I have UserControl, lets call it as CustomDataGrid, that contains DataGrid. Remained content doesn't matter. SelectedItem property of DataGrid must be SelectedItem property of CustomDataGrid. And I wanna be able to use Binding with this property, cause I use MVVM pattern. So I have to declare SelectedItem as DependencyProperty in CustomDataGrid. But I have no ideas haw can I make it properly...
This is how DepedencyProperty-s is declared usually:
public static readonly DependencyProperty SelectedItemProperty = DependencyProperty.Register(
"SelectedItem", typeof(Object), typeof(CustomDataGrid),
new FrameworkPropertyMetadata(default(Object), SelectedItemPropertyCallback)
{
BindsTwoWayByDefault = true,
DefaultUpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged
});
// Optionally
private static void SelectedItemPropertyCallback(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{
// dataGrid - `DataGrid` nested in `UserControl`
((CustomDataGrid)obj).dataGrid.SelectedItem = e.NewValue;
}
// Obviously, it has no any link with nested `dataGrid`. This is the problem.
public Object SelectedItem
{
get { return GetValue(SelectedItemProperty); }
set { SetValue(SelectedItemProperty, value); }
}
So, how can I declare SelectedItem property correctly?
You could leverage the binding framework for wiring such properties from underlying objects to outer containers
example assuming CustomDataGrid as UserControl
class CustomDataGrid : UserControl
{
public CustomDataGrid()
{
Binding b = new Binding("SelectedItem");
b.Source = this;
b.Mode = BindingMode.TwoWay;
dataGrid.SetBinding(DataGrid.SelectedItemProperty, b);
}
public object SelectedItem
{
get { return (object)GetValue(SelectedItemProperty); }
set { SetValue(SelectedItemProperty, value); }
}
// Using a DependencyProperty as the backing store for SelectedItem. This enables animation, styling, binding, etc...
public static readonly DependencyProperty SelectedItemProperty =
DependencyProperty.Register("SelectedItem", typeof(object), typeof(CustomDataGrid), new PropertyMetadata(null));
}
I have created a property called SelectedItem in CustomDataGrid and set a two way binding to SelectedItem of the actual dataGrid inside.
so this will wire up these properties and will propagate any changes to and fro.
XAML solution!
Use this DependencyProperty:
public object SelectedItem
{
get { return GetValue(SelectedItemProperty); }
set { SetValue(SelectedItemProperty, value); }
}
public static readonly DependencyProperty SelectedItemProperty = DependencyProperty.Register("SelectedItem", typeof(object), typeof(CustomDataGrid ), new FrameworkPropertyMetadata(null)
{
BindsTwoWayByDefault = true,
DefaultUpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged
});
Then make your outer CustomDataGrid UserControl XAML look like this:
<UserControl x:Class="CustomDataGrid">
<DataGrid ItemsSource="{Binding ItemsSource, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type CustomDataGrid}}}"
SelectedItem="{Binding SelectedItem, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type CustomDataGrid}}}">
</DataGrid>
</UserControl>
You can then use the CustomDataGrid control the same way as the DataGrid control when binding ItemsSource and SelectedItem to your view model.
I have a custom UserControl that contains a grid ...I wish to set the ItemsSource property of that grid by xaml code of of a data template in a resource dictionary...
then I have used dependency property... this is my implementation...
public partial class MyControlGrid : UserControl
{
// Dependency Property
public static readonly DependencyProperty MyItemSourceProperty =
DependencyProperty.Register("MyItemSource", typeof(ICollectionView),
typeof(MyControlGrid), new FrameworkPropertyMetadata(null, OnMyItemSourcePropertyChanged));
IDictionary<string, string> _columns = new Dictionary<string, string>();
private static void OnMyItemSourcePropertyChanged(DependencyObject obj,
DependencyPropertyChangedEventArgs args)
{
// When the color changes, set the icon color PlayButton
MyControlGrid muc = (MyControlGrid)obj;
ICollectionView value = (ICollectionView)args.NewValue;
if (value != null)
{
muc.MyGridControl.ItemsSource = value;
}
}
public ICollectionView MyItemSource
{
get
{
return (ICollectionView)GetValue(MyItemSourceProperty);
}
set
{
SetValue(MyItemSourceProperty, value);
//OnTargetPowerChanged(this, new DependencyPropertyChangedEventArgs(TargetPowerProperty, value, value));
// Old value irrelevant.
