs versus DependencyObjects.
Short summary:
DependencyObjects are not marked as
serializable
The DependencyObject class overrides and seals the Equals() and
GetHashCode() methods
A DependencyObject has thread affinity – it can only be accessed
on the thread on which it was
created
I prefer the POCO approach. A base class for PresentationModel (aka ViewModel) which implements INotifyPropertyChanged interface can be found here: http://compositeextensions.codeplex.com
According to the WPF performance guide, DependencyObjects definitely perform better than POCOs that implement INotifyPropertyChanged:
http://msdn.microsoft.com/en-us/library/bb613546.aspx
The choice is totally based on your business logic and UI abstraction level. If you dont want a good separation then DP will work for you.
DependencyProperties will be applicable mainly at the VisualElements level so it won't be good idea if we create lot of DPs for each of our business requirements. Also there is a greater cost for DP than a INotifyPropertyChanged. When you design a WPF/Silverlight try to design UI and ViewModel totally separate so that at any point of time we can change the Layout and UI controls (Based on theme and Styles)
Refer this post also - https://stackoverflow.com/questions/275098/what-applications-could-i-study-to-understand-datamodel-view-viewmodel . The link has a lot of reference to Model-View-ViewModel pattern, which is very relevant to this discussion.
From an expressiveness standpoint, I thoroughly enjoy using dependency properties and cringe at the thought of INotifyPropertyChanged. Apart from the string property names and possible memory leaks due to event subscription, INotifyPropertyChanged is a much more explicit mechanism.
Dependency properties imply "when this, do that" using easily-understood static metadata. It is a declarative approach that gets my vote for elegance.
Dependency properties are intended to supports binding (as a target) on UI elements not as a source to data binding, this is where INotifyProperty comes in. From a pure point of view you shouldn't use DP on a ViewModels.
"In order to be the source of a binding, a property does not need to be a dependency property; you can use any CLR property as a binding source. However, in order to be the target of a binding, the property must be a dependency property. For a one-way or two-way binding to be effective, the source property must support change notifications that propagate to the binding system and thus the target. For custom CLR binding sources, this means that the property must support INotifyPropertyChanged. Collections should support INotifyCollectionChanged."
All dependency objects cannot be serialised (This could hamper the use of ViewModels and DTO (POCO)'s.
There are differences between DP within Silverlight compared to WPF.
http://msdn.microsoft.com/en-us/library/cc221408(v=VS.95).aspx
http://msdn.microsoft.com/en-us/library/cc903933(VS.95).aspx
INotifyPropertyChanged when used also gives you the ability to add more logic in the code of your getters and setter of your properties.
DependencyProperty example:
public static DependencyProperty NameProperty = DependencyProperty.Register( "Name", typeof( String), typeof( Customer ) );
public String Name
{
set { SetValue( NameProperty, value ); }
get { return ( String ) GetValue( NameProperty ); }
}
In your getter and setter --- all you can do is simply call SetValue and GetValue respectively, b/c in other parts of the framework the getter/setter is not called, instead it directly calls SetValue, GetValue, so your property logic wouldnt reliably be executed.
With INotifyPropertyChanged, define an event:
public event PropertyChangedEventHandler PropertyChanged;
And then simply have any logic anywhere in your code, then call:
// ...
// Something cool...
// ...
if( this.PropertyChanged != null )
{
PropertyChanged( this, new PropertyChangedEventArgs( "Name" ) );
}
// More cool stuff that will reliably happen...
This could be in a getter/setter, or anywhere else.
Is it really a good idea to give the ViewModel dependencies to WPF?
.NET 4.0 will have System.Xaml.dll, so you won't have to take a dependency on an arbitrary framework to utilize it. See Rob Relyea's post about his PDC session.
My take
XAML is a language for describing objects, and WPF is a framework whose described objects are UI elements.
Their relationship is similar to C#, a language for describing logic, and .NET, a framework which implements particular kinds of logic.
XAML's purpose is declarative object graphs. The W*F technologies are great candidates for this paradigm, but XAML exists independently of them.
XAML and the entire dependency system were implemented as separate stacks for WF and WPF, probably to leverage the experience of different teams without creating a dependency (no pun intended) between them.
I too had to consider this decision recently.
I found that the INotifyPropertyChanged mechanism suited my needs better because it allowed me to glue my GUI to an existing business logic framework without duplicating state. The framework I was using had its own observer pattern and it was easy to forward one level of notification on to the next. I simply had a class which implemented the observer interface from my business logic framework and the INotifyPropertyChanged interface.
With DP you cannot define the backend that stores the state yourself. I would have had to let .net cache a copy of every item of state I was binding to. This seemed like an unnecessary overhead - my state is large and complicated.
So here I found INotifyPropertyChanged better for exposing properties from business logic to GUI.
That being said where I needed a custom GUI widget to expose a property and for changes to that property to affect other GUI widgets DP proved the simple solution.
So there I found DP useful for GUI to GUI notification.
Dependency properties are the glue of custom control creation. If you are interested in using Intelli-sense to show your properties in the properties window at XAML design time you must use Dependency properties. INPC will never show a property in the property window at design time.
It seems that Dependency Properties should be used in controls that you create such as Buttons. To use properties in XAML and use all the WPF features, those properties must Dependency Properties.
However, your ViewModel is better off using INotifyPropertyChanged. Using INotifyPropertyChanged will give you the ability to have getter/setter logic if you need to.
I recommend checking out Josh Smith's version of a base class for a ViewModel that already implements INotifyPropertyChanged:
http://joshsmithonwpf.wordpress.com/2007/08/29/a-base-class-which-implements-inotifypropertychanged/
I think this is an excellent example of how to do a ViewModel.
I think DependencyProperty and INotifyPropertyChanged are used for two different things in Binding : the first for enabling a property to be a target of a binding and receive the input from another property (use {Binding ...} to set the property), the last when you want the value of a property to be used as the source of a binding (name in the Binding Path Expression).
So the choice is merely technical.
I prefer a more direct approach, which I blogged about in Presentation Model Without INotifyPropertyChanged. Using an alternative to data binding, you can bind directly to CLR properties without any bookkeeping code. You just write plain-old .NET code in your View Model, and it gets updated when your Data Model changes.
There is only one thing why to prefer a DependencyObject - Binding will work better. Just try an example with a ListBox and TextBox, populate list with data from INotifyPropertyChanged property vs. DependencyProperty and edit current item from TextBox...
If you want to expose properties to other controls you must use Dependency properties... But good luck because they take a while to figure out...
In WPF:
Can someone please explain the relationship between DependencyProperty and Databinding?
I have a property in my code behind I want to be the source of my databinding.
When does a DependencyProperty (or does it) come into play if I want to bind this object to textboxes on the XAML.
The target in a binding must always be a DependencyProperty, but any property (even plain properties) can be the source.
The problem with plain properties is that the binding will only pick up the value once and it won't change after that because change notification is missing from the plain source property.
To provide that change notification without making it a DependencyProperty, one can:
Implement INotifyPropertyChanged on the class defining the property.
Create a PropertyNameChanged event. (Backward compatibility.)
WPF will work better with the first choice.
What is the DependencyProperty?
The DependencyProperty class is one of the most important design bases hidden deep in the .Net Framework WPF.
This class is protected by sealed from the .NET Framework.
This property differs from the one-dimensional general property in that it not only stores field values, but also takes advantage of the various functions provided within the class.
Most importantly, there is a full foundation for data binding. You can also send notifications whenever you bind something.
DependencyProperty
Wpf Xaml Binding
It's already a late answer, but I'll introduce the results of my research.