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,
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 read that I need to implement InotifyPropertyChanged but I can't have my control inherit from two different classes.
I'm a little confused as to how I do this.
Basically I wish to have a parent control have a property bound to a control in the UI (already done).
and then I wish to bind that value to a property in a child FrameworkElement.
however, the usercontrol is already inheriting from usercontrol, and multiple inheritance is not supported. How do i go about alerting other dependencyproperties that my dependency property has changed?
INotifyPropertyChanged is not a class. It is an interface, and C# allows you to inherit from as many interfaces as you'd like.
If that doesn't clear up your issue, please add more details. An example would be great.
I have a main window coupled with a view model.This main window uses a usercontrol which also has its own viewmodel.
What I would like to achieve is setting a binding in the main window's xaml between one of its viewmodel's custom property and one of the usercontrol's viewmodel's custom property.
How would one go about doing that?
Could you instead use the ViewModels as projections of a Model?
That is, could you have a class that holds the state (or actions) that both the VMs need to expose and have both the VMs reference this class?
If for some reason you have to couple views to something outside their own DataContext I believe you can only go up the visual tree by using RelativeSource FindAncestor in the binding. I don't think you can traverse down (e.g. Window -> Control).
If you really want to Bind them together you could make your ViewModel's properties Dependency Properties and your ViewModel derive from DependencyObject - then you could do..
var binding = new Binding("Something");
binding.Source = myViewModel1;
binding.Mode = BindingMode.TwoWay;
BindingOperations.SetBinding(viewModel2,ViewModelType.SomethingProperty,binding);
If this is a good design having your viewmodels derive from DependencyObject is another question..
You could also try looking at this library that allows binding to and from POCOs.
I ended up not using a modelview for my usercontrol, not as neat but at least it works and is less complicated datacontext wise.
Thanks to all.
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.
In WPF some properties of controls are dependency properties, others are normal properties.
Eg TextBox.SelectedText is a normal property and not a dependency property. I use MVVM and it happens often to me that I want to bind to some property, but I cant, because it is a normal property.
Can someone explain to me, what logic stands behind the decision whether a property is normal or a dependency property.
Also, can I work around this and somehow bind to the normal properties as if they were dependency properties?
Go through these links
When to use a WPF Dependency Property versus INotifyPropertyChanged
http://social.msdn.microsoft.com/forums/en-US/wpf/thread/65bf126f-e706-4d3e-8cc3-e0130a0ee6de
http://joshsmithonwpf.wordpress.com/2007/06/22/overview-of-dependency-properties-in-wpf/
WPF: What distinguishes a Dependency Property from a regular CLR Property?
How to set bindings on CLR Properties using DataResource
You will get better idea about what you are looking to find out
You can bind to normal properties, but if your property changes, your binding will not get notified. However, you can implement INotifyPropertyChanged in your classes and the binding will update your control automatically.