}
}
public MyControlGrid()
{
InitializeComponent();
}
}
this is the user control xaml code
<UserControl x:Class="GUI.Design.Templates.MyControlGrid"
Name="MyListControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:WpfTkit="http://schemas.microsoft.com/wpf/2008/toolkit"
xmlns:Templates="clr-namespace:Emule.GUI.Design.Templates">
<StackPanel>
<WpfTkit:DataGrid ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Templates:MyControlGrid}}, Path=MyItemSource}"
x:Name="MyGridControl"
<StackPanel>
this is the binding path expression I use
<basic:MyControlGrid MyItemSource="{Binding MyDataContextVisibleCollection}"/>
this dont work and wpf output window dont show me any errors
note that, naturally, if I bind this directly in the user controls work fine
<WpfTkit:DataGrid ItemsSource="{Binding MyDataContextVisibleCollection}"
Waths I wrong?
thanks
p.s. sorry for my english
this
answer show me the way
use of PropertyChangedCallback work fine with my code:
public static readonly DependencyProperty MyItemSourceProperty =
DependencyProperty.Register("MyItemSource", typeof(IEnumerable),
typeof(MyControlGrid), new FrameworkPropertyMetadata(null,
new PropertyChangedCallback(MyControlGrid.OnItemsSourceChanged)));
alternatively I have to remove comment on OnTargetPowerChanged and fire the property changed event
set
{
SetValue(MyItemSourceProperty, value);
//OnTargetPowerChanged(this, new DependencyPropertyChangedEventArgs(TargetPowerProperty, value, value));
// Old value irrelevant.
}
correct with
public ICollectionView MyItemSource
{
get
{
return (ICollectionView)GetValue(MyItemSourceProperty);
}
set
{
SetValue(MyItemSourceProperty, value);
OnItemsSourceChanged(this, new DependencyPropertyChangedEventArgs(MyItemSourceProperty, value, value));
}
}
I'm trying to make an intertia touch scrolling list in a UserControl in Silverlight 4 using Expression Blend 4. I've already made the dependency properties in my UserControl which i want to work like the ListBox does. ItemSource is the list of objects i want to show in my list and datatemplate is the way it should be shown.
How do i deal with these properties inside my UserControl? I have a StackPanel where all the datatemplates should be added showing the data ofc.
How do i apply the data in my IEnumerable to the DataTemplate when looping through the ItemSource to add them to the list (StackPanel).
public static readonly DependencyProperty ItemsSourceProperty = DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof(InertiaScrollBox), null);
public IEnumerable ItemsSource
{
get{ return (IEnumerable)GetValue(ItemsSourceProperty); }
set{ SetValue(ItemsSourceProperty, value); }
}
public static readonly DependencyProperty ItemTemplateProperty = DependencyProperty.Register("ItemTemplate", typeof(DataTemplate), typeof(InertiaScrollBox), null);
public DataTemplate ItemTemplate
{
get { return (DataTemplate)GetValue(ItemTemplateProperty); }
set { SetValue(ItemTemplateProperty, value); }
}
This was kinda hard to explain but hope you understand otherwise please ask. Thanks in advance
Dependency properties are rather useless if to not handle their changes.
