This question already has answers here:
What's the framework mechanism behind dependency properties?
(4 answers)
Closed 2 years ago.
Trying to understand how this code works:
Create dependency property,
public int YearPublished
{
get { return (int)GetValue(YearPublishedProperty); }
set { SetValue(YearPublishedProperty, value); }
}
public static readonly DependencyProperty YearPublishedProperty =
DependencyProperty.Register(
"YearPublished",
typeof(int),
typeof(SimpleControl),
new PropertyMetadata(2000));
Then use it in a form,
<xmlns:local="clr-namespace:WpfApplication1"
Title="MainWindow" Height="350" Width="525">
<StackPanel>
<local:SimpleControl x:Name="_simple" />
<TextBlock Text="{Binding YearPublished, ElementName=_simple}"
FontSize="30"
TextAlignment="Center" />
<Button Content="Change Value"
FontSize="20"
Click="Button_Click_1"/>
</StackPanel>
Then for Button_Click_1 do,
private void Button_Click_1(object sender, RoutedEventArgs e)
{
_simple.YearPublished++;
}
It works. Each time when you press the button, number must be changed from PropertyMetadata - from 2000++, but also I saw it on form in textBox.
Question: Why?
If I don't put any code for updating TextBlock in main Form, is it automatically updating or is there some hidden mechanism for it? Or maybe I do not fully understand how it works. Or maybe if its property there are features, that update the number on the form.
When you created a DependencyProperty,
DependencyProperty.Register(
"YearPublished",
typeof(int),
typeof(SimpleControl),
new PropertyMetadata(2000));
based on the YearPublished property, you are basically registering it with the DependencyProperty framework in a way that every time the property is read from or written to, it notifies all subscribers of the event that has taken place. You register it by specifying the name of the property i.e. "YearPublished", the property type, the type of the control where the property resides and, in this case the initial value of 2000.
By binding it to the TextBlock,
<TextBlock Text="{Binding YearPublished, ElementName=_simple}" />
you are letting the text block know when the property changes so it can update itself. When the YearPublished property changes, it notifies the text block of this change, which in turn retrieves the updated value and updates its Text property with it.
This is a very high level view, though, but enough to use it properly. To further understand the internals take a look at this MSDN post.
If the binding has the correct settings and the data provides the proper notifications, then, when the data changes its value, the elements that are bound to the data reflect changes automatically.
check this overview
Related
I am currently working on a MVVM project that uses a Window (with my ViewModel) and my own UserControl. The UserControl is nearly empty in the .xaml file because all of its functionality comes from code-behind, which draws different shapes. I wanted to bind a property from ViewModel to a DependencyProperty in the UserControl, but no matter what I do, i cannot get it to work. I have read tons of answers here and on different websites and noticed that it might be something with the UserControl's DataContext, but I eventually failed to fix the problem anyway. The way I raise the PropertyChanged event in my ViewModel is correct. I can successfully bind my property to other controls (like TextBoxes etc.), but not to my one. I would be grateful if you could explain to me why it is not working and how to fix that. Regards!
MainWindow.xaml binding:
<Grid Margin="10">
<local:FretboardControl Grid.Row="0" Fretboard="{Binding CurrentFretboard, Mode=TwoWay}"/>
</Grid>
FretboardControl.xaml:
<UserControl ...>
<Grid>
<TextBlock Text="{Binding Path=Fretboard, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=local:FretboardControl}}"/>
//the TextBlock above is just a test
<Canvas.../>
</Grid>
</UserControl>
FretboardControl.xaml.cs (code-behind):
public static readonly DependencyProperty FretboardProperty = DependencyProperty.Register
(nameof(Fretboard), typeof(Fretboard), typeof(FretboardControl), new PropertyMetadata(new Fretboard(), PropertyChangedCallback));
public Fretboard Fretboard {
get {
return GetValue(FretboardProperty) as Fretboard;
}
set {
SetValue(FretboardProperty, value);
}
}
protected static void PropertyChangedCallback(DependencyObject o, DependencyPropertyChangedEventArgs e) {
//breakpoint here. It is reached only once during runtime:
//at start, when the default value is inserted
if (o is FretboardControl) {
(o as FretboardControl).RefreshFretboard();
}
}
Okay, so apparently i probably found the seed of my problem. My CurrentFretboard setter raised the PropertyChanged event, but i did not change the reference of the object itself. The object was modified, but it was still the same object. I thought that this would not matter and it would be sent anyway to the binding, but it looks like the PropertyChangedCallback is called only if the reference was changed. I guess i can replace the reference on each set or just listen to the PropertyChanged event already in the UserControl. Thanks for help!
