I have searched on google about how to get started with the dependency property used in WPF/silverlight but didn't get any idea of the dependency property, can any one tell me about it , from beginner point of view, so that I get some idea about it and use it in my project
thanks in advance.
Can any one give me link or code example of simple application which explain in simple manner what is dependency Property is ???
I will be very thankfull
I find that implementing a DependencyProperty often involves four parts:
The DependencyProperty itself
Property with get and set
Static changed handler
Instance change handler
You can add a dependency property to a UserControl so that you can data bind to something in the DataContext where the UserControl is instantiated. For example you could add a property to a SoUserControl:
#region SampleProperty // Demo for SO 2424526
public static readonly DependencyProperty SamplePropertyProperty
= DependencyProperty.Register("SampleProperty", typeof(int), typeof(SoUserControl), new PropertyMetadata(OnSamplePropertyChanged));
/// <summary>
/// Demo for SO 2424526
/// Gets or sets dependency property.
/// </summary>
public int SampleProperty
{
get { return (int)GetValue(SamplePropertyProperty); }
set { SetValue(SamplePropertyProperty, value); }
}
/// <summary>
/// Handld changes to SamplePropertyProperty by calling a handler in the associated object.
/// </summary>
/// <param name="obj">object the property is associated with</param>
/// <param name="e">details of change</param>
static void OnSamplePropertyChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
(obj as SoUserControl).OnSamplePropertyChanged(e);
}
/// <summary>
/// Handle changes to the SamplePropertyProperty dependency property.
/// </summary>
/// <param name="e">details of change</param>
private void OnSamplePropertyChanged(DependencyPropertyChangedEventArgs e)
{
int SamplePropertyNewValue = (int)e.NewValue;
// do something with the internal logic of the control
}
#endregion
From my use with Dependency Properties. They become most important when binding. When you bind to display a regular property, the initial binding will work great, however, the UI will not always update when the property changes, in which case you could implement INotifyPropertyChanged on your class and raise a the NotifyPropertyChanged event, but a Dependency Property will update for you without implementing INotifyProperty Changed.
Triggers is another big one. If you wish to create a Trigger to fire off a WPF Animation whenever one of your properties is set to a certain value... then you need to be triggering off of a dependency property.
DepenedencyProperties can only be implemented on types that derive from DependencyObject.
UIElement derives from Visual which derives from DependencyObject, so most properties from .Net controls are dependency properties already. And when creating and registering my own DependencyProperties it's usually on my custom UserControls as that inherits from UIElement.
It was very frustrating for me when I first tried to create a dependency property on one of my regular classes (you're better off using INotifyPropertyChanged here)
Links:
http://msdn.microsoft.com/en-us/library/ms752914.aspx, http://www.wpftutorial.net/DependencyProperties.html
Have a look at Luis Abreu's blog :http://msmvps.com/blogs/luisabreu/
He's got a lot of info there about dependecy properties and how to use them.
I think you can compare it to the keyword yield.
it allows you to generate a lazy list, or an infinite generator.
every time it's needed one item is requested from that list, and the code continue with the flow.
A dependency property is the same idea for values or content. it computes the needed value, lazily, based on the other values it depends on.
it can cache the value, and when the underlying parts are "dirty", that is, something changed, it will propagate -- invalidate -- all the levels that depend on it.
the object needs to have the facilities of DependencyObject to participate (that is, so the engine will know about it, to introduce it to the chain.)
the mechanism is generic and unfixed, there isn't just one interface to comply with.
but it's somewhat more complex than what I said.
I have found a good explanation, to wire my understanding, in the following blog post:
http://blog.hackedbrain.com/2004/12/04/understanding-dependencyobject-and-dependencyproperty/
You should read a book, for starters.
I have read WPF In Action With Visual Sutdio 2005 from Manning.
As an answer for the negative punctuation just given to me, Dependency Property is part of a bigger plan called WPF and one can't understand Dependency Properties without the basics.
Therefore i think it's a waste of time to try to understand only what is a Dependency Property because i fear one will end up using it incorrectly.
Also, MSDN website has free information about WPF and Dependency Property.
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).
Do folks have any guidance on when a simple .NET property that fires INotifyPropertyChanged.PropertyChanged is sufficient in a view model? Then when do you want to move up to a full blown dependency property? Or are the DPs intended primarily for views?
