How to implement INotifyPropertyChanged on indexed child in SL4? - silverlight

My ViewModel class has a child property of type 'Messages' that has an indexed property, like:
public class ViewModel
{
// ...
public Messages Messages
{
get
{
if (_messages == null)
{
LoadMessagesAsync();
_messages = new Messages();
}
return _messages;
}
set
{
_messages = values;
PropertyChanged(new PropertyChangedArgs("Messages");
}
}
// ...
private void LoadMessagesAsync()
{
// Do the service call
Messages = theResult;
}
}
public class Messages
{
// ...
public String this[String name]
{
get { return _innerDictionary[name]; }
}
// ...
}
I don't think I need to fill in the rest of the gaps as it is all straight-forward.
The problem I am having is that the binding is not updating when I set the Messages property to a new object. Here is how I am referencing the property in XAML (with ViewModel as the DataContext):
<TextBlock Text="{Binding Messages[HelloWorld]}" />
It was my assumption that the binding would update when the PropertyChanged event was raised for the "Messages" property.
I've read elsewhere that my Messages class should raise a PropertyChanged event with either an empty string (""), "Item[]" or "Item["+name+"]" for the property name. However, since I am completely replacing the Messages object, this won't work as I never actually change the contents.
How do I make this work?
UPDATE
So I've done some digging into the behavior and into the BCL source code to see what's expected as a way to figure out how to make my code work. What I've learned is two-fold:
First, Silverlight data-binding is actually looking at the return object from the Messages property as the source of the binding. So raising PropertyChanged from ViewModel (sender is ViewModel) is not handled by the binding. I actually have to raise the event from the Messages class.
This is no different than using the following: Text={Binding Messages.HelloWorld}"
The reason that Myles' code work is that 'Data' returns 'this' so the binding is fooled into treating the parent class as the binding source.
That said, even if I make it so my child object raises the event, it still won't work. This is because the binding uses the System.Windows.IndexerListener as the binding target. In the SourcePropertyChanged method, the listener checks if the property name is "Item[]" but takes no action. The next statement delegates to the PropertyListener which checks the property name and only handles the event if it is equal to "Item[HelloWorld]".
So, unless I explicitly raise the event for each possible value within my collection, the UI will never update. This is disappointing because other articles and posts indicate that "Item[]" should work but looking at the source proves otherwise.
Nevertheless, I still hold out hope that there is a way to accomplish my goals.

OK the underlying problem here is that the Binding does not have a Path specified, therefore, the binding framework does not know which property name to look out for when handling PropertyChanged events. So I have fabricated a Path for the binding in order for change notification to work.
I have wrote the following code that proves the indexer binding is refreshed when the actual underlying dictionary changes:
ViewModel
public class BindingTestViewModel : AppViewModelBase, IBindingTestViewModel
{
private Dictionary<string, object> _data = new Dictionary<string, object>();
public BindingTestViewModel()
{
_data.Add("test","1");
_data.Add("test2", "21");
}
public object this[string index]
{
get
{
return _data[index];
}
set
{
_data[index] = value;
NotifyOfPropertyChange(() => Data);
}
}
public object Data
{
get
{
return this;
}
}
public void Refresh()
{
_data = new Dictionary<string, object>
{
{"test", "2"}, {"test2", "22"}
};
NotifyOfPropertyChange(() => Data);
}
}
My XAML:
<navigation:Page.Resources>
<Converters:IndexConverter x:Name="IndexConverter"></Converters:IndexConverter>
</navigation:Page.Resources>
<Grid x:Name="LayoutRoot">
<TextBox Height="23"
HorizontalAlignment="Left"
Margin="288,206,0,0"
Name="textBox1"
Text="{Binding Path=Data,Converter={StaticResource IndexConverter},ConverterParameter=test}"
VerticalAlignment="Top" Width="120" />
<Button x:Name="ReloadDict" Click="ReloadDict_Click" Width="50" Height="30" Content="Refresh" VerticalAlignment="Top"></Button>
</Grid>
The converter:
public class IndexConverter : IValueConverter
{
public object Convert(object value, Type targetType,
object parameter, CultureInfo culture)
{
var vm = value as BindingTestViewModel;
var index = parameter as string;
return vm[index];
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}

