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.
Related
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).
I'm currently working with the ArcGIS Runtime SDK .NET where I'd like to have the current viewport of the map bound to a property on my view model. However, the view port (expressed as Extent) does not have a setter, despite being a dependency property.
In order to set the viewport, I need to call a method on the control. But how do I do that from the view model? I already found similiar questions here on SO, but most of them were answered with The view model shouldn't be aware of the view. I agree with that, but unfortunately I can't change the fact that the setter of a property on a proprietary control is a different method than the actual property I can bind to and read values from.
There are many ways to do the job that might be considered MVVM friendly.
The one that I suggest is that you wrap your ArcGIS view in a custom control that you have full control over. This way you can expose your required dependency properties and handle their setters in your custom control.
I use this method almost every time I'm using a third-party component this way I make the component loosely coupled with my other code and I can replace them easily.
You can use an attached property to push make a VM request any listening view to go to a certain viewpoint. I'm using that in my sample here:
https://github.com/Esri/arcgis-runtime-demos-dotnet/blob/master/src/TurnByTurn/RoutingSample.Shared/CommandBinder.cs
In your VM you simply raise INPC for a viewpoint:
public Viewpoint ViewpointRequested
{
get { return m_ViewpointRequested; }
private set
{
m_ViewpointRequested = value;
RaisePropertyChanged("ViewpointRequested");
}
}
And then lastly just bind this to the attached property on the MapView:
<esri:MapView Map="{Binding Map}"
local:CommandBinder.RequestView="{Binding ViewpointRequested}" />
When should I use dependency properties in WPF?
They are static, so we save a lot on memory, compared to using .NET properties.
Other gains of using dependency properties over .NET properties are:
1) no need to check thread access
2) prompt a containing element to be rendered
etc...
So it seems I should ALWAYS use dependency properties in my projects where I use WPF?
Maybe for some trivial properties of helper classes here and there I could
get away with .NET properties...
Dependency Property is a broad concept to explain which might take couple of pages to write. So just to answer your main question, Dependency property is used where
You know the property is going to be the binding target i.e you are creating a user control/custom control and want property that should be binding driven.
You want automatic property change notifications (coerse and validation also).
We want value inheritance in styles,themes, parent or default value.
There is not need to create the properties on Model or ViewModel layer as dependency properties most of the time as that is not going to help much on memory saving front as most of the properties we define in model/VM will have values per instance as they will be constantly changing. Resolving Dependency property value is a burden in itself so making property dependency unnecessarily is not advisable.
Thanks
The main reason to create DependencyProperty is when you write you own WPF control.
DependencyProperties can be used as binding source and target, and can be animated.
The properties of all framework's controls are implemented as DependencyProperty, that why you can make powerful data binding in XAML.
However in most situation, like in with the MVVM pattern, you don't need dependency properties, INotifyPropertyChanged is enough.
The main difference is, that the value of a normal .NET property is read directly from a private member in your class, whereas the value of a DependencyProperty is resolved dynamically when calling the GetValue() method that is inherited from DependencyObject.
When you set a value of a dependency property it is not stored in a field of your object, but in a dictionary of keys and values provided by the base class DependencyObject. The key of an entry is the name of the property and the value is the value you want to set.
The advantages of dependency properties are
Reduced memory footprint:
It's a huge dissipation to store a field for each property when you think that over 90% of the properties of a UI control typically stay at its initial values. Dependency properties solve these problems by only store modified properties in the instance. The default values are stored once within the dependency property.
Value inheritance:
When you access a dependency property the value is resolved by using a value resolution strategy. If no local value is set, the dependency property navigates up the logical tree until it finds a value. When you set the FontSize on the root element it applies to all textblocks below except you override the value.
Change notification:
Dependency properties have a built-in change notification mechanism. By registering a callback in the property metadata you get notified, when the value of the property has been changed. This is also used by the databinding.
check the below url for more details about the magic behid it
dependency properties in WPF
CLR Property vs. Dependency Property
A CLR property reads directly from the private member of the class. The Get() and Set() methods of the class retrieve and store the values of the property.
Whereas when you set a value of a Dependency Property it is not stored in a field of your object, but in a dictionary of keys and values provided by the base class DependencyObject. The key of an entry is the name of the property and the value is the value you want to set.
Advantages of a Dependency Property
Less memory consumption
The Dependency Property stores the property only when it is altered or modified. Hence a huge amount of memory for fields are free.
Property value inheritance
It means that if no value is set for the property then it will return to the inheritance tree up to where it gets the value.
Change notification and Data Bindings
Whenever a property changes its value it provides notification in the Dependency Property using INotifyPropertyChange and also helps in data binding.
Participation in animation, styles and templates
A Dependency Property can animate, set styles using style setters and even provide templates for the control.
CallBacks
Whenever a property is changed you can have a callback invoked.
Resources
You can define a Resource for the definition of a Dependency Property in XAML.
