I am newbie in c# and i want to know why we have to implement INotifyPropertyChanged interface when we use the TwoWay binding ?? and for the OneWay also ??
Thank you
In brief to support OneWay/TwoWay bindings, the underlying data must implement INotifyPropertyChanged.
Then the OneWay/TwoWay binding is just to choose the bind direction, you can find more here :
Various wpf binding modes
INotifyPropertyChanged,like the name, is to notify your client that your property has changed, see MSDN
You will need it for updating your UI when properties change.
OneWay(Source to Target): Propertychange will cause UI update and UI operation will NOT cause propertychange. *
TwoWay(Both way): Proerty and UI are fully binded, any of them change will affect the other one.
Binding works as long as you implement INotifyPropertyChanged for your property in this case.
If you don't, nothing will change even if you use Twoway.
Implementing INotifyPropertyChanged just offers classes (others than the one implementing it) the possibility to react on property changes.
This is not possible if that interface is not implemented because if a class instance, say A sets a property on B, a third class instance C cannot track that information. Setting that property is now only a concern of A and B. If Chowever knows that B does implement INotifyPropertyChanged, it can simply add an event handler to the event PropertyChanged (which is part of the interface) and react on it - still hoping that B will throw the event as expected.
Bindings need that information to update the model (data) or the view depending where the change happened. So basically, they are C listening for changes of other objects (A & B).
Related
Say I have a property named Location of type Point in my ViewModel, to which my View binds. I'm using MVVM Light's ObservableObject and RaisePropertyChanged setup to raise property change notifications. The problem is that when I execute the following in the ViewModel:
this.Location = new Point(100,100);
my View updates correctly. But if I do:
this.Location.Offset(10,10);
my View doesn't update, means the Location property does not notify the View about the change. Why is this so? How do sub-properties of built-in types such as Point or Size deal with change notification?
The fundamental problem is that Point is mutable.
You could write your own ImmutablePoint that allows conversion to and from the existing Point class for API compatibility, but doesn't allow methods like Offset.
It's not an ideal solution, but it gets around the problem of changing the fields on a property rather than the property itself.
In MVVM land, your ViewModel is responsible for notifying the View when properties change. The majority of the .NET classes do not implement INotifyPropertyChanged themselves, so you won't get automatic wiring when using them. You'll have to raise PropertyChanged yourself on the Location property each time you modify any properties of Location
Alternatively, you could implement your own Point object that implements INotifyPropertyChanged.
You can add INotifyPropertyChanged to your class members automatically using either Castle Dynamic Proxy (which wraps your models in a proxy at runtime) or PropertyChanged.Fody (which modifies the model's IL at compile time). Google is your friend.
If I register to be notified of a property's change, it will only notify when the actual property is reassigned. So if I have a property that's a textbox, it won't notify if the text of the textbox changes, right? Is there any way to make it work this way?
If you are interested in specific properties of an object then you should be using a mechanism like INotifyPropertyChanged on the value itself. Unfortunately though in this case TextBox from WinForms doesn't implement INotifyPropertyChanged. In order to listen for changes to its Text property you need to subscribe to the TextChanged event
I have had a hard time imagining why a class would expose a property of type TextBox. Anyhow if you have ended up with such a design, you could simply add two properties, each for the TextBox itself and the text inside and then listen to TextBox's TextChanged event and fire your NotifyPropertyChanged thereupon.
I do feel however that you need to rethink your approach. If your class exposes a property of TextBox type, there should really be places where you actually assign a new TextBox to a class instance and upper layers listening to this change. I don't think that would be the case however. You'd almost certainly be looking for text changes and should therefore expose a property of type string.
I am starting with a new project based on MVVM approach.I will be loading some data asynchronously from some web service.I am confused about where to implement this INotifyPropertyChanged interface,in a viewmodel,model or both.
Also If I implement INotifyPropertyChanged on a model(let's call it ModelA).Then I create an ObservableCollection<ModelA>,Will not it be redundant as ObservableCollection itself keeps track of any data change made to the list?
ObservableCollection class only updates UI in case of following operations:
Item is added in collection.
Item is deleted from collection.
Collection is clear.
But if any property changes in underlying item i.e. ModelA in your case, it won't be reflected on UI unless your Model class is implementing INotifyPropertyChanged interface and raising PropertyChanged event on property change.
So, as a thumb rule you should implement INPC both at your ViewModel layer and also at Model layer.
Practically you need to implement INotifyPropertyChanged to objects which property will be displayed in UI using data binding. If you bind the UI directly to property of your model then you need to implement it in the model too.
ObservableCollection only notify UI when object added or removed from collection. To get the UI notified when property of one or more objects in that ObservableCollection changed you still need to implement INotifyPropertyChanged in that object class.
The answer is simple. If you need your model to implement INotifyPropertyChanged then it should. Redundant or not if you need to keep track on a single property you will be better of using that event rather than the ObservableCollection.
I am building and MVVM application where some of my Models contain the state of realworld-object that their are modelling. The state changes from outside events, e.g. my Model gets information over TCP/IP about a stock that changes, and updates its state to reflect that change.