There are a few approaches:
1. The dependency property
While you using the dependency property it makes the most sense in elements-classes that have a visual appearance (UIElements).
Pros:
WPF do the logic stuff for you
Some mechanism like animation use only dependency property
'Fits' ViewModel style
Cons:
You need to derive form DependencyObject
A bit awkward for simple stuff
Sample:
public static class StoryBoardHelper
{
public static DependencyObject GetTarget(Timeline timeline)
{
if (timeline == null)
throw new ArgumentNullException("timeline");
return timeline.GetValue(TargetProperty) as DependencyObject;
}
public static void SetTarget(Timeline timeline, DependencyObject value)
{
if (timeline == null)
throw new ArgumentNullException("timeline");
timeline.SetValue(TargetProperty, value);
}
public static readonly DependencyProperty TargetProperty =
DependencyProperty.RegisterAttached(
"Target",
typeof(DependencyObject),
typeof(Timeline),
new PropertyMetadata(null, OnTargetPropertyChanged));
private static void OnTargetPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
Storyboard.SetTarget(d as Timeline, e.NewValue as DependencyObject);
}
}
2. The System.ComponentModel.INotifyPropertyChanged
Usually, when creating a data object, you’ll use this approach. It is simple and neat solution for Data-like stuff.
Pros and Cons - complementary to 1. You need to to implement only one event (PropertyChanged).
Sample:
public class Student : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(PropertyChangedEventArgs e)
{
if (PropertyChanged != null)
PropertyChanged(this, e);
}
}
private string name;
public string Name;
{
get { return name; }
set {
name = value;
OnPropertyChanged(new PropertyChangedEventArgs("Name"));
}
}
3.PropertyNameChanged
Rising an event for each property with specified name(f.e. NameChanged). Event must have this name and it is up to you to handle/rise them. Similar approach as 2.
4. Get the binding
Using the FrameworkElement.GetBindingExpression() you can get the BindingExpression object
and call BindingExpression.UpdateTarget() to refresh.
First and second are the most likely depending what is your goal.
All in all, it is Visual vs Data.
As far as I know, DependencyProperty is only required when you need
PropertyValue inheritence
you need to allow the property to be set in Style setters
Use animation for the property
etc.
These features will not be available with normal properties.
DependencyProperty is required if you want to allow a binding to be set on the property. Usually this is for custom UIElements you create. You want to allow people to be able to bind data to your UIElements.
<local:MyUIElement MyProperty={Binding Path=SomethingToBindTo} />
To do this requires that MyProperty is a dependancy property
The main problem I see with INotifyPropertyChanged is if you viewmodel is complex containing many nested types it appears that you have to bubble the PropertyChanged event up through the hierarchy.
As the other answers have already said enough about when to create dependency property. i.e.
PropertyValue inheritence
you need to use binding on a property
Use animation for the property
The one more perspective/question on this is "In a WPF application is makes sense to create dependency properties in a control cause they are likely to change during user interaction like Height,width, text,content, background etc but what about other classes like Behaviors Classes(Non UI classes). Do properties in those classes need to be a dependency property?"
I won't say for very absolute or emphasis on some set of rules here but you should create your properties as DP. As from design perspective if a property is DP it's always in default form of WPF to use/bind.i.e.
As a DP is much more fast/natural in reflecting changes compare to a normal CLR property.
A DP has validation mechanism to validate the value assigned and a default structure to revert the value.
A DP has Coerce value callback to control the limits of property.
A DP has meta data associated with it unlike CLR property.
In terms of practices I've seen people doing many mistakes in nested bindings and then raising changes these kind of faults doesn't happen with a DP cause of it's design and compatibility of raising change itself. So with a little extra syntax you put a flexibility/performance/ easeness to your application. So go for it wherever affordable.
Still can't say sure for ViewModel classes/other helper classes. will update the answer if found convincing reasons in future.
Just a post worth reading on this topic
I have a complex WPF control that for some reasons (ie. performance) is not using dependency properties but simple C# properties (at least at the top level these are exposed as properties).
