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.
Related
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,
let say there is a textbox and i want to control the visibility of this control using MVVM, is there a sample on how to do this? First create a dependency property then get it hooked up in the ViewModel. Thanks.
Typically, you wouldn't need to use a dependency property in this case. Dependency properties really only need to be implemented for things like controls themselves, not for determining behavior. Behavior, such as the visibility of an element, can be handled directly via data binding.
Your ViewModel would just have some property, and you'd bind the TextBox.Visibility property directly to the ViewModel property.
The one "sticky point" is that you often will want to have some type of IValueConverter that will convert from your property type to a Visibility enum.
I'm working on PRISM application with modules, MVVM and so on. I understand PRISM pretty good now and I understand value of MVVM. All those things good to deliver business value which comes from testability, "uniformity" and so on.
But now I'm stuck with certain interaction issues. I already spent hours and hours trying to see how I set focus in Silverlight via MVVM. All this additional behaviors, attached properties, triggers. It just seems like bunch of junk code with MVVM being root cause.
For example, I need to create Lookup control which is basically textbox with button and popup window. This control itself needs lot of focus control, it needs to overlay view over parent (popups) and so on. It seems to be pretty easy to create it with code-behind, stick it into separate library and move on. My business forms will use this control inside my nice MVVM PRISM.
So, question is.. Is it justified to use code-behind in isolated islands like controls and keep MVVM and TDD for actual code that brings business value?
Is there line where you say "MVVM is not going to be used here" ?
I see absolutely nothing wrong with using Code Behind providing that the code is related to view-specific properties, such as setting Focus. Your ViewModel should never need to know about or care who or what has focus, since that is a View-Specific concept.
Usually I build UserControls in two ways: they are either built for a specific Model or ViewModel, or they are meant to be generic and have their values provided by whoever calls them.
In the case of the former, such as if I wanted a SearchResultsPopup, I would build the UserControl expecting to have something like a SearchResultsViewModel as the DataContext.
For example, my UserControl would expect to find the following properties on it's DataContext, and would use them in bindings to build the View.
ObservableCollection<SearchResult> Results
SearchResult SelectedResult
bool IsOpen
ICommand OkCommand
ICommand CancelCommand
I could then use the UserControl like this:
<local:SearchResultsPopup DataContext="{Binding MySearchResultsVM}" />
In the later situation, where I am creating something generic which can be used by any Model or ViewModel, I would use custom Dependency Properties to provide my UserControl with the values it needs to bind to.
So in this example, I would have DependencyProperties for
bool IsOpen
ICommand OkCommand
ICommand CancelCommand
And my XAML would look something like this:
<local:GenericPopup local:GenericPopup.IsOpen="{Binding IsPopupOpen}"
local:GenericPopup.SaveCommand="{Binding SavePopupCommand}"
local:GenericPopup.CancelCommand="{Binding HidePopupCommand}">
<local:MySearchResultsView ... />
</local:GenericPopup>
In summary, your UserControl is either a reflection of your ViewModel (meaning it becomes a View), or it is provided values by the View. The ViewModel doesn't care either way.
when I have a ViewModel, could there be cases where I can leave out INotifyPropertyChanged? I seem to get that this interface is used when you have multiple displays of a property in a GUI, so that when this property is edited, all the displays of the adjusted property get the new value. But what if you are fairly certain that UI will only have one display of the property?
I use NotifyPropertyWeaver - it generates property notification code for you. Then there is no reason why not to implement it in types that might potentially need it.
I'd always implement INotifyPropertyChanged because requirements might change.
The binding mode supports OneWay and OneTime so overhead can be reduced.
Inheriting from a base class will make it easy to implement the interface so I see no real advantages of not implementing INotifyPropertyChanged on a ViewModel.
It is OK to leave out INotifyPropertyChanged in two cases:
The property is not databound
The value of the property is never changed after the data binding is set up.
This is unlickly to be the case for a WPF ViewModel.
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.