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;
}
Related
What's the appropriate way to handle running an async operation when an item is selected from a two-way bound control such as a combobox (wpf data binding)?
When I have a two-way binding property (e.g. SelectedValue on ComboBox) I don't think I can use Stephen Cleary's NotifyTaskCompletion because when a user selects a value from the dropdown, the ComboBox itself would need to modify the bound Result property, which is the Task's result.
The only viable solution I've come up with is calling an async Task -method from the databound setter without awaiting the result. This should be fine as long as the async-method triggers a property changed event for whatever ui-related stuff is being done, and that any exceptions are picked up and propagated to the ui accordingly, right?
I assume this would be a common case in async WPF applications. How do you guys approach this?
My solution so far:
<ComboBox
ItemsSource="{Binding PossibleItems}"
DisplayMemberPath="Name"
SelectedValue="{Binding SelectedItem}"/>
...
public Item SelectedItem
{
get { return m_selectedItem; }
set
{
m_selectedItem = value;
OnPropertyChanged();
InitializeAsyncAndFirePropertyChanged(); // async Task method not awaited - gives compiler warning CS4014
}
}
public async Task InitializeAsyncAndFirePropertyChanged(ObservableCollection<RFEnvironment> possibleRfEnvironments)
{
//should check this method for exceptions and propagate them to the UI via databinding
OtherDataBoundProperty = await GetSomeStringFromWebAsync();
}
public string OtherDataBoundProperty
{
get { return m_otherDataBoundProperty; }
set
{
m_otherDataBoundProperty = value;
OnPropertyChanged();
}
}
Note: I have found similar questions asked, but none that addresses two-way bindings on controls such as a Combobox.
If you are using a functional reactive MVVM framework such as ReactiveUI, you would simply observe the SelectedItem property and kick off any operation you want when the property is set. e.g.:
this.WhenAnyValue(x => x.SelectedItem)
.Subscribe(async _ => await InitializeAsyncAndFirePropertyChanged());
A property itself should ne be kicking off background operations, but a view model may do this when a property is set.
Please refer to the docs for more information: https://reactiveui.net/docs/concepts/.
I have a similar issue with async call in property setter when using WCF databinding.
My solution is slightly better, because in your case when an exception occurs in InitializeAsyncAndFirePropertyChanged, no exception is thrown nor caught. The modified code is below. It uses task continuation to throw an exception and raising OnPropertyChanged. OnPropertyChanged call can stay in an original place, it depends on your needs.
public class MyViewModel: INotifyPropertyChanged
{
private readonly TaskScheduler _uiScheduler;
public MyViewModel()
{
_uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();
}
public Item SelectedItem
{
get { return m_selectedItem; }
set
{
m_selectedItem = value;
InitializeAsyncAndFirePropertyChanged()
.ContinueWith(t =>
{
if (t.Exception != null)
{
throw t.Exception;
}
OnPropertyChanged();
}, _uiScheduler);
}
}
public async Task InitializeAsyncAndFirePropertyChanged(ObservableCollection<RFEnvironment> possibleRfEnvironments)
{
//should check this method for exceptions and propagate them to the UI via databinding
OtherDataBoundProperty = await GetSomeStringFromWebAsync();
}
public string OtherDataBoundProperty
{
get { return m_otherDataBoundProperty; }
set
{
m_otherDataBoundProperty = value;
OnPropertyChanged();
}
}
.... other code to support INotifyPropertyChanged
}
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.
I have created a class Track which represents a song in the playlist:
public class Track
{
public Uri Path
{
get { return path; }
set { path = value; }
}
public TrackState State
{
get { return state; }
set { state = value; }
}
private Uri path;
private TrackState state;
}
Next I have created MainWindowController class which interacts between the UI window and the Track class:
public class MainWindowController : INotifyPropertyChanged
{
public ObservableCollection<Track> Playlist
{
get { return playlist; }
set
{
if (value != this.playlist)
{
playlist = value;
NotifyPropertyChanged("Playlist");
}
}
}
public int NowPlayingTrackIndex
{
set
{
if (value >= 0)
{
playlist[nowPlayingTrackIndex].State = TrackState.Played;
playlist[value].State = TrackState.NowPlaying;
this.nowPlayingTrackIndex = value;
}
}
}
private ObservableCollection<Track> playlist;
private int nowPlayingTrackIndex;
}
Basically, this class stores playlist collection and an index of the currently played track. Lastly, I have created the UI window in WPF:
<Window ...>
...
