I have a WPF application which uses a WPF user control.
The user control exposes a DependencyProperty to which I would like to bind to in my WPF application.
As long as my user control does not set its own DataContext this works and I am able to listen to changes in the DependencyProperty.
However the moment I set the DataContext the PropertyChanged being called is null.
What am I missing here?
Code sample:
https://skydrive.live.com/redir.aspx?cid=367c25322257cfda&page=play&resid=367C25322257CFDA!184
DependencyProperty has inheritance property, so if you don't set the UserControlDP's DataContext, the DataContext is inherited from the MainWindow's DataContext. In this case, the UserControlDP's DataContext in your code below is set as MainWindow_ViewModel. Thus, the binding is correctly executed.
<usercontrol:UserControlDP Width="200" Height="100"
TestValue="{Binding TestValueApp, Mode=TwoWay, NotifyOnSourceUpdated=True, NotifyOnTargetUpdated=True}"
Margin="152,54,151,157"></usercontrol:UserControlDP>
In the other case, UserControlDP's DataContext is set as UserControlDP_ViewModel, so the binding is broken. You can see the first exception message as the following at the debug window.
System.Windows.Data Error: 40 : BindingExpression path error: 'TestValueApp' property not found on 'object' ''UserControlDP_ViewModel' (HashCode=24672987)'. BindingExpression:Path=TestValueApp; DataItem='UserControlDP_ViewModel' (HashCode=24672987); target element is 'UserControlDP' (Name=''); target property is 'TestValue' (type 'Object')
Consider setting the DataContext on one of the elements contained within UserControl rather than on UserControl itself.
Thanks for the input and clarifying the details.
After giving it some thought I took the easy way out and removed the ViewModel from the control.
MVVM for the application but no MVVM for the user control.
This way I do not use any bindings in the user control, instead use Dependency Properties which are bound to in the Main application.
Related
I am trying to be a good soldier and design some simple User Controls for use in WPF MVVM applications. I am trying (as much as possible) to make the UserControls themselves use MVVM, but I don't think the calling app should know that. The calling app should just be able to slap down the user control, perhaps set one or two properties, and perhaps subscribe to events. Just like when they use a regular control (ComboBox, TextBox, etc.) I'm having a heck of a time getting the bindings right. Notice the use of ElementName in the below View. This is instead of using DataContext. Without further ado, here is my control:
<UserControl x:Class="ControlsLibrary.RecordingListControl"
...
x:Name="parent"
d:DesignHeight="300" d:DesignWidth="300">
<Grid >
<StackPanel Name="LayoutRoot">
<ListBox ItemsSource="{Binding ElementName=parent,Path=Recordings}" Height="100" Margin="5" >
<ListBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=FullDirectoryName}" />
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
</StackPanel>
</Grid>
In the code behind (if there is a way to avoid code behind, please tell me)...
public partial class RecordingListControl : UserControl
{
private RecordingListViewModel vm = new RecordingListViewModel();
public RecordingListControl()
{
InitializeComponent();
// I have tried the next two lines at various times....
// LayoutRoot.DataContext = vm;
//DataContext = vm;
}
public static FrameworkPropertyMetadata md = new FrameworkPropertyMetadata(new PropertyChangedCallback(OnPatientId));
// Dependency property for PatientId
public static readonly DependencyProperty PatientIdProperty =
DependencyProperty.Register("PatientId", typeof(string), typeof(RecordingListControl), md);
public string PatientId
{
get { return (string)GetValue(PatientIdProperty); }
set { SetValue(PatientIdProperty, value);
//vm.SetPatientId(value);
}
}
// this appear to allow us to see if the dependency property is called.
private static void OnPatientId(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
RecordingListControl ctrl = (RecordingListControl)d;
string temp = ctrl.PatientId;
}
In my ViewModel I have:
public class RecordingListViewModel : ViewModelBase
{
private ObservableCollection<RecordingInfo> _recordings = null;// = new ObservableCollection<string>();
public RecordingListViewModel()
{
}
public ObservableCollection<RecordingInfo> Recordings
{
get
{
return _recordings;
}
}
public void SetPatientId(string patientId)
{
// bunch of stuff to fill in _recordings....
OnPropertyChanged("Recordings");
}
}
I then put this control down in my main window and like so:
<Grid>
<ctrlLib:RecordingListControl PatientId="{Binding PatientIdMain}" SessionId="{Binding SessionIdMain}" />
<Label Content="{Binding PatientIdMain}" /> // just to show binding is working for non-controls
</Grid>
The error I get when I run all this is:
System.Windows.Data Error: 40 : BindingExpression path error: 'Recordings' property not found on 'object' ''RecordingListControl' (Name='parent')'. BindingExpression:Path=Recordings; DataItem='RecordingListControl' (Name='parent'); target element is 'ListBox' (Name=''); target property is 'ItemsSource' (type 'IEnumerable')
Clearly I have some sort of bindings problem. This is actually much further than I was getting. At least I'm hitting the code in the controls code behind:
OnPatientId.
