For some reason I am unable to bind to ViewModel properties within DataTemplates on some controls. The result of the binding itself is unpredictable, sometimes it work, sometimes it doesn't. For this reason I am thinking of exposing the ViewModel in some other way besides setting it as DataContext.
First thought was to add ViewModel to Resources collection. I am using TabControls for UI, so whenever a view needs to be displayed, it is done through Data templates.
<DataTemplate DataType="{x:Type vm:SomeViewModel}">
<vw:SomeView />
</DataTemplate>
In this situation the view is instantiated automatically, and its DataContext is set to ViewModel set in template. Is there a way I can make this ViewModel available to View's Resources (ex with key=viewModel), so that I can use it like this:
<TextBlock Text="{Binding SomeProperty, Source={StaticResource viewModel}}" />
I have tried adding it in code, in the Loaded event for the View:
this.Loaded += (s, e) =>
{
this.Resources.Add("viewModel", this.DataContext);
};
Above code is executed before the error pops up that says static resource is not found at run-time, so the resource was added to collection.
Any ideas what can I do?
You can define ViewModel as a resource in XAML like that:
<vm:SomeViewModel x:Key="ViewModel"/>
If you want to Bind to DataContext in a DataTemplate you can use the following:
{Binding Path=DataContext, ElementName=uc}
assuming that your window/usercontrol name is x:Name="uc" , or as #stukselbax wrote:
{Binding Path=DataContext, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=[UserControl|Window]}}
Related
I have successfully used ProxyElement to pass Data Context of my Data Grid to DataGridColumnHeaders. However, I am trying out something new and I just can't figure out what I am doing wrong over here.
Here is what I am trying to do: I am creating a UserControl and associating it to my ViewModel in my Resources file (see Resources.xaml code snippet below).
Resources.xaml:
<ResourceDictionary
xmlns:myVm="clr-namespace:..."
xmlns:myUserControl="clr-namespace:...">
<DataTemplate DataType={x:Type myVm:DummyModel}">
<myUserControl:DummyUserControl />
</DataTemplate>
</ResourceDictionary>
Now in my UserControl, I have a DataGrid with a DataGridComboBoxColumn. I am trying to access my data context to set its item source and in the past I was able to do it using proxy element. This time however I am not able to do so. (See DummyUserControl.xaml code snippet below)
DummyUserControl.xaml:
<UserControl x:Class="Client.MyControl.DummyUserControl"
...>
<UserControl.Resources>
<FrameworkElement x:Key="ProxyElement" x:Name="ProxyElement"
DataContext="{Binding}" />
</UserControl.Resources>
<DataGrid AutoGenerateColumns="False" ItemsSource="{Binding Products}">
<DataGridComboBoxColumn
Header="Company"
ItemsSource="{Binding Path=DataContext.ProductCompanies,
Source={StaticResource ProxyElement}}"
DisplayMemberPath="Name" SelectedValuePath="Id"
SelectedValueBinding="{Binding CompanyId}" />
</DataGrid>
</UserControl>
When I do this, my binding fails with the following message:
System.Windows.Data Error: 3 : Cannot find element that provides DataContext.
BindingExpression:(no path); DataItem=null; target element is 'FrameworkElement'
(Name='ProxyElement'); target property is 'DataContext' (type 'Object')
I have no idea what to do here. I remember reading that the datacontext for a datatemplate is automatically set, so I have no idea why the data context is null in this case. To prove it is null, I also tried setting the binding in the code-behind file and added a breakpoint to check its value (which was null).
Can anyone suggest what to do here?
Edit 1
I have also tried the following approaches:
Remove ProxyElement altogether and see if it can detect DataContext. To no surprise, this failed.
Tried binding to the templated parent. Fail.
Tried binding to the UserControl itself. Fail.
I also tried referencing the data context of the parent item where this view model is going to be displayed, which is in a TabItem of a TabControl.
All of the alternate bindings gave me same error as the error above.
Here is a (working but not preferred) solution to this problem. You will realize why it is not preferred by the end of it.
The key to this problem is understanding what and how data context of a data template works. Whenever you define a Data Template for a View Model, the data context for the view that follows, whether it is a user control or just xaml itself, is the View Model! This shouldn't be a surprise to anyone.
But this will surprise people: if you specify a User Control, the Data Context of the User Control is not set during construction of the User Control! In other words, in the constructor of User Control, Data Context is going to be null. Furthermore, any XAML code that relies on the Data Context at construction time, which in this case was my FrameworkElement resource called ProxyElement got its DataContext set to null because it gets constructed at construction time of the User Control!
