Create composite DataContext in XAML for a Usercontrol - wpf

I am trying to create a composite DataContext for a UserControl. Basically I have a control which has Order and Package properties and I wanted to create the composite object representing this datasource in XAML rather than in code.
This is how I am trying to display the UserControl (and create the DataContext):
<views:PackageDetailsControl>
<views:PackageDetailsControl.DataContext>
<vm:OrderPackagePair Package="{Binding Package, Mode=OneWay}"
Order="{Binding Order, Mode=OneWay}"/>
</views:PackageDetailsControl.DataContext>
</views:PackageDetailsControl>
The OrderPackagePair object is a simple dependency object that is created in XAML :
public class OrderPackagePair : DependencyObject
{
public OrderDetails Order
{
get { return (OrderDetails)GetValue(OrderProperty); }
set { SetValue(OrderProperty, value); }
}
public static readonly DependencyProperty OrderProperty =
DependencyProperty.Register("Order", typeof(OrderDetails), typeof(OrderPackagePair), new UIPropertyMetadata(null));
public PackageInfo Package
{
get { return (PackageInfo)GetValue(PackageProperty); }
set { SetValue(PackageProperty, value); }
}
public static readonly DependencyProperty PackageProperty =
DependencyProperty.Register("Package", typeof(PackageInfo), typeof(OrderPackagePair), new UIPropertyMetadata(null));
}
Order and Package are not bound correctly and are just null.
Yes I know there's probably a better way of doing this - but I cannot understand why this isn't working. Occasionally in Blend it'll work and then go blank again.

This will not work because DependencyObject(OrderPackagePair class) doesn't monitor internal changes of its dependency properties. As OrderPackagePair object remains the same, DataContext considered as unchanged.
On the opposite site, class Freezable is intented to notify subscribers that instance was changed when one of its dependency properties changed.
So, try to declare Freezable instead of DependencyObject as base class of OrderPackagePair.
------------- UPDATE --------
Yes, it works. In order to prove it I've implemented simple example.
Code of OrderPackagePairClass:
public class OrderPackagePair : Freezable
{
public OrderDetails Order
{
get { return (OrderDetails)GetValue(OrderProperty); }
set { SetValue(OrderProperty, value); }
}
public static readonly DependencyProperty OrderProperty =
DependencyProperty.Register("Order", typeof(OrderDetails), typeof(OrderPackagePair), new UIPropertyMetadata(null));
public PackageInfo Package
{
get { return (PackageInfo)GetValue(PackageProperty); }
set { SetValue(PackageProperty, value); }
}
public static readonly DependencyProperty PackageProperty =
DependencyProperty.Register("Package", typeof(PackageInfo), typeof(OrderPackagePair), new UIPropertyMetadata(null));
protected override Freezable CreateInstanceCore()
{