At first you should add PropertyChanged callbacks. In my example I add them inline and call the UpdateItems private method.
public static readonly DependencyProperty ItemsSourceProperty =
DependencyProperty.Register("ItemsSource", typeof(IEnumerable), typeof(InertiaScrollBox),
new PropertyMetadata((s, e) => ((InertiaScrollBox)s).UpdateItems()));
public static readonly DependencyProperty ItemTemplateProperty =
DependencyProperty.Register("ItemTemplate", typeof(DataTemplate), typeof(InertiaScrollBox),
new PropertyMetadata((s, e) => ((InertiaScrollBox)s).UpdateItems()));
Then you can call the LoadContent method of the DataTemplate class and set an item from the ItemsSource as the DataContext to the returned visual element:
private void UpdateItems()
{
//Actually it is possible to use only the ItemsSource property,
//but I would rather wait until both properties are set
if(this.ItemsSource == null || this.ItemTemplate == null)
return;
foreach (var item in this.ItemsSource)
{
var visualItem = this.ItemTemplate.LoadContent() as FrameworkElement;
if(visualItem != null)
{
visualItem.DataContext = item;
//Add the visualItem object to a list or StackPanel
//...
}
}
}
XAML:
<navigation:Page ... Title="{Binding Name}">
C#
public TablePage()
{
this.DataContext = new Table()
{
Name = "Finding Table"
};
InitializeComponent();
}
Getting a ag_e_parser_bad_property_value error in InitializeComponent at the point where the title binding is happening. I've tried adding static text which works fine. If I use binding anywhere else eg:
<TextBlock Text="{Binding Name}"/>
This doesn't work either.
I'm guessing it's complaining because the DataContext object isn't set but if I put in a break point before the InitializeComponent I can confirm it is populated and the Name property is set.
Any ideas?
You can only use data binding on properties that are supported by DependencyProperty. If you take a look at the docs for TextBlock for example you will find that the Text property has a matching TextProperty public static field of type DependencyProperty.
If you look at the docs for Page you will find that there is no TitleProperty defined, the Title property is therefore not a dependency property.
Edit
There is no way to "override" this however you could create an attached property:-
public static class Helper
{
#region public attached string Title
public static string GetTitle(Page element)
{
if (element == null)
{
throw new ArgumentNullException("element");
}
return element.GetValue(TitleProperty) as string;
}
public static void SetTitle(Page element, string value)
{
if (element == null)
{
throw new ArgumentNullException("element");
}
element.SetValue(TitleProperty, value);
}
public static readonly DependencyProperty TitleProperty =
DependencyProperty.RegisterAttached(
"Title",
typeof(string),
typeof(Helper),
new PropertyMetadata(null, OnTitlePropertyChanged));
private static void OnTitlePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
Page source = d as Page;
source.Title = e.NewValue as string;
}
#endregion public attached string Title
}
Now your page xaml might look a bit like:-
<navigation:Page ...
xmlns:local="clr-namespace:SilverlightApplication1"
local:Helper.Title="{Binding Name}">
Add the following to MyPage.xaml.cs:
public new string Title
{
get { return (string)GetValue(TitleProperty); }
set { SetValue(TitleProperty, value); }
}
public static readonly DependencyProperty TitleProperty =
DependencyProperty.Register("Title",
typeof(string),
typeof(Page),
new PropertyMetadata(""));
Once you add this property (dependency property) to your code behind, your code will work as normal.
I'm trying to bind to a Readonly property with OneWayToSource as mode, but it seems this cannot be done in XAML:
<controls:FlagThingy IsModified="{Binding FlagIsModified,
ElementName=container,
Mode=OneWayToSource}" />
I get:
The property 'FlagThingy.IsModified' cannot be set because it does not have an accessible set accessor.
IsModified is a readonly DependencyProperty on FlagThingy. I want to bind that value to the FlagIsModified property on the container.
To be clear:
FlagThingy.IsModified --> container.FlagIsModified
------ READONLY ----- ----- READWRITE --------
Is this possible using just XAML?
Update: Well, I fixed this case by setting the binding on the container and not on the FlagThingy. But I'd still like to know if this is possible.
Some research results for OneWayToSource...
Option # 1.