<ListBox
Name="PlaylistListBox"
ItemsSource="{Binding Source={StaticResource ResourceKey=PlaylistViewSource}}"
ItemTemplateSelector="{Binding Source={StaticResource ResourceKey=TrackTemplateSelector}}"
MouseDoubleClick="PlaylistListBox_MouseDoubleClick" />
...
</Window>
and the corresponding code behind:
...
private void PlaylistListBox_MouseDoubleClick(object sender, MouseButtonEventArgs e)
{
int index = this.PlaylistListBox.SelectedIndex;
this.windowController.NowPlayingTrackIndex = index;
}
...
Items source points to the static resource where the CollectionViewSource is defined. The ItemTemplateSelector defines which DataTemplate to use for list box items depending on the track state (NowPlaying or Played).
When the user double-clicks the playlist item, the NowPlayingTrackIndex in the MainWindowController gets updated and it updates the track state. The problem is, the DataTemplates for list box items do not get updated on the window, i.e. the the double-clicked list box item does not change the data template. Why?
I tried setting the PropertyChanged for track state but it didn't help. What am I missing? Thank you.
There are two issues to be addressed in your code.
First, you should know that ObservableCollection notify its observers about changes to its own elements, it doesn't know or care about changes to the properties of its elements. In other words, it doesn't watch for property change notification on the items within its collection. So changing the Track object property value in the PlayList collection doesn't watch by any mean. Here is an article about the subject.
Second, your MainWindowController doesn't broadcast for NowPlayingTrackIndex property value change at all. You should call NotifyPropertyChanged("NowPlayingTrackIndex") to notify interesting parties about the change of the current playing track. This may solve your problem but more elegant way, and my suggestion, would be implementing a custom ObservableCollection class (something like TrackObservableCollection) that contains NowPlaying property rather than implementing it in the MainWindowController class which looks like an unnecessary intermediation.
Short Version
If I update the Model object that my ViewModel wraps, what's a good way to fire property-change notifications for all the model's properties that my ViewModel exposes?
Detailed Version
I'm developing a WPF client following the MVVM pattern, and am attempting to handle incoming updates, from a service, to data being displayed in my Views. When the client receives an update, the update appears in the form of a DTO which I use as a Model.
If this model is an update to an existing model being shown in the View, I want the associated ViewModel to update its databound properties so that the View reflects the changes.
Let me illustrate with an example. Consider my Model:
class FooModel
{
public int FooModelProperty { get; set; }
}
Wrapped in a ViewModel:
class FooViewModel
{
private FooModel _model;
public FooModel Model
{
get { return _model; }
set
{
_model = value;
OnPropertyChanged("Model");
}
}
public int FooViewModelProperty
{
get { return Model.FooModelProperty; }
set
{
Model.FooModelProperty = value;
OnPropertyChanged("FooViewModelProperty");
}
}
The Problem:
When an updated model arrives, I set the ViewModel's Model property, like so:
instanceOfFooVM.Model = newModel;
This causes OnPropertyChanged("Model") to fire, but not OnPropertyChanged("FooViewModelProperty"), unless I call the latter explicitly from Model's setter. So a View bound to FooViewModelProperty won't update to display that property's new value when I change the Model.
Explicitly calling OnPropertyChanged for every exposed Model property is obviously not a desirable solution, and neither is taking the newModel and iterating through its properties to update the ViewModel's properties one-by-one.
What's a better approach to this problem of updating a whole model and needing to fire change notifications for all its exposed properties?