throw new NotImplementedException();
}
}
XAML:
<Window x:Class="WindowTest.MainWindow"
xmlns:self="clr-namespace:WindowTest"
Name="RootControl">
<StackPanel Margin="10" DataContextChanged="StackPanel_DataContextChanged">
<StackPanel.DataContext>
<self:OrderPackagePair Package="{Binding Path=DataContext.PackageInfo, Mode=OneWay, ElementName=RootControl}"
Order="{Binding Path=DataContext.OrderDetails, Mode=OneWay, ElementName=RootControl}"/>
</StackPanel.DataContext>
<Button Margin="10" Content="Change Package" Click="Button_Click"/>
</StackPanel>
</Window>
And code behind:
public partial class MainWindow : Window, INotifyPropertyChanged
{
public MainWindow()
{
InitializeComponent();
this.DataContext = this;
}
private OrderDetails _orderDetails;
public OrderDetails OrderDetails
{
get
{
return this._orderDetails;
}
set
{
this._orderDetails = value;
this.OnPropertyChanged("OrderDetails");
}
}
private PackageInfo _packageInfo;
public PackageInfo PackageInfo
{
get
{
return this._packageInfo;
}
set
{
this._packageInfo = value;
this.OnPropertyChanged("PackageInfo");
}
}
private void Button_Click(object sender, RoutedEventArgs e)
{
this.PackageInfo = new PackageInfo(DateTime.Now.ToString());
}
private void StackPanel_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
{
Trace.WriteLine("StackPanel.DataContext changed");
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string name)
{
var safeEvent = this.PropertyChanged;
if (safeEvent != null)
{
safeEvent(this, new PropertyChangedEventArgs(name));
}
}
}
When you click the button, model changes PackageInfo property (for simplicity model and view are implemented in the same class). Dependency property OrderPackagePair.Package reacts on new value and overwrites its value. Due to Freezable nature, OrderPackagePair notifies all subscribers that it was changed and handler StackPanel_DataContextChanged is called. If you get back to DependencyObject as base class of OrderPackagePair - handler will be never called.
So, I suppose your code doesn't work because of other mistakes. You should carefully work with DataContext. For example, you wrote:
<views:PackageDetailsControl>
<views:PackageDetailsControl.DataContext>
<vm:OrderPackagePair Package="{Binding Package, Mode=OneWay}"
Order="{Binding Order, Mode=OneWay}"/>
</views:PackageDetailsControl.DataContext>
</views:PackageDetailsControl>
and certainly this is one of the problems. Binding expression is oriented on current DataContext. But you set DataContext as OrderPackagePair instance. So you binded OrderPackagePair.Package to OrderPackagePair.Package (I suppose, that your goal is to bind OrderPackagePair.Package to Model.Package). And that's why nothing happened.
In my example in binding expression I explicitly tell to which DataContext I want to bind:
Package="{Binding Path=DataContext.PackageInfo, Mode=OneWay, ElementName=RootControl}"