I don't necessarily like answering my own questions but I have found a solution to my problem. I've been able to keep my original flow and address the idiosyncrocies of SL4 data-binding. And the code seems a bit cleaner, too.
What it boils down to is that I don't replace the child object anymore. That seems to be the key. Instead, I create a single instance and let that instance manage changing the internal list of items as needed. The child object notifies the parent when it has changed so the parent can raise the PropertyChanged event. The following is a brief example how I've gotten it to work:
public class ViewModel
{
// ...
public Messages Messages
{
get
{
if (_messages == null)
{
lock (_messagesLock)
{
if (_messages == null)
{
_messages = new Messages();
_messages.ListChanged += (s, e) =>
{
NotifyPropertyChanged("Messages");
};
}
}
}
return _messages;
}
}
}
public class Messages
{
// ...
public String this[String name]
{
get
{
if (_innerDictionary == null)
{
_innerDictionary = new Dictionary<String, String>();
LoadMessagesAsync();
}
return _innerDictionary[name];
}
}
// ...
private void LoadMessagesAsync()
{
// Do the service call
_innerDictionary = theResult;
NotifyListChanged();
}
// ...
public event EventHandler ListChanged;
}
For brevity, I've left out the obvious parts.

Related

Bind Visibility to ReactiveCommand CanExecute

I have several Tiles (TileLayoutControl Class) in my xaml (only shown 2 in this example) whose Visibility are binded to Boolean Properties and converted through BooleanToVisibilityConverter.
This works just fine. My question is
Can I bind the visibility to the Command instead so that I can remove the need of those several Boolean Properties?
Something like binding the Visibility to Command.CanExecute
If Yes, How can I achieve that? Any help will be really appreciated! Thanks.
<dxlc:Tile Command="{Binding Tile1Command}"
Visibility="{Binding Path=IsTile1Visible , Converter={StaticResource BooleanToVisibilityConverter}}"/>
<dxlc:Tile Command="{Binding Tile2Command}"
Visibility="{Binding Path=IsTile2Visible , Converter={StaticResource BooleanToVisibilityConverter}}"/>
ViewModel
private bool _isTile1Visible;
public bool IsTile1Visible
{
get { return _isTile1Visible; }
set { this.RaiseAndSetIfChanged(ref _isTile1Visible, value); }
}
public ReactiveCommand Tile1Command { get; private set; }
Tile1Command = new ReactiveCommand();
Tile1Command.Subscribe(p => PerformTile1Operation());
Yes, just use RxUI bindings:
<dxlc:Tile x:Name="Tile1" />
Then in your View constructor (make sure to implement IViewFor<Tile1ViewModel> to get this extension):
this.BindCommand(ViewModel, x => x.Tile1Command);
this.WhenAnyObservable(x => x.ViewModel.Tile1Command.CanExecuteObservable)
.BindTo(this, x => x.Tile1.Visibility);
You could also solve this in the ViewModel level, though that's not what I would do - in the ViewModel ctor:
Tile1Command = new ReactiveCommand(/* ... */);
Tile1Command
.Select(x => x ? Visibility.Visible : Visibility.Collapsed)
.ToProperty(this, x => x.Tile1Visibility, out tile1Visibility);
ReactiveCommand is an ICommand implementation that is simultaneously a RelayCommand implementation...
Assume that the ReactiveCommand has been declared like this...
public ReactiveCommand FileCommand { get; private set; }
...and has been instantiated in a View Model like this...
SomeText = "";
FileCommand = new ReactiveCommand(this.WhenAny(vm => vm.SomeText, s => !string.IsNullOrWhiteSpace(s.Value)));
FileCommand.Subscribe(param => MessageBox.Show("Processing"));
... which means if the property SomeText is empty, then the command cannot be executed, otherwise the command can be executed. And if the command is executed, a message box will get displayed.
If your objective is to simply eliminate the boolean IsTile1Visible, you can make a Xaml declaration like this...
<Button Content="File"
Command="{Binding FileCommand}"
Visibility="{Binding FileCommand, Converter={genericMvvm1:CommandToVisibilityConverter}}" />
where the visibility is bound to the same command and uses a value converter...
and the value converter looks like this...
public class CommandToVisibilityConverter : MarkupExtension, IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
try
{
ICommand iCommand = value as ICommand;
if (iCommand != null)
{
if (iCommand.CanExecute(parameter))
{
return Visibility.Visible;
}
return Visibility.Collapsed;
}
}
catch
{
}
return value;
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
return null;
}
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
}
The value converter simply dereferences the command into a basic ICommand and converts it into a visibility. Note that since this converter inherits from Markup Extension, there's no need to declare it as a static resource in the Xaml's object graph.
NOTE: the same functionality can be achieved by using 'code-behind' available in ReactiveUI, but the Xaml/ValueConverter appeals to developers who do not want their View Models to deal explicitly with the 'Visibility' property.
You could potentially do that, but it would require subclassing the command so that it also implements INotifyPropertyChanged, and the underlying condition would need to raise PropertyChange for the CanExecute property whenever it changes.
It won't work without that, as ICommand doesn't implement INotifyPropertyChanged - it uses CanExecuteChanged instead.
Note that you could simplify the property, however, by just handling it yourself in the constructor:
// In constructor:
Tile1Command = new ReactiveCommand();
Tile1Command.Subscribe(p => PerformTile1Operation());
IReactiveObject self = this as IReactiveObject;
Tile1Command.CanExecuteChanged += (o,e) => self.RaisePropertyChanged(new PropertyChangedEventArgs("IsTile1Visible"));
Then your property becomes:
// Use command directly here...
public bool IsTile1Visible
{
get { return Tile1Command.CanExecute; }
}