This question already has answers here:
How to implement two-way binding on a property?
(2 answers)
Closed 6 years ago.
I have some strange problem with DependecyProperty-binding.
To keep the question simpler i´ve created some dummy-control, that has the same unwanted behaviour
I have a UserControl, that has a DependencyProperty defined in code behind:
public static readonly DependencyProperty TestValueProperty = DependencyProperty.Register("TestValue", typeof(string), typeof(Test), new PropertyMetadata(default(string)));
public string TestValue
{
get { return (string)GetValue(TestValueProperty); }
set { SetValue(TestValueProperty, value); }
}
This property is used in XAML:
<Label Content="{Binding TestValue}" />
This control should be used in another control like this:
<views:Test TestValue="{Binding Settings.Setting123}" />
Settings is defined in viewmodel as property.
But the content of Settings.Setting123 is not visible in my usercontrol´s label.
When writing some fixes value instead of the binding it works fine:
<views:Test TestValue="Test" />
But of course i do not want a fixed value, but the content of the bound object.
Any hint what is going wrong here?
Thanks in advance!
You didn't share enough code for anybody to recreate the issue, but reading between the lines, I'm guessing that Label is in your UserControl XAML. If TestValue is a property of your UserControl, this will probably work:
<Label Content="{Binding TestValue, RelativeSource={RelativeSource AncestorType=UserControl}}" />
However, one reason you might have done that (and had it semi-work, with literal strings) is if you made your UserControl its own DataContext. In that case, then the problem is that you made your UserControl its own DataContext. If you did that, that Binding on the bound one is being evaluated in the context of the UserControl, which does not have a Settings.Setting123 property.
What a control's DataContext means, is that when you have a Binding on one of the controls properties or inside its XAML, that's where the Binding goes to look for the property you bind to. You're explicitly telling it to look in the wrong place.
If you make your UserControl its own DataContext, you can't bind anything to it. That's why you shouldn't do that. It's like one of those machines that does nothing but unplug itself from the wall. Instead, use {RelativeSource AncestorType=UserControl} bindings as above inside the UserControl XAML.
I shouldn't have to guess. You claim you created a minimal verifiable example, but didn't bother sharing it. If you share it, we can solve your problem with confidence.
I'm new to Windows 8.1 development, XAML, and C#, so if this question is rudimentary, please forgive me.
I've got a <Page> in my app that contains a <ListView>, like so:
<ListView ItemsSource="{Binding Mode=TwoWay}" x:Name="ListView_Statistical">
<ListView.ItemTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<TextBlock Style="{StaticResource SubheaderTextBlockStyle}" Width="100" Margin="10,20">
<Run Text="X/Y " />
<!--<Run Text="{Binding Source={StaticResource ThisPage}, Path=i}" />-->
</TextBlock>
<TextBox HorizontalAlignment="Left" Text="{Binding xVal}" PlaceholderText="X" InputScope="Number" FontSize="28" Width="100" Margin="0,10,10,10" />
<TextBox HorizontalAlignment="Left" Text="{Binding yVal}" PlaceholderText="Y" InputScope="Number" FontSize="28" Width="100" Margin="0,10,10,10" />
</StackPanel>
</DataTemplate>
</ListView.ItemTemplate>
</ListView>
In the Code Behind, I set up its DataContext like so:
ListView_Statistical.DataContext = this.statisticalPoints;
this.statisticalPoints is defined as such:
public ObservableCollection<StatisticalPoint> statisticalPoints
{
get { return (ObservableCollection<StatisticalPoint>)GetValue(statisticalPointsProperty); }
set {
SetValue(statisticalPointsProperty, value);
NotifyPropertyChanged("statisticalPoints");
}
}
// Using a DependencyProperty as the backing store for statisticalPoints. This enables animation, styling, binding, etc...