Related

Binding to dependency property in a ViewModel

I have dependency property in SliderViewModel where this view model implements DependencyObject and is set as the data context for BRSliderUserControl. How can I bind to the dependency property in the view model from AmplitudeOptionsUserControl. Is it possible to do so. My guess is I need to create an other dependency property in BRSliderUserControl and then send the update value to the view model. Is this the right way though?
SliderViewModel.cs
public Class SliderViewModel:DependencyObject
{
public AnalysisViewType AnalysisTypeValue
{
get { return (AnalysisViewType)GetValue(AnalysisTypeDependencyProperty); }
set { SetValue(AnalysisTypeDependencyProperty, value); }
}
public static readonly DependencyProperty AnalysisTypeDependencyProperty =
DependencyProperty.Register("AnalysisTypeValue", typeof(AnalysisViewType), typeof(SliderViewModel),
new PropertyMetadata(AnalysisViewType.Unassigned, OnAnalysisTypeChanged));
private static void OnAnalysisTypeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
//Do something here
}
}
BRSliderUserControl.xaml.cs
public BRSliderUserControl()
{
InitializeComponent();
SliderViewModel sliderViewModel = new SliderViewModel();
this.DataContext = sliderViewModel;
}
Now how can I bind to that dependency property from another user control?
AmplitudeOptionsControl.xaml
//This does not work..
<lib:BRSliderUserControl
Grid.Row="5"
Grid.Column="0"
Grid.ColumnSpan="3"
AnalysisTypeValue="{Binding AmplitudeOptionsVM.AnalysisType,RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type UserControl}}}"
/>
You don't need dependency property in your View Models. Your ViewModel should implement INotifyPropertyChanged interface, and your properties should raise NotifyPropertyChanged event when the value changes. There are many helpers around which makes this a bit easier.
You can use Dependency property if you want, but it makes your view models dependent on WPF, although binding to Dependency properties seems to be much faster (see here: https://learn.microsoft.com/en-us/dotnet/framework/wpf/advanced/optimizing-performance-data-binding)
You can see a discussion here: INotifyPropertyChanged vs. DependencyProperty in ViewModel
Also, since your DataContext is of type SliderViewModel, which has a public property named AnalysisTypeValue, in your XAML you should bind like this
... AnalysisTypeValue = {Binding AnalysisTypeValue}
Move the dependency property to the code-behind of the UserControl class:
public class BRSliderUserControl
{
public AnalysisViewType AnalysisTypeValue
{
get { return (AnalysisViewType)GetValue(AnalysisTypeDependencyProperty); }
set { SetValue(AnalysisTypeDependencyProperty, value); }
}
public static readonly DependencyProperty AnalysisTypeDependencyProperty =
DependencyProperty.Register("AnalysisTypeValue", typeof(AnalysisViewType), typeof(BRSliderUserControl),
new PropertyMetadata(AnalysisViewType.Unassigned, OnAnalysisTypeChanged));
private static void OnAnalysisTypeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
//Do something here
}
public BRSliderUserControl()
{
InitializeComponent();
SliderViewModel sliderViewModel = new SliderViewModel();
this.DataContext = sliderViewModel;
}
}
Add a plain CLR property to the view model:
public class SliderViewModel : INotifyPropertyChanged
{
private AnalysisViewType _analysisTypeValue;
public AnalysisViewType AnalysisTypeValue
{
get { return _analysisTypeValue; }
set { _analysisTypeValue = value; NotifyPropertyChanged(); }
}
public event PropertyChangedEventHandler PropertyChanged;
private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Bind the dependency property in the view to the source property of the view model:
<lib:BRSliderUserControl
Grid.Row="5"
Grid.Column="0"
Grid.ColumnSpan="3"
AnalysisTypeValue="{Binding AnalysisTypeValue}" />