// Control definition
public partial class FlagThingy : UserControl
{
public static readonly DependencyProperty IsModifiedProperty =
DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());
}
<controls:FlagThingy x:Name="_flagThingy" />
// Binding Code
Binding binding = new Binding();
binding.Path = new PropertyPath("FlagIsModified");
binding.ElementName = "container";
binding.Mode = BindingMode.OneWayToSource;
_flagThingy.SetBinding(FlagThingy.IsModifiedProperty, binding);
Option # 2
// Control definition
public partial class FlagThingy : UserControl
{
public static readonly DependencyProperty IsModifiedProperty =
DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());
public bool IsModified
{
get { return (bool)GetValue(IsModifiedProperty); }
set { throw new Exception("An attempt ot modify Read-Only property"); }
}
}
<controls:FlagThingy IsModified="{Binding Path=FlagIsModified,
ElementName=container, Mode=OneWayToSource}" />
Option # 3 (True read-only dependency property)
System.ArgumentException: 'IsModified' property cannot be data-bound.
// Control definition
public partial class FlagThingy : UserControl
{
private static readonly DependencyPropertyKey IsModifiedKey =
DependencyProperty.RegisterReadOnly("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());
public static readonly DependencyProperty IsModifiedProperty =
IsModifiedKey.DependencyProperty;
}
<controls:FlagThingy x:Name="_flagThingy" />
// Binding Code
Same binding code...
Reflector gives the answer:
internal static BindingExpression CreateBindingExpression(DependencyObject d, DependencyProperty dp, Binding binding, BindingExpressionBase parent)
{
FrameworkPropertyMetadata fwMetaData = dp.GetMetadata(d.DependencyObjectType) as FrameworkPropertyMetadata;
if (((fwMetaData != null) && !fwMetaData.IsDataBindingAllowed) || dp.ReadOnly)
{
throw new ArgumentException(System.Windows.SR.Get(System.Windows.SRID.PropertyNotBindable, new object[] { dp.Name }), "dp");
}
....
This is a limitation of WPF and it is by design. It is reported on Connect here:
OneWayToSource binding from a readonly dependency property
I made a solution to dynamically be able to push read-only dependency properties to the source called PushBinding which I blogged about here. The example below does OneWayToSource Bindings from the read-only DP's ActualWidth and ActualHeight to the Width and Height properties of the DataContext
<TextBlock Name="myTextBlock">
<pb:PushBindingManager.PushBindings>
<pb:PushBinding TargetProperty="ActualHeight" Path="Height"/>
<pb:PushBinding TargetProperty="ActualWidth" Path="Width"/>
</pb:PushBindingManager.PushBindings>
</TextBlock>
PushBinding works by using two Dependency Properties, Listener and Mirror. Listener is bound OneWay to the TargetProperty and in the PropertyChangedCallback it updates the Mirror property which is bound OneWayToSource to whatever was specified in the Binding.
Demo Project can be Downloaded Here.
It contains source code and short sample usage.