public static readonly DependencyProperty statisticalPointsProperty =
DependencyProperty.Register("statisticalPoints", typeof(ObservableCollection<StatisticalPoint>), typeof(EnterCalc), new PropertyMetadata(0));
I'm not sure if making this a DependencyProperty is necessary, or if making it follow INotifyPropertyChanged is necessary, but they don't seem to hurt.
Anyway, so in my constructor, I add a bunch of stuff to my statisticalPoints:
this.statisticalPoints = new ObservableCollection<StatisticalPoint>();
this.statisticalPoints.Add(new StatisticalPoint() { xVal = 1.0, yVal = 2.0 });
this.statisticalPoints.Add(new StatisticalPoint() { xVal = 33.0, yVal = 44.0 });
this.statisticalPoints.Add(new StatisticalPoint() { xVal = 555.0, yVal = 666.0 });
this.statisticalPoints.Add(new StatisticalPoint() { xVal = 0.7, yVal = 0.8 });
And when I load up the page, I do indeed see five rows in my ListView, populated as defined in my initialization of this.statisticalPoints.
The part I'm having trouble with is this:
I change the first value in the first <TextBox> in the ListView, then hit my save button... but ListView.Items doesn't have my change reflected, and I can't figure out how to look at the <TextBox> itself.
What I really want to do is have this bunch of statistical points modifiable by my user and be able to save their changes. To do that, I feel like I need to read the value in the <TextBox>es, but I can't figure out how to do that.
Alternatively, if the 'right way' to do this is to keep the data in this.statisticalPoints up-to-date when changes are made in the <TextBox>es, then I thought that a TwoWay binding Mode would do it, but neither ListView.Items nor this.statisticalPoints is changed when I make changes in the <TextBox>.
I do not have event handlers set up in those <TextBox> elements, as you can see, but do I need them, or am I missing something obvious?
Thanks in advance for any help you can give me!
To solve your initial problem, make the binding for each text box Mode="TwoWay". For reasons beyond my comprehension, the mode is default OneWay on pretty much everything in Windows Store Apps.
Making the ItemsSource binding two way does next to nothing, as the UI isn't changing the collection itself (by changing, I mean completely replacing). To iterate over your collection, just iterate over this.statisticalPoints and it will have the current data.
Now, you have a ton of other misconceptions so to try and run through them:
You never showed your save button, but the bindings either update your source or they don't. A save button is usually used to persist changes from the view model to the model.
Speaking of view models, you don't appear to have one. You shouldn't be directly setting the data context of controls, and certainly shouldn't have so much in code-behind. Create a proper view model object for your page, and bind ItemsSource to a public property of that view model.
NotifyPropertyChanged on a collection usually is unnecessary unless you are replacing the collection in code.
Having it won't hurt though, except, the setter of the backing property of a DependencyProperty (DP) is never called by the framework, so putting it there is just weird
And you don't really need a DP at all. DPs are there so a parent control can bind data to your special user control. Until you are using user controls, and really understand how DPs work, you shouldn't need to use them.
I have a GridView that has several buttons. One of them is defined by the following template:
<DataTemplate x:Name="SubjectItemTemplate">
<Canvas Width="340" Height="170" VerticalAlignment="Top">
<Controls:ThreeImageButton HorizontalAlignment="Center" VerticalAlignment="Top" Margin="0,0,0,0"
NormalStateImageSource="{Binding NormalImage}"
HoverStateImageSource="{Binding HoverImage}"
PressedStateImageSource="{Binding PressedImage}" Command="{Binding Path=NavigateToUnitsPage}"
CommandParameter="{Binding}" Canvas.Left="0" Canvas.Top="0">
</Controls:ThreeImageButton>
</Canvas>
</DataTemplate>
Now I have a custom control as you can see, called ThreeImageButton. The button works fine when I use it on its own. But when I have it in the DataTemplate it won't bind properties to the code behind.