How to pass initialization parameters to UserControl in WPF (using MVVM)

I have a UserControl called ActionsTreeView I built using MVVM practices where I have an IPluginsProvider interface that populates the data in my UserControl. I want to be able to provide an object implementating this IContentProvider interface as a parameter to initialize my UserControl's ViewModel.
Here is my approach so far, which isn't working. I am wondering if I'm going down the right path? I declare a DependencyProperty in my user control which is visible to my mainWindow where I want to instantiate this UserControl. This code just attempts to pass the PluginsProvider object to my UserControl which needs it to build its ViewModel.
My PluginProvider DependencyProperty setter in my UserControl never gets hit because my My PropertyChanged handler is always null in MainWindow.xaml.cs I think I have the code right, but not sure I'm going down the right road and what I'm missing to make this connection?
ActionsTreeView.xaml.cs
public partial class ActionsTreeView: UserControl
{
public static readonly DependencyProperty PluginProviderProperty = DependencyProperty.Register("PluginProvider", typeof(Models.IPluginsProvider), typeof(ActionsTreeView), new FrameworkPropertyMetadata(null, OnPluginProviderChanged));
private ViewModels.ActionsTreeViewModel vm;
public ActionsTreeView()
{
//Wire-up our ViewModel with the data provider and bind it to DataContext for our user control
//This is a Mock-up until I figure out a way to get the real provider here
Models.IPluginProvider pluginSource = new Models.MockPluginProvider();
vm = new ViewModels.ActionsTreeViewModel(pluginSource );
this.DataContext = vm;
InitializeComponent();
}
private static void OnPluginProviderChanged(DependencyObject source, DependencyPropertyChangedEventArgs e)
{
((ActionsTreeView)source).PluginProvider = (Models.IPluginsProvider)e.NewValue;
}
public Models.IPluginsProvider PluginProvider
{
get
{
return (Models.IPluginsProvider)GetValue(PluginProviderProperty);
}
set
{
SetValue(PluginProviderProperty, value);
vm.SetPluginSource(PluginProvider);
}
}...
MainWindow.xaml.cs
public partial class MainWindow : Window, INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
public MainWindow()
{
InitializeComponent();
this.ActionProvider = new Models.PluginsProvider(Library.Action.AvailableActions);
}
private Models.IPluginsProvider _actionProvider;
public Models.IPluginsProvider ActionProvider
{
get { return _actionProvider; }
set
{
_actionProvider = value;
OnPropertyChanged("ActionProvider");
}
}
protected void OnPropertyChanged(string property)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null) //HANDLER IS ALWAYS NULL
{
handler(this, new PropertyChangedEventArgs(property));
}
}
}
Using my UserControl in MainWindow.xaml
<Grid>
<UserControls:ActionsTreeView PluginProvider="{Binding ActionProvider}" />
</Grid>
I don't think you can pass a parameter in the ctor in xaml.
If you create control in code behind you can pass the parameter in the ctor(Param param)
Not sure if this fits in the MVVM model but I use it a lot in regular code behind
Use a frame in the XAML for a place to put the UserControl
Seems like you are missing the binding source
<Grid>
<UserControls:ActionsTreeView PluginProvider="{Binding ActionProvider, RelativeSource={RelativeSource FindAncestor, AncestorType=Window}}" />
</Grid>
since your property ActionProvider is declared in MainWindow so during binding you are required to refer the same source unless you've set it as data context of the window
alternative to above you can also do the below if there is no other data context used in the MainWindow then you can use the original binding you have PluginProvider="{Binding ActionProvider}"
public MainWindow()
{
InitializeComponent();
this.ActionProvider = new Models.PluginsProvider(Library.Action.AvailableActions);
DataContext = this;
}
I've set the DataContext to this which will effectively resolve the value of ActionProvider in binding from the instance this
Extra
you may also choose to remove INotifyPropertyChanged from MainWindow as it is already DependencyObject and capable of property notification and declare a DependencyProperty for ActionProvider
eg
public Models.IPluginsProvider ActionProvider
{
get { return (Models.IPluginsProvider)GetValue(ActionProviderProperty); }
set { SetValue(ActionProviderProperty, value); }
}
// Using a DependencyProperty as the backing store for ActionProvider. This enables animation, styling, binding, etc...
public static readonly DependencyProperty ActionProviderProperty =
DependencyProperty.Register("ActionProvider", typeof(Models.IPluginsProvider), typeof(MainWindow), new PropertyMetadata(null));
so you don't need to worry for the notification change manually, you might be required to use this if the above solution does not work for you otherwise it is good to have.