The goal is to make it possible to bind to some of those top level properties -> I guess I should declare them as DPs.(right? or is there some other way to achieve this? )
I started reading on MSDN about DependencyProperties and DependencyObjects and found an example:
public class MyStateControl : ButtonBase
{
public MyStateControl() : base() { }
public Boolean State
{
get { return (Boolean)this.GetValue(StateProperty); }
set { this.SetValue(StateProperty, value); }
}
public static readonly DependencyProperty StateProperty = DependencyProperty.Register(
"State", typeof(Boolean), typeof(MyStateControl),new PropertyMetadata(false));
}
If I'm right - this code enforces the property to be backed up by DependencyProperty which restricts it to be a simple property with a store(from functional point of view, not technically) instead of being able to calculate the property value each time getter is called and setting other properties/fields each time setter is called.
What can I do about that? Is there any way I could make those two worlds meet at some point?
//edit
I guess I have to tell you a little more about what I want to do and what my limitations are. So:
I have TabControl that is bound to a collection of ViewModel(I'm using MVVM pattern) objects. Every tab is meant to be an editor for one object of that collection. Objects can be of different types so I have multiple definitions each with a different DataType property. Now I have that complex WPF Control that I want to use as a part of one of those DataTemplates. If I use usual TextBox I can simply bind to its Text property, but I can't do the same with Text property of that custom control simply because its Text property is not a dependency property.
In this scenario I have :
no direct access to the control itself nor to its events
no code behind file that I can use to do that kind of thing
I can see however a dirty solution -
In the Window class I would have to subscribe to CollectionChanged event of the collection that is bound to the TabControl.
Whenever an item is added to that collection use ItemContainerGenerator to obtain a copy of I suppose TabItem and use it to find the right copy of 'complex control'
Regiter items handlers to 'complex controls' events to do the job
This is wrong because:
this is agains MVVM - I have to play with tab control to do the job instead of doing it in the ViewModel class
this couples in an unwanted way the view and viewmodel
I think you are mixing up Dependency Properties and implementing INotifyPropertyChanged on your classes.
You don't need your property to be a dependency property, you just need your class to implement INotifyPropertyChanged and call OnPropertyChanged whenever the state of your object changes in a way that would affect the value you want to expose to binding.
So let's say you have a property Sum that you want to bind to. The Sum property simple adds two other properties (or fields, or whatever) together. When anything happens that affects the Sum calculation, you want to notify that the Sum value has changed, so the any controls bound to Sum get updated.
public int Sum => Value1 + Value2;
public int Value1
{
set
{
// changing this affects "Sum", so I need to notify that the binding should update
_value1 = value;
OnPropertyChanged("Sum");
}
}
public int Value2
{
set
{
// changing this affects "Sum", so I need to notify that the binding should update
_value2 = value;
OnPropertyChanged("Sum");
}
}
It seems to me that you've been saddled with a WPF user control that was built by someone who didn't intend it to be used with data binding. I would assume that this is for one of two reasons: a) there's some logical reason that you shouldn't be able to bind to this property, or b) the original author of this control didn't know what he was doing. You haven't provided enough information for me to know which of those two conditions is the one you're really working under.
But in general, the reason you can't expose calculated properties for binding is that calculated properties generally don't have a setter. It doesn't make sense to set a property whose value is calculated.
If there are other properties whose values need to be updated when this one changes, the right approach (or at least the one consonant with the design of dependency properties) is to handle those updates in the dependency property's callback function, which is kind of what the callback function is for.
Please, don't judge strictly if this question was discussed previously or indirectly answered in huge nearby prism and mvvm blogs.
In WPF implementation of RelayCommand or DelegateCommand classes there is a such eventhandler
/// <summary>
/// Occurs whenever the state of the application changes such that the result
/// of a call to <see cref="CanExecute"/> may return a different value.
/// </summary>
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
but in SL subset of namespaces there are no CommandManager class. And this is where I'm stuck. I haven't yet found an workaround for this in MVVM adoptation for SL (PRISM is so complex for me yet). Different simple HelloWorldMVVM apps don't deal with at all.
Thanks in advance and sorry for my English -)
There is no support for commands in Silverlight. When the user manipulates controls in your view you will have to write code (e.g. event handlers) that modifies the view-model in the code-behind for your view. This might be something as simple as calling a method on the view-model when the user clicks a button in the view. By using PRISM you are able to create attached properties in the XAML for your view to get rid of these event-handlers, but if you would rather not use PRISM you can simply stick to using event handlers.
One other aspect you will have to handle is modifying the view when the view-model changes. In particular you will want to enable and disable controls based on the state of the view-model. To achieve this you will have to bind the IsEnabled property of a control to something in the view-model that reflects if a certain operation is allowed. Implementing custom IValueConverter objects that converts to boolean values are often useful. For instance, if your view-model has a property that represents a count and you want a particular control in the view to only be enabled when the count is greater than zero you can create a value converter that converts to true when the number is greater than zero and use this value converter in the binding.