Before, I didn't have the ElementName in the User Control and was using DataContext and was getting a binding error indicating that PatientIdMain was being considered a member of the user control.
Can someone point me to an example of using a User Control with MVVM design in a MVVM application? I would think this is a fairly common pattern.
Let me know if I can provide more details.
Many thanks,
Dave
Edit 1
I tried har07's idea (see one of the answers). I got:
If I try:
ItemsSource="{Binding ElementName=parent,Path=DataContext.Recordings}"
I get
System.Windows.Data Error: 40 : BindingExpression path error: 'Recordings' property not found on 'object' ''MainViewModel' (HashCode=59109011)'. BindingExpression:Path=DataContext.Recordings; DataItem='RecordingListControl' (Name='parent'); target element is 'ListBox' (Name=''); target property is 'ItemsSource' (type 'IEnumerable')
If I try:
ItemsSource="{Binding Recordings}"
I get
System.Windows.Data Error: 40 : BindingExpression path error: 'Recordings' property not found on 'object' ''MainViewModel' (HashCode=59109011)'. BindingExpression:Path=Recordings; DataItem='MainViewModel' (HashCode=59109011); target element is 'ListBox' (Name=''); target property is 'ItemsSource' (type 'IEnumerable')
I think his first idea (and maybe his second) are very close, but recall, Recordings is defined in the ViewModel, not the view. somehow I need to tell XAML to use viewModel as source. That's what setting the DataContext does, but as I said in the main part, that creates problems elsewhere (you get binding errors related to binding from the MainWindown to properties on the control).
Edit 2. If I try har07's first suggestion:
ItemsSource="{Binding ElementName=parent,Path=DataContext.Recordings}"
AND add in the code behind for the control:
RecordingListViewModel vm = new RecordingListViewModel();
DataContext = vm;
I get:
System.Windows.Data Error: 40 : BindingExpression path error: 'PatientIdMain' property not found on 'object' ''RecordingListViewModel' (HashCode=33515363)'. BindingExpression:Path=PatientIdMain; DataItem='RecordingListViewModel' (HashCode=33515363); target element is 'RecordingListControl' (Name='parent'); target property is 'PatientId' (type 'String')
in other words, the control seems fine, but the binding of the dependency proprerties to the main window seem messed up. The compiler assumes that PatientIdMain is part of RecordingListViewModel.
Various posts indicated that I couldn't set DataContext for this very reason. It would mess up bindings to the main window. See for example:
Binding to a dependency property of a user control WPF/XAML and check out Marc's answer.
You should not set x:Name in a UserControl, since the control has only one Name property, and normally that would be set through the code which uses the control. So, you can't use an ElementName Binding in order to bind to properties of the UserControl itself. Another way to bind to properties of the UserControl inside its content would be to use
{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type my:RecordingsControl}}, Path=Recordings}
In the same way, the client code which uses the control sets its DataContext, either explicitly or through inheritance. Therefore, the instance of the vm you create in the constructor is discarded and replaced by the inherited DataContext as soon as the control is displayed.
You can do two things to solve this: either create the vm outside the UserControl, e.g. as a property of the main window's ViewModel, and use the UserControl like this:
<my:RecordingsControl DataContext="{Binding RecordingListVM}"/>
This way, you wouldn't need any code behind, and the Binding above would simply change to
{Binding Recordings}
Or, create a Recordings property in the UserControl's code behind file, and bind to it as I showed in the first code example.
What you get if binding statement changed this way :
ItemsSource="{Binding ElementName=parent,Path=DataContext.Recordings}"
or this way :
ItemsSource="{Binding Recordings}"
If one of above binding way solve current binding error ("BindingExpression path error: 'Recordings' property not found..."), but lead to another binding error please post the latter error message.
I think the correct binding statement for this part is as mentioned above.
UPDATE :
Responding to your edit. Try to set DataContext locally at StackPanel level, so you can have UserControl set to different DataContext :
public RecordingListControl()
{
InitializeComponent();
RecordingListViewModel vm = new RecordingListViewModel();
LayoutRoot.DataContext = vm;
}
again I can see you've tried this but I think this is the correct way to solve particular binding error ("BindingExpression path error: 'PatientIdMain' property not found..."), so let me know if this solve the error but lead to another binding error.
One possible answer is this (inspired by Dependency property binding usercontrol with a viewmodel)
Simply use DataContext for the UserControl and don't use any of the ElementName business.
I.e.
public partial class RecordingListControl : UserControl
{
public RecordingListViewModel vm = new RecordingListViewModel();
public RecordingListControl()
{
InitializeComponent();
**DataContext = vm;**
}
....