How can I know that some property was binding?

How can I know that some property was binding?
For example a property (Class implemeted from NotificationObject):
public string Title
{
set
{
_title=value;
this.RaisePropertyChanged(() => this.Title);
}
get
{
return _title;
}
}
Using:
<TextBlock Text={Binding Title}>
I need to know when a property is not used by anyone to release dispose resources.
There's no easy way to know if a control is bound to a specific property of your ViewModel, but you can know if someone is subscribed to the PropertyChanged event (just check if it's not null). The binding engines subscribes to this event, so if something is bound to at least one property of your ViewModel, the PropertyChanged event handler won't be null.
You can tell if someone has requested your property by setting a flag, although not sure if this will meet your needs:
private bool _isTitleBound = false;
public string Title
{
set
{
_title = value;
this.RaisePropertyChanged(() => Title);
}
get
{
_isTitleBound = true;
return _title;
}
}
You could also consider lazy instantiation, which would result in your disposable objects only being created when the property getter was called. If the property getters are never called, your disposable objects will never be created. Also, if this is a one-time binding consider using lazy instantiation with disposal of your object. For example:
public MyThing Thing
{
get
{
MyThing thing = CreateMyThing();
Dispatcher.CurrentDispatcher.BeginInvoke(
DispatcherPriority.Background,
new Action(() => thing.Dispose());
return thing;
}
}
private MyThing CreateMyThing()
{
//create and return MyThing instance;
}