Right now, I have
x:Name="MyThreeImageButton"
in the custom button definition. And I connect to the code-behind like this:
<TextBlock Text="{Binding ElementName=MyThreeImageButton, Path=NormalStateImageSource}"/>
(This is just a test to display the text, in the actual code I would assign an image source to another property that is referred to by an element).
Right now, nothing is displayed in the TextBlock. What is the correct binding syntax I'm supposed to use to reach my properties?
Thanks!
Edit: I am setting the variable in the InitializeComponent function and I am using SetValue on the DependencyProperty.
Edit: Let me add the following information to be more clear
Scenario I:
In DataTemplate for GridView:
<UserControl CustomParameter="Literal Text">
In UserControl:
<TextBlock Text="{Binding CustomParameter}">
in UserControl .cs: this.DataContext = this
works!
Scenario II:
In DataTemplate for GridView:
<UserControl CustomParameter="{Binding ValueFromDataItem">
In UserControl:
<TextBlock Text="{Binding CustomParameter}">
in UserControl .cs: this.DataContext = this
nope!
I see,
So setting up a two-way binding to a custom property in a user control can be tricky because a user control cannot bind to a CLR property. Not only that but setting the data context on a user control has an unexpected effect on the binding inside it.
You can solve these problems with a little slight of code. Basically back your CLR properties with dependency properties and set the data context on a child element instead of the root user control.
Take a look at this sample. Let's pretend you have the following MainPage. That MainPage will eventually use our custom user control. So let's set the stage.
Here's the code-behind:
public sealed partial class MainPage : Page
{
public MainPage()
{
this.InitializeComponent();
this.DataContext = new /* your view model */
{
Title = Guid.NewGuid().ToString(),
};
}
}
In the code above I am simulating a complex view model with a simple anonymous class. It would be silly for you to implement your own like this, but at the same time it is silly for me to build a simple sample with the complete scaffolding. I bring this up only so it does not confuse you - as it could look like I am suggesting this approach in prod.
Here's the XAML:
<Grid Background="{StaticResource ApplicationPageBackgroundThemeBrush}">
<local:MyUserControl Text="{Binding Title}" />
</Grid>
In the XAML above, there is absolutely nothing special. I already have reference to the user control in the local namespace and I simply declare it here.
Okay, now that we have a consumer of the control, it's worth pointing out that in testing developers can mistakenly think that their binding is working because they test with literal values. Literal values bind fine. It's binding from the underlying view model that hick-ups.
Let's say another thing, some developers tend to avoid dependency properties because the require a little more typing. People remember that [kbd]propdp[/kbd] is a handy Visual Studio snippet that stubs out a dependency property for you.
Take a look at this user control. It has two controls, a TextBox and a TextBlock which are there to demonstrate the OneWay and TwoWay functionality of this binding approach. We also implement INotifyPropertyChanged on the user control. For the most part, adding a view model in the case of a user control is overkill because the user control already acts like a view model. It's up to the developer, but it seems dumb to me.
Here's the code behind:
public sealed partial class MyUserControl : UserControl, INotifyPropertyChanged
{
public MyUserControl()
{
this.InitializeComponent();
}
// text property
public string Text
{
get { return (string)GetValue(TextProperty); }
set { SetValueDp(TextProperty, value); }
}
public static readonly DependencyProperty TextProperty =
DependencyProperty.Register("Text", typeof(string), typeof(MyUserControl), null);
// bindable
public event PropertyChangedEventHandler PropertyChanged;
void SetValueDp(DependencyProperty property, object value,
[System.Runtime.CompilerServices.CallerMemberName] String propertyName = null)
{
SetValue(property, value);
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
In the ode above, I have create a "Text" property and backed it with a dependency property. For a matter of reuse I have also implemented SetValueDp() which could be used again and again if I had more than a single property. Even though this demo has but one, I wanted to include this because the repetitive logic should certainly be abstracted out like this.