Now I would like the change to propagate to my View, and the way to do that, is to let my ViewModel know about the change.
I can think of two ways of doing this.
One, I let the Model implement INotifyPropertyChanged, and fire the event when the property changes. However, this seems to be frowned upon for some reason.
Two, I implement an event for each property that can change in the Model, events that the ViewModel explicitly can bind to.
What is the preferred way? Are there other ways (better ways) of doing it?
Edit:
I have now read, both in the comment from slugster and here, that having the Model have a state is not the purpose of the model.
However, in John Gossmans original MVVM post, we find: "The Model is defined as in MVC; it is the data or business logic, completely UI independent, that stores the state and does the processing of the problem domain."
Option 2 is viable, albeit dirty. You can mitigate the dirtiness somewhat by exposing the public events and functions via an interface, and the viewmodel uses the model only via that interface.
Having said that, the model should be treated as a conduit of information, rather than a container for information. This means that ideally your model should not contain any state information. If you have a persistently open channel in your model that needs to notify other components when something is received then I suggest you use the EventAggregator in Prism - this is a weak event pub/sub system, your viewmodel can subscribe to an event of your choosing, and your model can publish that event when it needs to (along with an appropriate payload).
The model classes typically provide property and collection change
notification events through the INotifyPropertyChanged and
INotifyCollectionChanged interfaces. This allows them to be easily
data bound in the view. Model classes that represent collections of
objects typically derive from the ObservableCollection class.
MSDN
I wouldn't say that a Model typically implements INotifyPropertyChanged, but I wouldn't categorically refuse to do so. In simpler - non-PRISM - environments it's totally fine in my opinion.
And then?
To clarify: INotifyPropertyChanged is an interface for the notification in case of a property change, as the name says and most people know. However, it is not more than this, especially it is not WPF specific! Furthermore, as according to the the MVVM principles, the Model should be agnostic of the ViewModel. Therefore, it is clear, that you should use a Publisher(Model)-Subscriber(ViewModel)-pattern (events). That's all. So, as an answer to your question: How to set up the communication between Model/ViewModel depends on your style and the specific task:
If you just want to react on changed properties, just use INotifyPropertyChanged, as that's what it was made for. In the ViewModel, just subscribe to the PropertyChanged-event of the Model and process the changes, which is basically just 'map them forward', that is raise PropertChanged for the ViewModel-property, which is effected by the change of the model property.
Alternatively, if you need to wrap the model changes in a more specific event, lets say 'MyDataUpdated', then I don't see any problem about that as well. Just listen to this event in the ViewModel, and handle it as you wish. That's perfectly fine to.
I hope this helps.
EDIT: As a side-note: I recommend using the PropertyWeaver extension, so that you donÄt mess up your model with all the PropertyChanged-stuff.
I will go with option1. Keep a reference to the model in your viewmodel, however do not use any properties directly. Instead, populate the viewmodel's own properties from the model.Your view model can listen to the property changing notifications for the concerned properties from the Model, and repopulate its own properties from the model. The model is still UI independent, it is merely notifying property changed.
In my opinion, doing your own publisher-subscriber logic other than inotifypropertychanged could be just an overkill as there might not be any benefits doing that. However, this depends on whether you can afford to have a model reference in the view model and my answer is based on the assumption that you can have a reference of model in view model.
For for data binding you can use **Binding**
Data Binding Overview
I'm trying to figure out the best way to bubble up PropertyChanged events from nested Properties in my ModelView. Say I have my ModelView PersonModelView which has a Property PersonModelView.Address. Address in turn has a property City. When I bind to City in my view, I would do something like {Binding Address.City}.
My problem is that even if Address implements INotifyPropertyChanged, the binding will not get updated because it is handling PropertyChanged on the PersonModelView, not Address. I think I have two options: (1) change the source of the binding (or change the DataContext) to the Address property or (2) have the PersonModelView handle PropertyChanged on the Address object and refire its own PropertyChanged with something like Address.City.
How are you guys solving this? (I'm using MVVM light toolkit at the mo, but am interested in any approaches)
If Address implements INotifyPropertyChanged and correctly raises PropertyChanged events on its City property then the binding should notice that the property it is bound to has changed.
Here's a SO thread containing a solution on how to bubble up these notifications:
When nesting properties that implement INotifyPropertyChanged must the parent object propogate changes?
However, IIRC WPF has the intelligence to automatically monitor Address for INotifyPropertyChanged notifications when a control's binding is set to Address.City without PersonViewModel needing to re-broadcast the Address object's update notifications.
Does your Address object implement INotifyPropertyChanged? If not, I think that would fix the issue that you are seeing.
Edit:
Sorry, just noticed that you mentioned in your post that you tried this already. Have you tried subscribing to the PropertyChanged event of the Address object in your PersonViewModel? OnChanged, you could perform a PropertyChanged on your Address object.
Check out PropertyChangedPropagator, it can handle dependencies on nested view models' properties including dynamically changeable nested view models:
http://www.codeproject.com/Articles/775831/INotifyPropertyChanged-propagator