According to the docs:
The PropertyChanged event can indicate all properties on the object have changed by using either null or String.Empty as the property name in the PropertyChangedEventArgs.
One option is to listen to your own events, and make a helper routine to raise the other notifications as required.
This can be as simple as adding, in your constructor:
public FooViewModel()
{
this.PropertyChanged += (o,e) =>
{
if (e.PropertyName == "Model")
{
OnPropertyChanged("FooViewModelProperty");
// Add other properties "dependent" on Model here...
}
};
}
Whenever your Model property is set, subscribe to its own PropertyChanged event. When your handler gets called, fire off your own PropertyChanged event. When the Model is set to something else, remove your handler from the old Model.
Example:
class FooViewModel
{
private FooModel _model;
public FooModel Model
{
get { return _model; }
set
{
if (_model != null)
{
_model.PropertyChanged -= ModelPropertyChanged;
}
if (value != null)
{
value.PropertyChanged += ModelPropertyChanged;
}
_model = value;
OnPropertyChanged("Model");
}
}
public int FooViewModelProperty
{
get { return Model.FooModelProperty; }
set
{
Model.FooModelProperty = value;
OnPropertyChanged("FooViewModelProperty");
}
}
private void ModelPropertyChanged(object sender, PropertyChangedEventArgs e)
{
// Here you will need to translate the property names from those
// present on your Model to those present on your ViewModel.
// For example:
OnPropertyChanged(e.PropertyName.Replace("FooModel", "FooViewModel"));
}
}
Implements INotifyPropertyChanged
Public Event PropertyChanged As PropertyChangedEventHandler Implements INotifyPropertyChanged.PropertyChanged
RaiseEvent PropertyChanged(Me, New PropertyChangedEventArgs(String.Empty))
For VB.net if anybody else needs it. If you have already implemented "INotifyPropertyChanged" then the last line is all you need.
I have an INotifyProperty Screen item that I have bound to a wpf control.
Ok... I Simplified everything and am posting more code. I have a MainViewModel with the selected screen property.
public Screen SelectedScreen
{
get { return this.selectedScreen; }
set
{
this.selectedScreen = value;
this.OnPropertyChanged("SelectedScreen");
}
}
I have a textbox that is bound to this property:
<TextBlock Text="{Binding Path=SelectedScreen.ScreenNumber}" />
This all works initially. I have created another control that is changing the selected screen with the following code.
public Screen SelectedScreen
{
get { return (Screen)GetValue(SelectedScreenProperty); }
set
{
this.SetValue(SelectedScreenProperty, value);
for (int x = 0; x < this.Screens.Count; ++x)
this.Screens[x].IsSelected = false;
value.IsSelected = true;
}
}
public ObservableCollection<Screen> Screens
{
get { return (ObservableCollection<Screen>)GetValue(ScreensProperty); }
set { this.SetValue(ScreensProperty, value); }
}
public static readonly DependencyProperty SelectedScreenProperty =
DependencyProperty.Register("SelectedScreen",
typeof(Screen),
typeof(ScreenSelection));
public static readonly DependencyProperty ScreensProperty =
DependencyProperty.Register("Screens",
typeof(ObservableCollection<Screen>),
typeof(ScreenSelection),
new UIPropertyMetadata(new ObservableCollection<Screen>()));
This screen selection control is working. When I change screens and put a breakpoint on the set property of SelectedScreen it is called which then calls the SelectedScreen property of the MainViewModel. So the event is firing, but the textbox isn't updated even though it binds correctly the first time.
Does the class which contains the SelectedScreen property implement INotifyPropertyChanged? When the SelectedScreen property changes, the containing class should raise the PropertyChanged event, and typically, WPF should update the Binding.
Thank you gehho for looking at this. I figured it out and there is no way you had enough information to be able too. I was inheriting from ViewModelBase in the MainViewModel that was inheriting from ObservableObject where I implemented INotifyPropertyChanged. The problem is that I implemented the methods for INotifyPropertyChanged in both classes and WPF was listening to the wrong one. Very obscure. Very annoying. Very lasjkdf;ashdoh