I read this
Simple Binding of Data from code behind to XAML
What's the link between INotifyPropertyChanged and Databinding above not clear from the sample code.
You will want to review Data Binding in the MSDN documentation for a more clear description of how data binding is working in Silverlight and WPF. When you define a data binding declaratively in XAML, an instance of the Binding class is created that connects the properties of binding targets and data sources. It is this Binding class that is performing the monitoring and propagation of changes between the target of the binding and the data source.
While it is technically possible to bind to CLR objects that do not implement the INotifyPropertyChanged interface, it is definitely recommended that "for changes to the source object to propagate to the target, the source must implement the INotifyPropertyChanged interface. INotifyPropertyChanged has the PropertyChanged event, which tells the binding engine that the source has changed so that the binding engine can update the target value." See How Data Binding References are Resolved for further details.
if you have a piece of code that needs to update the UI and the UI is bound to a Poperty like in that example, INotifyPropertyChanged is how we can update the UI without having to do anything extra.
When you bind your property in xaml (XP) to property in code (CP), the binding contains link to object, which contains CP and the name of CP. If the object inherits INotifyPropertyChanged, binding system subscribes on PropertyChanged event and every single time it rises, binding system receives property-name string from propertyChangedEventArgs object and updates bindings for all XPs which are binded to CP with that name.
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'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.
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.
My question is in the title. I read that a binding target must be a DP but a reply I got from a moderator on the WPF forum seemed to indicate that it was sufficient for my class to implement INotifyPropertyChanged in order to designate it as a binding target.
Targets of data bindings are required to be dependency properties. WPF data binding sources do not have to be dependency properties. Obviously there must exist some kind of mechanism for the source object to notify the outside world when the bound property has changed. For that you have to implement the INotifyPropertyChanged interface and has an event named PropertyChanged, which identifies the particular event that's changed with a string.
for more please have a look at this links
http://msdn.microsoft.com/en-us/library/ms522664.aspx
http://www.charlespetzold.com/blog/2006/03/210946.html
I have a business object that comes from the core of the application. That object does not inherit from INotifyPropertyChanged. It contains some property that my XAML code bind to it.
I only want to update the properties not the UI dynamically (OneWayToSource style).
By example, if I change the text of a text box, the source item does not get updated.
It is a limitation of silverlight that if the object does not implement INotifyPropertyChanged or use DepencyProperties that the source of the binding cannot be updated?
The source property does not need to be a dependency property nor does the class that exposes it need to implement INotifyPropertyChanged.
If you have the binding on the TextBox set to use the TwoWay mode editing the text box should update the bound property even if it is a plain "vanila" property. Note by default the focus has to leave the TextBox in order for the binding to update.
If your business object has a set method on the property you want to update, then the value should be updated, provided the value you enter doesn't trigger an exception.
Not implementing INotifyPropertyChanged hinders just visual feedback.