i am working on MVVM and i am not much familiar with it and i would like to know where to implement INotifyPropertyChanged?
I implemented it in ViewModel (Which is correct from my understanding) and i am in a situation where i need to modify the retrieved data in the View and once the data is modified save it in a property and once the propriety in the View is assigned/changed i want the ComboBox pick up that itemsource which will be the property thats changed and holds the modified Data.
So what do i do in this situation? should i implement INotifyPropertyChanged in the View and use:
PropertyChanged += new PropertyChangedEventHandler(PropertyChanged_implimentation); ?
What do i do?
Typically you would implement INotifyPropertyChanged on your view models. You may also choose to implement it on your models, if you need to notify any consumer of their property changes (for example view models).
You wouldn't typically implement INotifyPropertyChanged on your view, as you would use XAML binding to communicate between your view and underlying view model.
INPC is required to notify your view to update itself whenever a property value changes in your view model. The binding engine will update the bound property values in your view model whenever a control value in your view changes.
A good introduction to MVVM can be found here, and I would also strongly recommend using an MVVM framework for any kind of serious app.
Related
This is the link i am following for learning MVVM in WPF and i have a question:-
https://www.tutorialspoint.com/mvvm/mvvm_first_application.htm
DataContext of the WPF window is set to a VIEWMODEL object.
Itemsource of a List-DataTemplate is set to a List from the same VIEWMODEL Object.
The Model contains an implementation of INotifyPropertyChanged.
When i update the view,the INotifyPropertyChanged of MODEL gets fired ie VIEW is directly updating the MODEL while what i have understood till now is that VIEW interacts with the VIEWMODEL only via Bindings and Commands and never with the MODEL DIRECTLY.It is always the ViewModel which interacts with the Model to fetch data for the View.But here,the View is updating the Model DIRECTLY.This is confusing me owing to my limited knowledge.
Please guide.
If the view model exposes the model through a property, the view may actually bind directly to the model through this property. This doesn't really violate the MVVM pattern and is perfectly fine, especially if the model class implements the INotifyPropertyChanged interface. If it does, you can say that the model is kind of a (child) view model.
A "real" model such as a domain object or a service shouldn't have any knowledge about WPF and how you raise change notifications to a view. Therefore it rarely makes sense to bind directly to such objects but if your models are "WPF aware" and implements view related interfaces, you can bind to them without any issues.
It is a common error to bind the Model thru lists to the View. The correct approach would be always to create a ViewModel of that Model (the list element) and bind to it.
For example:
Otherwise you are opening the door to including data on ModelB that should be stored in the ViewModelB.
I have a single viewmodel representing my applications main view. This viewmodel contains an ObservableCollection of model objects which a grid on my view is bound to. Each item in the collection implements IDataErrorInfo which facilitates the standard WPF validation mechanism i.e an error in the model causes the view control to highlight red (in this instance a cell).
My problem is, I want to perform a piece of logic in the viewmodel which needs access to the number of errors on the page (or rather IF there are errors or not). I can't see a way to access this from within the viewmodel itself, all the WPF validation seems to be contained in the view, is this right?
I don't think there is anything "built in" to achieve this. One option is to expose a property on each model in the OC such as IsValid, which you would set from within the model's IDataErrorInfo indexed property. Your VM can then use a bit of Linq to check for any models in the OC where IsValid=false.
If you are intending to have your VM expose some kind of "PageHasErrors" property, then you'll probably need to have the VM subscribe to each model's PropertyChange event (specifically the IsValid property), assuming the model implements INPC. Within the VM's event handler you would then update the "PageHasErrors" property based on whether any of the models IsValid=false.
I've seen many Tutorials about the MVVM-Patern but I still don't get why I need to get a Dependency-Property or an INotiyfyPropertyChanged-Property if I want to send information from the ViewModel back to the View.
Dependency properties provide built in change notification for when a property changes which means WPF knows when a controls value has changed.