Silverlight MVVM binding updates fire in undesired order

Scenario: In a Silverlight 4 MVVM project, we have a ListBox control containing items, the selected item is two-way-bound to the appropriate property in the ViewModel. Another control (for example reasons, I've stripped it down to a single TextBox) is data bound to the selected item's content. The value should update on leave/focus lost.
Problem: When the value in the TextBox is changed and we leave that TextBox by pressing the Tab key, everything works as desired - the value is updated. However, if the user clicks on a different item in the ListBox, then the SelectedItem setter is fired before the content of TextBox setter is fired, leaving no chance to handle the user input.
You can see in debugger, when adding breakpoints to the property setters, that the new ListView selection is applied first, before the TextBox update is processed.
Desired behavior: We need to know that the currently selected item was modified before the user has selected another item. It's not desired to have a custom update trigger which would notify on each key press (we know that's possible).
Can you help?
Code (a very simple example):
ViewModel
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(propertyName));
}
}
public class ItemViewModel : ViewModelBase
{
private string _content;
public ItemViewModel(string initContent)
{
_content = initContent;
}
public string Content
{
get
{
return _content;
}
set
{
if (_content != value)
{
_content = value;
OnPropertyChanged("Content");
}
}
}
}
public class MainViewModel : ViewModelBase
{
private ObservableCollection<ItemViewModel> _items =
new ObservableCollection<ItemViewModel>();
private ItemViewModel _selectedViewModel;
public ObservableCollection<ItemViewModel> Items
{
get
{
return _items;
}
}
public ItemViewModel SelectedItem
{
get
{
return _selectedViewModel;
}
set
{
if (_selectedViewModel != value)
{
_selectedViewModel = value;
OnPropertyChanged("SelectedItem");
}
}
}
}
XAML
<Grid x:Name="LayoutRoot" Background="White">
<ListBox Height="100"
HorizontalAlignment="Left"
Margin="12,12,0,0"
VerticalAlignment="Top"
ItemsSource="{Binding Items}"
SelectedItem="{Binding SelectedItem, Mode=TwoWay}"
DisplayMemberPath="Content"
Width="220" />
<TextBox Height="23"
HorizontalAlignment="Left"
Margin="12,118,0,0"
Text="{Binding SelectedItem.Content, Mode=TwoWay}"
VerticalAlignment="Top"
Width="220" />
</Grid>
XAML Code Behind
public MvvmTestView()
{
InitializeComponent();
Loaded += new RoutedEventHandler(MvvmTestView_Loaded);
}
void MvvmTestView_Loaded(object sender, RoutedEventArgs e)
{
MainViewModel viewModel = new MainViewModel();
viewModel.Items.Add(new ItemViewModel("Hello StackOverflow"));
viewModel.Items.Add(new ItemViewModel("Thanks to Community"));
DataContext = viewModel;
}
UPDATE 1
I present a self designed solution for you to check out, which will be probably be the accepted one, I still want to encourage you to make comments and give your hints. Thanks.
You could add a behavior to your textbox to updated the binding every time the text is changed in the textbox. Maybe this solved your problems.
Here´s the code for the Behavior class:
public class UpdateTextBindingOnPropertyChanged : Behavior<TextBox> {
// Fields
private BindingExpression expression;
// Methods
protected override void OnAttached() {
base.OnAttached();
this.expression = base.AssociatedObject.GetBindingExpression(TextBox.TextProperty);
base.AssociatedObject.TextChanged+= OnTextChanged;
}
protected override void OnDetaching() {
base.OnDetaching();
base.AssociatedObject.TextChanged-= OnTextChanged;
this.expression = null;
}
private void OnTextChanged(object sender, EventArgs args) {
this.expression.UpdateSource();
}
}
Heres the XAML:
xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
xmlns:local="Namespace of the class where UpdateTextBindingOnPropertyChanged is defined"
<TextBox Text="{Binding SelectedItem.Content, Mode=TwoWay}">
<i:Interaction.Behaviors>
<local:UpdateTextBindingOnPropertyChanged />
</i:Interaction.Behaviors>
</TextBox >
This is one solution we currently came up with. It has the advantage that it separates different tasks to the appropriate layer. For example, the View enforces an update of the binding, while the ViewModel tells the View to do so. Another advantage is that its handled synchronously, which would for example allow to check the content right before switching away, and the call-stack remains unchanged without raising "External Code" (Going over Dispatcher or even DispatcherTimer would do so) which is better for maintenance and flow control. A disadvantage is the new Event which has to be bound and handled (and finally unbound. I present an anonymous handler only for example reasons).