Wrote this:
Usage:
<TextBox Text="{Binding Text}"
p:OneWayToSource.Bind="{p:Paths From={x:Static Validation.HasErrorProperty},
To=SomeDataContextProperty}" />
Code:
using System;
using System.Windows;
using System.Windows.Data;
using System.Windows.Markup;
public static class OneWayToSource
{
public static readonly DependencyProperty BindProperty = DependencyProperty.RegisterAttached(
"Bind",
typeof(ProxyBinding),
typeof(OneWayToSource),
new PropertyMetadata(default(Paths), OnBindChanged));
public static void SetBind(this UIElement element, ProxyBinding value)
{
element.SetValue(BindProperty, value);
}
[AttachedPropertyBrowsableForChildren(IncludeDescendants = false)]
[AttachedPropertyBrowsableForType(typeof(UIElement))]
public static ProxyBinding GetBind(this UIElement element)
{
return (ProxyBinding)element.GetValue(BindProperty);
}
private static void OnBindChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
((ProxyBinding)e.OldValue)?.Dispose();
}
public class ProxyBinding : DependencyObject, IDisposable
{
private static readonly DependencyProperty SourceProxyProperty = DependencyProperty.Register(
"SourceProxy",
typeof(object),
typeof(ProxyBinding),
new PropertyMetadata(default(object), OnSourceProxyChanged));
private static readonly DependencyProperty TargetProxyProperty = DependencyProperty.Register(
"TargetProxy",
typeof(object),
typeof(ProxyBinding),
new PropertyMetadata(default(object)));
public ProxyBinding(DependencyObject source, DependencyProperty sourceProperty, string targetProperty)
{
var sourceBinding = new Binding
{
Path = new PropertyPath(sourceProperty),
Source = source,
Mode = BindingMode.OneWay,
};
BindingOperations.SetBinding(this, SourceProxyProperty, sourceBinding);
var targetBinding = new Binding()
{
Path = new PropertyPath($"{nameof(FrameworkElement.DataContext)}.{targetProperty}"),
Mode = BindingMode.OneWayToSource,
Source = source
};
BindingOperations.SetBinding(this, TargetProxyProperty, targetBinding);
}
public void Dispose()
{
BindingOperations.ClearAllBindings(this);
}
private static void OnSourceProxyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
d.SetCurrentValue(TargetProxyProperty, e.NewValue);
}
}
}
[MarkupExtensionReturnType(typeof(OneWayToSource.ProxyBinding))]
public class Paths : MarkupExtension
{
public DependencyProperty From { get; set; }
public string To { get; set; }
public override object ProvideValue(IServiceProvider serviceProvider)
{
var provideValueTarget = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
var targetObject = (UIElement)provideValueTarget.TargetObject;
return new OneWayToSource.ProxyBinding(targetObject, this.From, this.To);
}
}
Have not tested it in styles and templates yet, guess it needs special casing.
Here is another implementation for binding to Validation.HasError
public static class OneWayToSource
{
public static readonly DependencyProperty BindingsProperty = DependencyProperty.RegisterAttached(
"Bindings",
typeof(OneWayToSourceBindings),
typeof(OneWayToSource),
new PropertyMetadata(default(OneWayToSourceBindings), OnBinidngsChanged));
public static void SetBindings(this FrameworkElement element, OneWayToSourceBindings value)
{
element.SetValue(BindingsProperty, value);
}
[AttachedPropertyBrowsableForChildren(IncludeDescendants = false)]
[AttachedPropertyBrowsableForType(typeof(FrameworkElement))]
public static OneWayToSourceBindings GetBindings(this FrameworkElement element)
{
return (OneWayToSourceBindings)element.GetValue(BindingsProperty);
}
private static void OnBinidngsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
((OneWayToSourceBindings)e.OldValue)?.ClearValue(OneWayToSourceBindings.ElementProperty);
((OneWayToSourceBindings)e.NewValue)?.SetValue(OneWayToSourceBindings.ElementProperty, d);
}
}
public class OneWayToSourceBindings : FrameworkElement
{
private static readonly PropertyPath DataContextPath = new PropertyPath(nameof(DataContext));
private static readonly PropertyPath HasErrorPath = new PropertyPath($"({typeof(Validation).Name}.{Validation.HasErrorProperty.Name})");
public static readonly DependencyProperty HasErrorProperty = DependencyProperty.Register(