Getting 'ConvertBack(...)' to fire on WPF ComboBox selection change

I have a ComboBox with its Items property bound to a collection of objects. I also have SelectedItem property bound to the entire collection, with a ValueConverter designed to examine the elements in the collection and return the 1 item to be selected. This part works.
What doesn't work is when the user makes a selection change on the ComboBox, the ConvertBack(...) method of the ValueConverter is not being called. I need ConvertBack(...) called because I need to take the user's selection, re-examine the collection, and edit the old selected item and newly selected item appropriately.
I know this approach is a awkward, but it's the way it is. Here's the relevant code:
ComboBox:
<ComboBox ItemsSource="{Binding}" SelectedItem="{Binding Path=., Converter={StaticResource ResourceKey=DataInputAssetChoiceSelectedItemConverter}}" />
ValueConverter:
public class DataInputAssetChoiceSelectedItemConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (value != null)
{
foreach (CustomObject Choice in (Collection<CustomObject>)value)
{
if (Choice.IsSelected)
{
return Choice;
}
}
return ((Collection<CustomObject>)value).First();
}
return null;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{ //breakpoint...execution never gets here!
return null;
}
}
So why doesn't ConvertBack(...) ever get called? Is it just something I'm misunderstanding about ComboBox? I've tried this approach using SelectedItem, SelectedValue, SelectedIndex, and have tried messing with UpdateSourceTrigger, various binding modes, DataTriggers, and can never seem to get ConvertBack(...) to be called. Is using the SelectionChanged event the only option? If so, why?
You are not binding to a property, so the Binding can't set anything. You are binding directly to the DataContext object, and the Binding won't update that.
If you had {Binding Path=SomeProperty, Converter=...} then the ConvertBack would be called. As it stands though, it won't be called.
You're right, it is awkward, but only because you're trying to add some management to the collection in a value converter instead of on the collection itself. I think it would help if your collection was more aware that its items have an IsSelected property:
public CustomCollection : Collection<CustomObject> {
CustomObject _current;
public CustomObject CurrentSelection {
get { return _current; }
set {
if (_current == value)
return;
if (_current != null)
_current.IsSelected = false;
if (value != null)
value.IsSelected = true;
_current = value;
}
}
}
Just add a little extra to make sure that _current is at least the first element in the collection.
<ComboBox ItemsSource="{Binding}" SelectedItem="{Binding CurrentSelection}">
Now you shouldn't need the converter anymore. There are some considerations missing, however. You may want to use ObservableCollection<T> instead and raise the PropertyChanged event when CurrentSelection is changed, so that if anything else is bound to that property or it changed in code, all bindings will update appropriately.
Edit: Wrapping the model
One easy way to wrap the collection instead of making a custom collection like above:
public class CollectionWrapper : INotifyPropertyChanged {
public event PropertyChangedEventHandler PropertyChanged = (o,e)=>{};
// never have to check for null
public CollectionWrapper(Collection<CustomObject> collection) {
Items = collection;
}
// unlikely to change, so let's prevent it for now
public Collection<CustomObject> Items {
get;
private set;
}
CustomObject _current;
public CustomObject CurrentSelection {
get { return _current; }
set {
if (_current == value)
return;
if (_current != null)
_current.IsSelected = false;
if (value != null)
value.IsSelected = true;
_current = value;
PropertyChanged(this, new PropertyChangedEventArgs("CurrentSelection"));
}
}
}
Then this object becomes your data context and the ComboBox bindings change to this:
<ComboBox ItemsSource="{Binding Items}" SelectedItem="{Binding CurrentSelection}">