How to get there?
In ViewModelBase, implement a new ForceBindingUpdate event:
public abstract class ViewModelBase : INotifyPropertyChanged
{
// ----- leave everything from original code ------
public event EventHandler ForceBindingUpdate;
protected void OnForceBindingUpdate()
{
var handler = ForceBindingUpdate;
if (handler != null)
handler(this, EventArgs.Empty);
}
}
In MainViewModel, update the setter of the SelectedItem property:
set // of SelectedItem Property
{
if (_selectedViewModel != value)
{
// Ensure Data Update - the new part
OnForceBindingUpdate();
// Old stuff
_selectedViewModel = value;
OnPropertyChanged("SelectedItem");
}
}
Update the MvvmTestView Code Behind to implement the new event:
void MvvmTestView_Loaded(object sender, RoutedEventArgs e)
{
// remains unchanged
Mvvm.MainViewModel viewModel = new Mvvm.MainViewModel();
viewModel.Items.Add(new Mvvm.ItemViewModel("Hello StackOverflow"));
viewModel.Items.Add(new Mvvm.ItemViewModel("Thanks to Community"));
// Ensure Data Update by rebinding the content property - the new part
viewModel.ForceBindingUpdate += (s, a) =>
{
var expr = ContentTextBox.GetBindingExpression(TextBox.TextProperty);
expr.UpdateSource();
};
// remains unchanged
DataContext = viewModel;
}
Last but not least, the minimal XAML Update: Give the TextBox a name by adding x:Name="ContentTextBox" Attribute to the TextBoxs XAML.
Done.
Actually, I don't know if this is the cleanest solution, but it gets close to what we had in mind.
Maybe you could handle TextBox LostFocus then (instead of listening to every key press)?
Other idea would be to keep a proxy property on the ViewModel instead of directly binding to SelectedItem.Content and writing some code to make sure the item is updated.
Solution №1
public class LazyTextBox: TextBox
{
//bind to that property instead..
public string LazyText
{
get { return (string)GetValue(LazyTextProperty); }
set { SetValue(LazyTextProperty, value); }
}
public static readonly DependencyProperty LazyTextProperty =
DependencyProperty.Register("LazyText", typeof(string), typeof(LazyTextBox),
new PropertyMetadata(null));
//call this method when it's really nessasary...
public void EnsureThatLazyTextEqualText()
{
if (this.Text != this.LazyText)
{
this.LazyText = this.Text;
}
}
}
Solution №2 (works as magic :) )
public class MainViewModel : ViewModelBase
{
private ObservableCollection<ItemViewModel> _items =
new ObservableCollection<ItemViewModel>();
private ItemViewModel _selectedViewModel;
public ObservableCollection<ItemViewModel> Items { get { return _items; } }
public ItemViewModel SelectedItem
{
get { return _selectedViewModel; }
set
{
if (_selectedViewModel != value)
{
if (SelectedItem != null)
{
SelectedItem.Content = SelectedItem.Content;
}
_selectedViewModel = value;
// A little delay make no harm :)
var t = new DispatcherTimer();
t.Interval = TimeSpan.FromSeconds(0.1);
t.Tick += new EventHandler(t_Tick);
t.Start();
}
}
}
void t_Tick(object sender, EventArgs e)
{
OnPropertyChanged("SelectedItem");
(sender as DispatcherTimer).Stop();
}
}
I know that in MVVM we do not want to put code in code behind. But in this instance it hurts nothing as it is entirely maintained in the UI and SOP is maintained.
By putting a ghost element to take focus we can swap the focus back in forth forcing
the text box to commit its contents. So in the code behind we take care of the focus wiggle.
But yet we still are using a relay command Update Command to execute the save. So the order is good as the Click event fires wiggling the view. And then the relay command UpdateCommand will fire and the textbox is committed and ready for update.
<MenuItem Header="_Save"
Command="{Binding UpdateCommand}" Click="MenuItem_Click">
</MenuItem>
private void MenuItem_Click(object sender, RoutedEventArgs e)
{
UIElement elem = Keyboard.FocusedElement as UIElement;
Keyboard.Focus(ghost);
Keyboard.Focus(elem);
}
Solution #3
public abstract class ViewModelBase : INotifyPropertyChanged
{
private List<string> _propNameList = new List<string>();
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string propertyName)
{
var handler = PropertyChanged;
if (handler != null)
_propNameList.Add(propertyName);
var t = new DispatcherTimer();
t.Interval = TimeSpan.FromSeconds(0);
t.Tick += new EventHandler(t_Tick);
t.Start();
}
void t_Tick(object sender, EventArgs e)
{
if (_propNameList.Count > 0)
{
var handler = PropertyChanged;
if (handler != null)
handler(this, new PropertyChangedEventArgs(_propNameList[0]));
_propNameList.Remove(_propNameList[0]);
}
}
}
PS: it's the same timer.. but this solution is more generic..