Here's the XAML:
<Grid Background="Black" DataContext="{Binding ElementName=userControl}">
<StackPanel>
<TextBox Text="{Binding Text, Mode=TwoWay}"
MinHeight="100" Padding="15" FontWeight="Light" FontSize="50" />
<TextBlock Text="{Binding Text}"
MinHeight="100" Padding="15" FontWeight="Light" FontSize="50" />
</StackPanel>
</Grid>
In the XAML above, I do nothing special insofar as binding. The syntax simply binds to the Text property using the Mode appropriate to the control. Just like you would do normally. However, what's worth noticing is that the DataContext is NOT set on the user control. Instead, it is set on the Grid. As a point of fact, any control in the tree other than the user control could be used like this. Just don't set the data context of the user control.
That is it by the way.
I have tested it to make sure it works. Demonstrating both one and two way binding is pretty handy here. I might even turn this into a blog in case other developers want to find it and don't discover this question. Thanks for your question!
Best of luck!
As the comments alluded to, your DataTemplate is placing the datacontext of the items to whatever object you are adding to your list. This is not the same as the surrounding user control's data context. If you want to reference that datacontext's commands, do the following in the DataTemplate's bindings:
{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, Path=DataContext.NormalImage}
What this is saying is to go out and find the user control ancestor and use its datacontext and then look for the NormalImage property. If you run into problems, check your output window for binding errors. It is very helpful in finding binding problems.
I have a kinda awful problem with my WPF application right now...
I have a custom UserControl used to edit details of a component. It should start by being not enabled, and become enabled as soon as the user chose a component to edit.
The problem is: the IsEnabled property does not even change.
Here is my code:
<my:UcComponentEditor Grid.Column="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
IsEnabled="{Binding EditorEnabled}"
DataContext="{Binding VmComponent}" />
EditorEnabled is a property in my ViewModel (VmComponent), and is by default false, becomes true when the user chose a component or created one
Just for the record, in my ViewModel:
private Boolean _editorEnabled = false;
public Boolean EditorEnabled
{
get { return _editorEnabled; }
set
{
_editorEnabled = value;
OnPropertyChanged("EditorEnabled");
}
}
When I try to launch my app, the UserControl is starting... enabled.
I added breakpoints everywhere, the EditorEnabled is false from the beginning.
I also did a horribly stupid thing to try to figure out what's happening: I created a converter (so useful -- converting a boolean to boolean -- eh), put a breakpoint on it, and... The code is never reached.
<my:UcComponentEditor Grid.Column="1" HorizontalAlignment="Stretch" VerticalAlignment="Stretch"
IsEnabled="{Binding EditorEnabled, Converter={StaticResource BoolConverter}}"
DataContext="{Binding VmComponent}" />
That probably means that the property isEnabled is never set, since the converter is never reached.
Do you see any kind of problem there? I started working in WPF about one week ago and therefore I may have missed something essential...
Thank you very much for your time :-)
You should add a DependencyProperty for the binding to work properly. See here for more information.
Code-behind:
public static readonly DependencyProperty EditorEnabledDependencyProperty = DependencyProperty.Register("EditorEnabled", typeof(bool), typeof(UcComponentEditor), new PropertyMetadata(false));
public bool EditorEnabled
{
get { return (bool)base.GetValue(UcComponentEditor.EditorEnabledDependencyProperty); }
set { base.SetValue(UcComponentEditor.EditorEnabledDependencyProperty, value); }
}
The issue I think is that there is a binding on the DataContext property of the user control. Which means the EditorEnabled property should be a property in the VmComponent object. At least that's what my problem was.
To get around it, I specified a proper source to the binding of IsEnabled. Once I did that the control started working as expected.
Hope that helps.
Encapsulating your control in a DockPanel (for example) will remove the need for a DependencyProperty.
You can then simply do your binding with the dockpanel instead of the custom control. Setting the variable bound to IsEnabled on the Dockpanel will automatically enable or disable the items contained in the Dockpanel.