Current Date in Silverlight XAML TextBlock

I am coming from Flex where you can do just about anything inside of curly braces. I am trying to get a TextBlock to display today's Date and Time without just coding it in C#. I have tried many different variations of the following with no luck.
TextBlock Text="{Source=Date, Path=Now, StringFormat='dd/MM/yyyy'}"
I know I could probably just set a property MyDate and bind to that but why can't I bind directly to the DateTime.Now property?
Binding in Silverlight requires a Source object or a Dependency object. From that source object you can bind to Properties (hence by definition you are binding to instance members) or Dependency Properties.
Since DateTime.Now is a static property you cannot bind to it in Silverlight directly, hence some code is needed. The next best thing is to use code to:-
ensure as much of what you need can be expressed in XAML
to do so in an as de-coupled manner as possible.
Hence we can analyse that we need two things.
Expose the static members of DateTime as instance properties of some object
Have some way to format the DateTime to a desirable output.
To handle the first item I would create a StaticSurrogate class, where I would create instance properties for the static properties that we need access to:-
public class StaticSurrogate
{
public DateTime Today { get { return DateTime.Today; } }
public DateTime Now { get { return DateTime.Now; } }
}
Now we need a way to format a Date time. A value converter is the right tool for this job, borrowing heavily from this Tim Heuer Blog :-
public class FormatConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (parameter != null)
{
string formatterString = parameter.ToString();
if (!String.IsNullOrEmpty(formatterString))
{
return String.Format(culture, String.Format("{{0:{0}}}", formatterString), value);
}
}
return (value ?? "").ToString();
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
throw new NotImplementedException();
}
}
With these two classes in place we can now do the rest in Xaml, first we need instances of these classes in our resources:-
<UserControl.Resources>
<local:StaticSurrogate x:Key="Static" />
<local:FormatConverter x:Key="Formatter" />
</UserControl.Resources>
Now we can wire up the TextBlock :-
<TextBlock Text="{Binding Today, Source={StaticResource Static},
Converter={StaticResource Formatter}, ConverterParameter='dd MMM yyy'}" />
Note that this approach has the following advantages:-
we do not need to add code to the UserControl on which the TextBlock is placed, nor do we have to fiddle around with any data context.
The Static resources could be placed in the App.Resources which would make the creation of the TextBlock entirely independent of having to add anything else to the UserControl.
The formatting used to display the date can be independently modified.
Access to additional static properties can easily be added to the StaticSurrogate class.
Even if you could declare DateTime.Now in Silverlight's XAML (since you can in WPF - http://soumya.wordpress.com/2010/02/12/wpf-simplified-part-11-xaml-tricks/), you have the issue that your time won't update. If you use a local timer that updates on the second you can ensure that your time will update as well.
public class LocalTimer : INotifyPropertyChanged
{
private DispatcherTimer timer;
public LocalTimer()
{
timer = new DispatcherTimer();
timer.Interval = TimeSpan.FromSeconds(1.0);
timer.Tick += new EventHandler(TimerCallback);
this.TimeFormat = "hh:mm:ss";
this.DateFormat = "dddd, MMMM dd";
}
private void TimerCallback(object sender, EventArgs e)
{
PropertyChanged(this, new PropertyChangedEventArgs("FormattedDate"));
PropertyChanged(this, new PropertyChangedEventArgs("FormattedTime"));
}
public bool Enabled
{
get { return this.timer.IsEnabled; }
set { if (value) this.timer.Start(); else this.timer.Stop(); }
}
public string FormattedDate { get { return DateTime.Now.ToString(this.DateFormat); } set {} }
public string FormattedTime { get { return DateTime.Now.ToString(this.TimeFormat); } set{} }
public string TimeFormat { get; set; }
public string DateFormat { get; set; }
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
}
Declare an instance of this in xaml ala:
<local:LocalTimer x:Key="theTime" Enabled="True" />
and use the binding to ensure that your time is always reflected.
<TextBlock Text="{Binding Source={StaticResource theTime}, Path=FormattedDate, Mode=OneWay}" x:Name="TodaysDate" />
<TextBlock Text="{Binding Source={StaticResource theTime}, Path=FormattedTime, Mode=OneWay}" x:Name="CurrentTime" />
xmlns:sys="clr-namespace:System;assembly=mscorlib"
Text="{Binding Source={x:Static sys:DateTime.Today}, StringFormat='Today is {0:dddd, MMMM dd}'}"

Silverlight TabControl bound to ObservableCollection<string> not updating when collection changed