If you try to adapt a WPF example of an MVVM application you will have to get rid of all uses of commands and substitute your own code. The code in your example is not meaningful in Silverligt, but in WPF it is involved in the process of determining if a control in the view is enabled, visible etc.
Silverlight 4.0 supports the ICommand interface and thus will provide a WPF like commanding infrastructure.
I am new to WPF and trying to wrap my head around WPF's framework, what it does and does not do for you.
To clarify this, I would like to know what is the difference between this:
public List<MyCustomObject> MyCustomObjects
{
get { return (List<MyCustomObject>)GetValue(MyCustomObjectsProperty); }
set { SetValue(MyCustomObjectsProperty, value); }
}
public static readonly DependencyProperty MyCustomObjectsProperty =
DependencyProperty.Register("MyCustomObjects", typeof(List<MyCustomObject>),
typeof(Main), new UIPropertyMetadata(new List<MyCustomObject>()));
and this:
public ObservableCollection<MyCustomObject> MyCustomObjects { get; set; }
public Main ()
{
MyCustomObjects = new ObservableCollection<<MyCustomObject>();
}
Ok, we must put some order into things, there's a few concepts mixed in together here.
First of all, you're asking what the difference is between a field-backed property and a dependency property. Google would be your best friend, however I recommend this blog post by WPF's vanguard Josh Smith: Overview of dependency properties in WPF
In short: dependency properties support the richness that is WPF: Styling, animation, binding, metadata, and more.
Secondly, you're asking what the difference is between a List and an ObservableCollection. Well the latter provides change notifications (in the forms of events) on any change to the collection (addition, removal, change of order, clearing, etc.), and the former does not. You can read more about that here: The ObservableCollection Class
In short: ObservableCollection provides change notifications which are required for the UI to automatically reflect changes in the view model.
In addition to Aviad and Reed's answers, I would like to point out a serious bug in your first code sample :
public static readonly DependencyProperty MyCustomObjectsProperty =
DependencyProperty.Register("MyCustomObjects", typeof(List<MyCustomObject>),
typeof(Main), new UIPropertyMetadata(new List<MyCustomObject>()));
The new List<MyCustomObject>() used as the default value will be created only once, so by default all instances of your type will share the same List<MyCustomObject> instance, which is probably not what you want... The only sensible default value here is null
In the first case, you're setting up a Dependency Property containing a List<T> instance.
In the second, you're making a normal CLR property, but having it setup as an ObservableCollection<T>.
For WPF Data Binding, there are some differences here.
Typically, you want all of your properties in the DataContext (which is the object that, by default, things "bind" to) to either implement INotifyPropertyChanged or to be a Dependency Property. This lets the binding framework know when changes are made to that object. Normally, though, you'd only use a Dependency Property if your working with a custom control - it's usually a better idea to have your object to which your data bound be a separate class, assigned to the DataContext. (For details here, see Josh Smith on MVVM or my recent detailed post on MVVM...)
However, with a collection, you typically also want the binding system to know when the items within the collection change (ie: an item is added). ObservableCollection<T> handles this by implementing INotifyCollectionChanged.
By using the second approach (using an ObservableCollection<T>), your UI can tell when items were added or removed from the collection - not just when a new collection is assigned. This lets things work automatically, like a ListBox adding elements when a new item is added to your collection.
1:
You're using a dependency property to "tell" the framework when that property is changed. This will have the following consequences for your binding:
MyCustomObjects.Add(new MyCustomObject()); //Wont update the view through databinding
MyCustomObjects = new List<MyCustomObject>(); //Will update the view through databinding
You could gain the same databinding functionality by implementing INotifyPropertyChanged on which ever class exposes the property, but dependency properties a capable of much more than just notifying about changes. These are rather advanced features though, which you aren't likely to come across in your average joe app :)
2:
You're using an observable collection, which implements INotifyCollectionChanged for you, to tell the databinding whenever the content of the collection has changed. This will have the opposite consequences than #1:
MyCustomObjects.Add(new MyCustomObject()); //Will update the view through databinding
MyCustomObjects = new ObservableCollection<MyCustomObject>(); //Won't update the view through databinding