Your ViewModel types do not, by default, provide any mechanism for change notification so if they don't support either of these options how is the view supposed to know when a property in your viewModel has been changed?
You need your viewModel to use either of these options so that the view can be notified when a property value changes.
This means if a property value is changed in code, the user interface is updated and if a property is changed by user input your viewModel (and ultimately your model) is also updated to reflect these changes. (basically both sides of a binding require a way of communicating a property change to each other).
The INotifyPropertyChanged interface is the preferred method as it means your viewModels are not specific to WPF and can be used by other user interface technologies. also, dependency properties can only be used in types that derive from DependencyObject.
First: You do not need to use INotifyPropertyChanged or DependencyObject at all.
But, and this is the central point in using Binding, there is some Pub / Sub Mechanism in
the Binding, which is listening to those PropertyChanged events and doing the update
of the view in case a relevant property for Binding has changed.
Here is more information on that:
SO on how binding works
Pub Sub aka Publish Subscribe Pattern
INotiyfyPropertyChanged - This property we used in the viewmodel so that if there happens any changes in the UI this property will reflect those changes.
As i was reading about ObservableCollection i came to know that it implements INotifyPropertyChanged which means whenever the view changes it's underlying collection also gets updated automatically which is exactly what is the purpose of TwoWay databinding. So, what is the difference between ObservableCollection and TwoWay databinding?
Thanks in advance :)
The two "things" TwoWay databinding and ObservableCollection are different things which can collaborate, but are not directly linked. Databinding is a concept and ObservableCollection is an implementation of an interface (INotifyCollectionChanged) which is used in the implementation of the concept of databinding in the .NET framework. In itself, INotifyCollectionChanged is a small part of this implementation.
In fact you can use TwoWay databinding without ObservableCollections and ObservableCollections without databinding.
Databinding is a mecanism allowing you to bind a business object property (Often a ViewModel property) to an UI property. OneWay Databinding provides support for update of UI when the object is changed (which requires implementation of INotifyPropertyChanged and/or INotifyCollectionChanged on the business object). TwoWay Databinding provides twoway support as its name suggest it: Object => UI (like OneWay do) AND UI => Object. UI to Object updates does not requires implementation of INotifyCollectionChanged nor INotifyPropertyChanged.
Observable collection now is interesting because it implements INotifyCollectionChanged, which makes it a good candidate to create databindable collections. But I use often INotifyCollectionChanged without databinding, and I am pretty sure you can find other collections which implement INotifyCollectionChanged (like PagedCollectionView)
Does it answer your question?
If you bind to your collection like this {Binding Path=MyCollection, Mode=TwoWay} then reference to collection will be updated but not its contents. Observable collection allows to track its contents changes, but not reference change (MyCollection.Clear() will trigger UI update, but MyCollection = anotherInstance will not).
The question is a little bit like "what's the difference between a horse and a cart?"
An ObservableCollection can be used for TwoWay databinding (to an ItemsControl).
It is more or less designed to make databinding easy.
Whenever a Control updates an ObservableCollection or the other way around, there is a DataBinding in the middle.
Object needs to implement INotifyPropertyChanged so you are able to bind to properties of this object.
TwoWayDatabinding tells databinding engine it needs to update properties both ways.
You are comparing apples and airplanes. Those are two completly different things.
Also ObservableCollection also implements INotifyCollectionChanged that allows to notify its items changed, not only properties.
I am building an object model with strongly typed collection classes (e.g. CustomerCollection). I want to support full two-way binding on both the collection itself and all of the data models in the collection.
For the models it seems like implementing INotifyPropertyChanged is the right way to wire up the models. But what inferface/base class should I use so that WPF knows when my collection's contents change?
ObservableCollection<T> - designed specifically for WPF binding.
I would recommend typing your properties that you expose as IList, or IEnumerable (generic or not, your choice), rather than ObservableCollection since it ties you into that implementation, and there are a number of situations that this becomes annoying.
The specific interface you need your collections to implement is INotifyCollectionChanged.