nameof(HasError),
typeof(bool),
typeof(OneWayToSourceBindings),
new FrameworkPropertyMetadata(default(bool), FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
internal static readonly DependencyProperty ElementProperty = DependencyProperty.Register(
"Element",
typeof(UIElement),
typeof(OneWayToSourceBindings),
new PropertyMetadata(default(UIElement), OnElementChanged));
private static readonly DependencyProperty HasErrorProxyProperty = DependencyProperty.RegisterAttached(
"HasErrorProxy",
typeof(bool),
typeof(OneWayToSourceBindings),
new PropertyMetadata(default(bool), OnHasErrorProxyChanged));
public bool HasError
{
get { return (bool)this.GetValue(HasErrorProperty); }
set { this.SetValue(HasErrorProperty, value); }
}
private static void OnHasErrorProxyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
d.SetCurrentValue(HasErrorProperty, e.NewValue);
}
private static void OnElementChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
if (e.NewValue == null)
{
BindingOperations.ClearBinding(d, DataContextProperty);
BindingOperations.ClearBinding(d, HasErrorProxyProperty);
}
else
{
var dataContextBinding = new Binding
{
Path = DataContextPath,
Mode = BindingMode.OneWay,
Source = e.NewValue
};
BindingOperations.SetBinding(d, DataContextProperty, dataContextBinding);
var hasErrorBinding = new Binding
{
Path = HasErrorPath,
Mode = BindingMode.OneWay,
Source = e.NewValue
};
BindingOperations.SetBinding(d, HasErrorProxyProperty, hasErrorBinding);
}
}
}
Usage in xaml
<StackPanel>
<TextBox Text="{Binding Value, UpdateSourceTrigger=PropertyChanged}">
<local:OneWayToSource.Bindings>
<local:OneWayToSourceBindings HasError="{Binding HasError}" />
</local:OneWayToSource.Bindings>
</TextBox>
<CheckBox IsChecked="{Binding HasError, Mode=OneWay}" />
</StackPanel>
This implementation is specific to binding Validation.HasError
Here's another attached property solution based on SizeObserver detailed here Pushing read-only GUI properties back into ViewModel
public static class MouseObserver
{
public static readonly DependencyProperty ObserveProperty = DependencyProperty.RegisterAttached(
"Observe",
typeof(bool),
typeof(MouseObserver),
new FrameworkPropertyMetadata(OnObserveChanged));
public static readonly DependencyProperty ObservedMouseOverProperty = DependencyProperty.RegisterAttached(
"ObservedMouseOver",
typeof(bool),
typeof(MouseObserver));
public static bool GetObserve(FrameworkElement frameworkElement)
{
return (bool)frameworkElement.GetValue(ObserveProperty);
}
public static void SetObserve(FrameworkElement frameworkElement, bool observe)
{
frameworkElement.SetValue(ObserveProperty, observe);
}
public static bool GetObservedMouseOver(FrameworkElement frameworkElement)
{
return (bool)frameworkElement.GetValue(ObservedMouseOverProperty);
}
public static void SetObservedMouseOver(FrameworkElement frameworkElement, bool observedMouseOver)
{
frameworkElement.SetValue(ObservedMouseOverProperty, observedMouseOver);
}
private static void OnObserveChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
{
var frameworkElement = (FrameworkElement)dependencyObject;
if ((bool)e.NewValue)
{
frameworkElement.MouseEnter += OnFrameworkElementMouseOverChanged;
frameworkElement.MouseLeave += OnFrameworkElementMouseOverChanged;
UpdateObservedMouseOverForFrameworkElement(frameworkElement);
}
else
{
frameworkElement.MouseEnter -= OnFrameworkElementMouseOverChanged;
frameworkElement.MouseLeave -= OnFrameworkElementMouseOverChanged;
}
}
private static void OnFrameworkElementMouseOverChanged(object sender, MouseEventArgs e)
{
UpdateObservedMouseOverForFrameworkElement((FrameworkElement)sender);
}
private static void UpdateObservedMouseOverForFrameworkElement(FrameworkElement frameworkElement)
{
frameworkElement.SetCurrentValue(ObservedMouseOverProperty, frameworkElement.IsMouseOver);
}
}
Declare attached property in control
<ListView ItemsSource="{Binding SomeGridItems}"
ut:MouseObserver.Observe="True"
ut:MouseObserver.ObservedMouseOver="{Binding IsMouseOverGrid, Mode=OneWayToSource}">
WPF will not use the CLR property setter, but seems it does some odd validation based on it.