Silverlight 3 app with a TabControl bound to an ObservableCollection using an IValueConverter. Initial the binding works (converter called) on app startup. Changes, Clear() or Add(), to the bound collection are not reflected in the TabControl... converter not called.
note: the bound ListBox reflects the changes to the bound collection while the TabControl does not.
Ideas?
/jhd
The XAML binding...
<UserControl.Resources>
<local:ViewModel x:Key="TheViewModel"/>
<local:TabConverter x:Key="TabConverter" />
</UserControl.Resources>
<StackPanel DataContext="{StaticResource TheViewModel}">
<ListBox ItemsSource="{Binding Classnames}" />
<controls:TabControl x:Name="TheTabControl"
ItemsSource="{Binding Classnames, Converter={StaticResource TabConverter}, ConverterParameter=SomeParameter}"/>
<Button Click="Button_Click" Content="Change ObservableCollection" />
</StackPanel>
The ViewModel...
namespace DatabindingSpike
{
public class ViewModel
{
private ObservableCollection<string> _classnames = new ObservableCollection<string>();
public ViewModel()
{
_classnames.Add("default 1 of 2");
_classnames.Add("default 2 of 2");
}
public ObservableCollection<string> Classnames
{
get { return _classnames; }
set { _classnames = value; }
}
}
}
The converter (for completeness)...
namespace DatabindingSpike
{
public class TabConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var source = value as ObservableCollection<string>;
if (source == null)
return null;
var param = parameter as string;
if (string.IsNullOrEmpty(param) || param != "SomeParameter")
throw new NotImplementedException("Null or unknow parameter pasased to the tab converter");
var tabItems = new List<TabItem>();
foreach (string classname in source)
{
var tabItem = new TabItem
{
Header = classname,
Content = new Button {Content = classname}
};
tabItems.Add(tabItem);
}
return tabItems;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
}
Update 8/19
The concise answer is you have to implement INotifyPropertyChanged on the view model and notify listeners when the Property/Collection is changed.
Implement INotifyPropertyChanged on the ViewModel
* implement the interface INotifyPropertyChanged
* define the event (public event PropertyChangedEventHandler PropertyChanged)
* subscribe to the CollectionChanged event (Classnames.CollectionChanged += ...)
* fire the event for listeners
Best,
/jhd
ViewModel update per above... ValueConverter now called on all changes to the Property/Collection
public class ViewModel : INotifyPropertyChanged
{
private readonly ObservableCollection<string> _classnames = new ObservableCollection<string>();
public ViewModel()
{
Classnames.CollectionChanged += Classnames_CollectionChanged;
}
public event PropertyChangedEventHandler PropertyChanged;
private void Classnames_CollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
{
NotifyPropertyChanged("Classnames");
}
private void NotifyPropertyChanged(string info)
{
PropertyChangedEventHandler handler = PropertyChanged;
if (handler != null)
{
foreach (PropertyChangedEventHandler d in handler.GetInvocationList())
{
d(this, new PropertyChangedEventArgs(info));
}
}
}
public ObservableCollection<string> Classnames
{
get { return _classnames; }
}
}
The XAML binding...
<UserControl.Resources>
<local:ViewModel x:Key="TheViewModel"/>
<local:TabConverter x:Key="TabConverter" />
</UserControl.Resources>
<StackPanel DataContext="{StaticResource TheViewModel}">
<ListBox ItemsSource="{Binding Classnames}" />
<controls:TabControl x:Name="TheTabControl"
ItemsSource="{Binding Classnames, Converter={StaticResource TabConverter}, ConverterParameter={StaticResource TheViewModel}}"/>
<Button Click="Button_Click" Content="Change Classnames" />
</StackPanel>
The ValueConverter (basically unchanged
public class TabConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
var source = value as ObservableCollection<string>;
if (source == null)
return null;
//also sorted out the binding syntax to pass the ViewModel as a parameter
var viewModel = parameter as ViewModel;
if (viewModel == null)
throw new ArgumentException("ConverterParameter must be ViewModel (e.g. ConverterParameter={StaticResource TheViewModel}");
var tabItems = new List<TabItem>();
foreach (string classname in source)
{
// real code dynamically loads controls by name
var tabItem = new TabItem
{
Header = "Tab " + classname,
Content = new Button {Content = "Content " + classname}
};
tabItems.Add(tabItem);
}
return tabItems;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
I realize this is a slightly old question at this point, but I don't know that anyone has explained why you need to do the INotifyPropertyChanged on the bound property on your view model.
The ItemsControl itself needs to be bound to an ObservableCollection for the collection change events to cause the ItemsControl to re-evaluate. Your converter is returning a distinct List (or Observable) collection each time it is called rather than holding on to a single ObservableCollection and adding items to it. Therefore, these collections never have any of the collection changed events raised on them... they're always new, each time the binding is re-done.
Raising PropertyChanged forces the binding to be re-evaluated and re-runs your converter, returning a distinct collection and reflecting your changes.
I feel a better approach may be to do the conversion in your ViewModel rather than in a Converter. Expose an ObservableCollection of TabItem that you bind directly to and that you modify in place. The TabControl should then see changes made directly to your collection without the need to raise PropertyChanged and re-evaluate the entire binding.
[Edit - Added my approach]
ViewModel:
public class TabSampleViewModel
{
private ObservableCollection _tabItems = new ObservableCollection();
public TabSampleViewModel()
{
AddTabItem("Alpba");
AddTabItem("Beta");
}
public ObservableCollection<TabItem> TabItems
{
get
{
return _tabItems;
}
}
public void AddTabItem( string newTabItemName )
{
TabItem newTabItem = new TabItem();
newTabItem.Header = newTabItemName;
newTabItem.Content = newTabItemName;
TabItems.Add( newTabItem );
}
}
View:
<controls:TabControl ItemsSource="{Binding TabItems}"/>
Expose
public ObservableCollection<TabItem> Classnames
{
get { return _classnames; }
set { _classnames = value; }
}
If you debug the valueconverter you'll see it's not being called as often as you think it is.
The problem could be that your ValueConverter returns a List<TabItem> instead of an ObservableCollection<TabItem>. Try that one line change and see if it helps.

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