WPF Binding Not Working

I am pretty sure I am doing something dreadfully wrong, but can't figure it out.
I created a simple wrapper around a class and added a dependency property so I could bind to it. However, the binding gives no errors, but does nothing.
In order to simplify things I changed the class to TextBox, and got the same results.
public class TextEditor : TextBox
{
#region Public Properties
#region EditorText
/// <summary>
/// Gets or sets the text of the editor
/// </summary>
public string EditorText
{
get
{
return (string)GetValue(EditorTextProperty);
}
set
{
//if (ValidateEditorText(value) == false) return;
if (EditorText != value)
{
SetValue(EditorTextProperty, value);
base.Text = value;
//if (PropertyChanged != null) PropertyChanged(this, new PropertyChangedEventArgs("EditorText"));
}
}
}
public static readonly DependencyProperty EditorTextProperty =
DependencyProperty.Register("EditorText", typeof(string), typeof(TextEditor));
#endregion
#endregion
#region Constructors
public TextEditor()
{
//Attach to the text changed event
//TextChanged += new EventHandler(TextEditor_TextChanged);
}
#endregion
#region Event Handlers
private void TextEditor_TextChanged(object sender, EventArgs e)
{
EditorText = base.Text;
}
#endregion
}
When I run the following XAML the first gives results, but the second one (EditorText) doesn't even hit the EditorText property.
<local:TextEditor IsReadOnly="True" Text="{Binding Path=RuleValue, Mode=TwoWay}" WordWrap="True" />
<local:TextEditor IsReadOnly="True" EditorText="{Binding Path=RuleValue, Mode=TwoWay}" WordWrap="True" />
You're doing extra work in your CLR property. There is no guarantee that your CLR property will be used by WPF so you shouldn't be doing this. Instead, use metadata on your DP to achieve the same effect.
public string EditorText
{
get { return (string)GetValue(EditorTextProperty); }
set { SetValue(EditorTextProperty, value); }
}
public static readonly DependencyProperty EditorTextProperty =
DependencyProperty.Register(
"EditorText",
typeof(string),
typeof(TextEditor),
new FrameworkPropertyMetadata(OnEditorTextChanged));
private static void OnEditorTextChanged(DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
{
var textEditor = dependencyObject as TextEditor;
// do your extraneous work here
}

WPF CommandParameter Binding Problem

I'm having some trouble understanding how command parameter binding works.
When I create an instance of the widget class before the call to InitializeComponent it seems to work fine. Modifications to the parameter(Widget) in the ExecuteCommand function will be "applied" to _widget. This is the behavior I expected.
If the instance of _widget is created after InitializeComponent, I get null reference exceptions for e.Parameter in the ExecuteCommand function.
Why is this? How do I make this work with MVP pattern, where the bound object may get created after the view is created?
public partial class WidgetView : Window
{
RoutedCommand _doSomethingCommand = new RoutedCommand();
Widget _widget;
public WidgetView()
{
_widget = new Widget();
InitializeComponent();
this.CommandBindings.Add(new CommandBinding(DoSomethingCommand, ExecuteCommand, CanExecuteCommand));
}
public Widget TestWidget
{
get { return _widget; }
set { _widget = value; }
}
public RoutedCommand DoSomethingCommand
{
get { return _doSomethingCommand; }
}
private static void CanExecuteCommand(object sender, CanExecuteRoutedEventArgs e)
{
if (e.Parameter == null)
e.CanExecute = true;
else
{
e.CanExecute = ((Widget)e.Parameter).Count < 2;
}
}
private static void ExecuteCommand(object sender, ExecutedRoutedEventArgs e)
{
((Widget)e.Parameter).DoSomething();
}
}
<Window x:Class="CommandParameterTest.WidgetView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="WidgetView" Height="300" Width="300"
DataContext="{Binding RelativeSource={RelativeSource Self}}">
<StackPanel>
<Button Name="_Button" Command="{Binding DoSomethingCommand}"
CommandParameter="{Binding TestWidget}">Do Something</Button>
</StackPanel>
</Window>
public class Widget
{
public int Count = 0;
public void DoSomething()
{
Count++;
}
}
InitializeCompenent processes the xaml associated with the file. It is at this point in time that the CommandParameter binding is first processed. If you initialize your field before InitializeCompenent then your property will not be null. If you create it after then it is null.
If you want to create the widget after InitializeCompenent then you will need to use a dependency property. The dependency proeprty will raise a notification that will cause the CommandParameter to be updated and thus it will not be null.
Here is a sample of how to make TestWidget a dependency property.
public static readonly DependencyProperty TestWidgetProperty =
DependencyProperty.Register("TestWidget", typeof(Widget), typeof(Window1), new UIPropertyMetadata(null));
public Widget TestWidget
{
get { return (Widget) GetValue(TestWidgetProperty); }
set { SetValue(TestWidgetProperty, value); }
}
Even with the dependency property, you still need to call CommandManager.InvalidateRequerySuggested to force the CanExecute of the Command being evaluated.

Resources