Overriding Metadata
You can define certain behaviours of a Dependency Property using PropertyMetaData. Thus, overriding a metadata from a derived property will not require you to redefine or re-implement the entire property definition.
Perhaps you should take another look at the Dependency Properties Overview page at MSDN.
Personally, I would only ever create a DependencyProperty when I really need to. For the most part, I use normal CLR properties in data type and view model classes to bind to... this is absolutely fine, as long as I implement the INotifyPropertyChanged interface.
So for all of my usual data bindings, I use normal CLR properties. I only declare a Dependency Property in order to provide some added functionality in a UserControl.
Dependency properties are used when you want data binding in a UserControl, and is the standard method of data binding for the WPF Framework controls. DPs have slightly better binding performance, and everything is provided to you when inside a UserControl to implement them.
Otherwise, you typically use INotifyPropertyChanged for binding elsewhere, since it's easier to implement in stand-alone classes and has less overhead. As far as your original assumptions:
There is a local instance of your variable, you definitely do not save any overhead over a property since there is significant data binding logic built-in.
They must be accessed on the main thread.
I am new to WPF/MVVM and the examples I have found do not seem to cover an issue I am facing.
I have a screen for managing a rather complex business configuration object. In MVVM I think this means I should have the following:
A XAML View with close to zero logic
A ViewModel class that has the screen logic
My usual business classes fill the role of Model and have all business logic
In my situation there are business rules that say changes to fieldA of my business class might have various side effects, for example changing the value of fieldB, or populating an entire list of sub-objects.
I could be wrong, but I think I should keep these rules encapsulated in the business class, as these rules are not really about the screen so much as the entity.
Naturally, these side-effects need to make their way back onto the screen immediately.
So from the user's perspective, he might edit fieldA, and see fieldB updated on the View.
I understand how to databind from the View to the ViewModel.
But in my case, it seems that I need two layers of databinding: one between the View and ViewModel, and another between the ViewModel and the Model.
Given that I have essentially the same problem twice, I think one solution should apply. So I have made my Model class into a DependencyObject, and I have made its properties into DependencyProperties.
Looking at fieldA for example, it would appear in all three layers:
View as a visual component databound to ViewModel.FieldA, for example text="{Binding FieldA, Mode=TwoWay}"
ViewModel as a DependencyProperty bound "upward" to the View, and "downward" to the Model
Model as a DependencyProperty
I prefer not to directly couple my View XAML to the business object by skipping part #2, this does not seem like a clean application of the pattern to me. Perhaps that is misguided.
I essentially seem to need a "pass-through DependencyProperty" in my ViewModel.
My questions:
Is this the right general approach or am I thinking about it all wrong?
Are there examples out there using this pass-through approach?
Can someone give a code example of the proper way to create a pass-through binding between the ViewModel and Model FieldA DependencyProperties?
I struggled with this issue myself, and I imagine it is a very common snag when it comes to MVVM. My answer was to avoid polluting the domain with DependencyObject or INotifyPropertyChanged as it somewhat negates the validity of using a ViewModel.
The goal of a ViewModel is to expose a model in a manner that is relevant to a particular view. It gets confusing when the VM essentially needs to expose an entire domain object. I refer to these as "Editor" view models. These are the most tempting to pass through properties from the domain object. In these cases I give the VM a reference to a domain object (composition) and pass through getters and setters. The ViewModel adopts INotifyPropertyChanged and IDataErrorInfo rather than DependencyProperty to signal the UI if the UI needs to refresh or display validation errors. If the domain raises a validation error, then the VM catches it and prepares it into the data error info details for the view.
I would agree with Steve that you should avoid using DependencyObject and DependencyProperty in your model/domain classes, and the view model classes should adopt INotifyPropertyChanged and IDataErrorInfo for binding purposes.
I would add that in your view model classes, I would avoid using DependencyProperty except for properties that you need to utilize in xaml logic, such as DataTriggers.
For handling changes that are triggered within a model layer class, I would also have a reference to the model/domain object in the view model class, and pass through getters and setters to the model/domain class just as Steve mentioned. I would add that the model/domain class will need to raise an event that the view model class will need to subscribe to, so that OnPropertyChanged() for one or more properties can be called in your view model class based on a change that happened in your business logic.
Firstly, I would not recommend using dependency properties (DP) for view models, or models. DPs are properties that have been designed with UI concepts in mind, such as precedence rules, DataContext support, default values and more. You do not need these concepts within a view models, so you should use INotifyPropertyChanged instead.
Having a view-model that is simply a pass-through to a model layer adds no value at all. So don't do it! You should never add layers, structures or concepts to your code just because you think you should. Simplicity is something that you should always strive for.
So, if you can implement a model layer, with INotifyPropertyChanged simply bind that to your view.
However ... in some cases you may not be able to implement INotifyPropertyChanged in your model. It might be generated from a web service for example. In this case you will need a view model that performs pass-through functionality, but also adds change notification via INotifyPropertyChanged.
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