May be in your situation this can be ok:
public bool IsModified
{
get { return (bool)GetValue(IsModifiedProperty); }
set { throw new Exception("An attempt ot modify Read-Only property"); }
}
Hmmm... I'm not sure I agree with any of these solutions. How about specifying a coercion callback in your property registration that ignores external change? For instance, I needed to implement a read-only Position dependency property to get the position of a MediaElement control inside a user control. Here's how I did it:
public static readonly DependencyProperty PositionProperty = DependencyProperty.Register("Position", typeof(double), typeof(MediaViewer),
new FrameworkPropertyMetadata(0d, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | FrameworkPropertyMetadataOptions.Journal, OnPositionChanged, OnPositionCoerce));
private static void OnPositionChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
var ctrl = d as MediaViewer;
}
private static object OnPositionCoerce(DependencyObject d, object value)
{
var ctrl = d as MediaViewer;
var position = ctrl.MediaRenderer.Position.TotalSeconds;
if (ctrl.MediaRenderer.NaturalDuration.HasTimeSpan == false)
return 0d;
else
return Math.Min(position, ctrl.Duration);
}
public double Position
{
get { return (double)GetValue(PositionProperty); }
set { SetValue(PositionProperty, value); }
}
In other words, simply ignore the change and return the value backed by a different member that does not have a public modifier. -- In the above example, MediaRenderer is actually the private MediaElement control.
The way I worked around this limitation was to expose only a Binding property in my class, keeping the DependencyProperty private altogether. I implemented a "PropertyBindingToSource" write-only property (this one not a DependencyProperty) which can be set to a binding value in the xaml. In the setter for this write-only property I call to BindingOperations.SetBinding to link the binding to the DependencyProperty.
For the OP's specific example, it would look like this:
The FlatThingy implementation:
public partial class FlatThingy : UserControl
{
public FlatThingy()
{
InitializeComponent();
}
public Binding IsModifiedBindingToSource
{
set
{
if (value?.Mode != BindingMode.OneWayToSource)
{
throw new InvalidOperationException("IsModifiedBindingToSource must be set to a OneWayToSource binding");
}
BindingOperations.SetBinding(this, IsModifiedProperty, value);
}
}
public bool IsModified
{
get { return (bool)GetValue(IsModifiedProperty); }
private set { SetValue(IsModifiedProperty, value); }
}
private static readonly DependencyProperty IsModifiedProperty =
DependencyProperty.Register("IsModified", typeof(bool), typeof(FlatThingy), new PropertyMetadata(false));
private void Button_Click(object sender, RoutedEventArgs e)
{
IsModified = !IsModified;
}
}
Notice that the static readonly DependencyProperty object is private. In the control I added a button whose click is handled by Button_Click.
The use of the FlatThingy control in my window.xaml:
<Window x:Class="ReadOnlyBinding.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:ReadOnlyBinding"
mc:Ignorable="d"
DataContext="{x:Static local:ViewModel.Instance}"
Title="MainWindow" Height="450" Width="800">
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Text="{Binding FlagIsModified}" Grid.Row="0" />
<local:FlatThingy IsModifiedBindingToSource="{Binding FlagIsModified, Mode=OneWayToSource}" Grid.Row="1" />
</Grid>
Note that I've also implemented a ViewModel for binding to that is not shown here. It exposes a DependencyProperty named "FlagIsModified" as you can glean from the source above.
It works great, allowing me to push information back into the ViewModel from the View in a loosely coupled manner, with the direction of that information flow explicitly defined.
You're doing the binding in the wrong direction right now. OneWayToSource will try and update FlagIsModified on container whenever IsModified changes on the control you are creating. You want the opposite, which is to have IsModified bind to container.FlagIsModified. For that you should use the binding mode OneWay
<controls:FlagThingy IsModified="{Binding FlagIsModified,
ElementName=container,
Mode=OneWay}" />
Full list of enumeration members: http://msdn.microsoft.com/en-us/library/system.windows.data.bindingmode.aspx