Then, in the MainWindow, you need to bind the user control like so:
<ctrlLib:RecordingListControl
Name="_ctrlRecordings"
PatientId="{Binding RelativeSource={RelativeSource
AncestorType=Window},Path=DataContext.PatientIdMain, Mode=TwoWay}"
SessionId="{Binding RelativeSource={RelativeSource
AncestorType=Window},Path=DataContext.SessionIdMain, Mode=TwoWay}" />
I won't mark this as answer because (besides the audacity of answer one's own question) I don't really like the answer. It forces the application programmer, to remember to put in all this nonsense about AncestorType and RelativeSource. I don't think one has to do this with standard controls, so why here?
I'm sure I am doing something wrong.
Some context: WPF application, MVVM, Entity Framework, etc.
In my ViewModel for a window, I have two properties: MemberCount and EventCount. Both are int.
In my view, I have a 5 instances of a custom control. This custom control has three dependency Properties: ImageData, ButtonText, and ButtonSubText. All are string.
What I want to do is bind the control in such a way that MemberCount is bound to the ButtonText property on one control, and EventCount is Bound to ButtonText property on another control.
So I do something along the lines of:
<control:MyControl ... ButtonText="{Binding MemberCount, Mode=OneWay}" ButtonSubText="Members" Background="{DynamicResource MetroRed}" />
The DataContext of my window is set to the correct ViewModel.
No matter what I try, I cannot get the member count or event count to show up on the control. I get an error in the output window:
System.Windows.Data Error: 40 : BindingExpression path error: 'MemberCount' property
not found on 'object' ''MyControl' (Name='button')'. BindingExpression:Path=MemberCount;
DataItem='MyControl' (Name='button'); target element is 'MyControl' (Name='button');
target property is 'ButtonText' (type 'String')
I am getting used to DataBinding in WPF, but for the life of me, I can't figure this one out.
What do I need to do to be able to bind a property from my view model to a property on my custom control?
i think the problem is the datacontext or binding within your usercontrol. you should post your usercontrol code.
<MyUserControl x:Name="uc">
<TextBlock Text="{Binding ElementName=uc,Path=ButtonText}/>
</MyUserControl>
check my answer here, there is a similar problem.
Can you explain the following WPF code:
DataContext="{Binding Path=PlacementTarget,RelativeSource={x:Static RelativeSource.Self}}">
I find it extremely confusing. What is placement target and what is relative source?
This looks like a hack that is used for popup-elements such as ContextMenus and Popup-windows.
The problem with these elements is, that they are disconnected from the visual tree of your window. Therefore the DataContext is not available. The PlacementTarget is a link to an element of the visual-tree.
Mostly you will find a binding path like PlacementTarget.Tag where in the source element the Tag property has been set to the DataContext but in some situations, the element itself is also meaningful, such as in your example.
Assuming that the above code is used in a ToolTip or a ContextMenu, the DataContext will be set to the control that "owns" the element.
Look at the post from (Gishu +1) for an explanation of the mechanics.
Every FrameworkElement has a DataContext that is an arbitrary object. The default source for a data binding is that DataContext. You can use RelativeSource.Self to change the source for a binding to the FrameworkElement itself instead of its DataContext. So the RelativeSource part just moves you "up one level" from the DataContext of the FrameworkElement to the FrameworkElement itself. Once you are at the FrameworkElement you can specify a path to any of its properties. If the FrameworkElement is a Popup, it will have a PlacementTarget property that is the other FrameworkElement that the Popup is positioned relative to.
In short, if you have a Popup placed relative to a TextBox for example, that expression sets the DataContext of the Popup to the TextBox and as a result {Binding Text} somewhere in the body of the Popup would bind to the text of the TextBox.
This is binding the DataContext of a thing (UI Control? need to see more of the code snippet ) to its own PlacementTarget property value.
RelativeSource is used to indicate the source object relative to the binding target. The path property indicates the name of the property on the source object.
I've seen this syntax show up, and have tried to google for it's definition to no avail; what does it mean when a dp is bound this way?
<Grid>
<ContentControl Content="{Binding}"/>
</Grid>
I was under the assumption that you have to bind to some property on the DataContext, or another element, but this appears to bind to nothing.
I believe it means you are binding to the root of whatever the binding context is. So if you use this syntax in a datatemplate that is part of some sort of list control, you would be binding to the root level of whatever the parent control (the list control) was binding to.
I believe {Binding} refers to the DataContext itself.
edit (clarification): By DataContext I mean the current level DataContext. For example, if your window's DataContext is bound to a List, then setting ItemsSource on a ListBox control in your window to {Binding} would bind the ListBox to the List itself, not a property of the List, like Count.
{Binding} is for {Binding [CurrentDataContext]}
{Binding} means that you want to Bind to the the current DataContext which could be set on the object itself. If no DataContext is set on the current object, then it will walk up the VisualTree and find the closest Parent that has a DataContext.
In WPF, how do I get the control (FrameworkElement) that is bound to a property in a viewmodel's end? I want to create a drop shodow effect on the control.
Getting a control instance in the ViewModel class is not really a good practice(as per MVVM). You can always have DataTrigger or converter in the XAML side to do that.