So when does the DataContext get set? Simply after the User Control is created. In pseudo code, this following describes the logic behind drawing a ViewModel:
Draw ViewModel x;
DataTemplate in ResourceDictionary says ViewModel x can be drawn using UserControl abc
Let's create a new instance of UserControl abc
Let's now assign the DataContext of abc to the ViewModel itself.
Let's return the newly created instance of UserControl abc
So what do we do to solve the problem in this question?
UserControl.xaml:
<UserControl ...
DataContextChanged="DaCoHasChanged">
<UserControl.Resources>
<FrameworkElement x:Key="ProxyElement" /> <!--Remove DataContext="{Binding}"-->
</UserControl.Resources>
<DataGrid ...>
<DataGridComboBoxColumn
ItemsSource="{Binding Path=DataContext.ProductCompanies,
Source={StaticSource ProxyElement}}"
... />
</DataGrid>
</UserControl>
UserControl.xaml.cs:
private void DaCoHasChanged(object sender, DependencyPropertyChangedEventArgs e)
{
var proxyElement = Resources["ProxyElement"] as FrameworkElement;
proxyElement.DataContext = e.NewValue; // instead of e.NewValue, you could
// also say this.DataContext
}
I am trying to figure out a way of getting rid of the code in the code-behind file. But till then, if someone else hits this problem, then they might be able to get inspired from this solution.
Credit to the concept behind this solution goes to: How to set the DataContext for a View created in DataTemplate from ViewModel
Try this
<DataGrid AutoGenerateColumns="False" ItemsSource="{Binding Products}">
<DataGridTemplateColumn Header="Company">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<ComboBox
ItemsSource="{Binding Path="{Binding DataContext.ProductCompanies,RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}"
DisplayMemberPath="Name" SelectedValuePath="Id"
SelectedValueBinding="{Binding CompanyId}" />
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
</DataGrid.Columns>
</DataGrid>
Now as I got your problem I think the problem is in DataGridComboBoxColumn I dont know why it not Binding using RelativeResource . Try it with DataGridTemplateColumn and you would not require any ProxyElement I hope this will help.
I'm trying to understand. When I'm connecting View to ViewModel like this:
<DataTemplate DataType="{x:Type local:MyViewModel}">
<local:MyView />
</DataTemplate>
What does it mean?
It looks like the View is set to be the DataTemplate of the ViewModel. BUT the ViewModel doesn't have a Property of DataTemplate. So what exactly is going on in there?
A demonstration of the question - How do I do that by code (Connecting the View and ViewModel this specific way. I can't write ViewModel.DataTemplate = View)?
Thank you.
It means "To whatever control whose Content data is MyViewModel place MyView there". You are not setting DataTemplate of viewmodel (That does not mean anything) you are setting the DataTemplate for the control whose Data is MyViewModel.
Take for example an ItemsControl with an Items Source of
ObservableCollection<Employee> Employees
where each Employee is represented by a DataTemplate for Example :
<DataTemplate TargetType="{x:Type local:Employee}">
<StackPanel>
<TextBlock Text="{Binding FirstName}" />
<TextBlock Text="{Binding LastName}" />
</StackPanel>
</DataTemplate>
So in the same manner a MyViewModel.cs such as Employee.cs as a visual representation based on a DataTemplate .
and represented for example as such :
<ContentControl Content="{Binding MyViewModelProperty}" />
The way how it works is very simple. Your definition of DataTemplate is something like a definition of how a data will look like. In your example the data that you want to represent visually are of type:
DataType="{x:Type local:MyViewModel}"
By defining DataTemplate in control, window or other resources, e.g.
<UserControl.Resources> ...your template... <UserControl.Resources>,
you say "Hey, I want that all my data of type local:MyViewModel will look like this...". Inside the template you define a root control, that will be put in all places where your local:MyViewModel have been used. Normally when you place local:MyViewModel in Grid, ContentControl or other containers, you will see its string representation like "xxxx.xxxxx.MyViewModel" instead of visual.
To to see a graphical representation you must define a DataTemplate. It will replace the string "xxxx.xxxxx.MyViewModel" - representing your data and put there a visual control you defined in the template. Then when it is done, this representation - control from your DataTemplate will get DataContext property set to your View Model, here it will be local:MyViewModel instance.
That will give you a possibility to use binding in your DataTemplate, to bind in you DataTemplate directly to properties from you ViewModel.
Is that more clear now?
As a continuation of the question Linking DataContext with another property in WPF.
At the very end of the research I was very surprised to find out that when one writes something like that:
<Label Content="{Binding Path=Name}" />
The DataContext against which the Content property is binded is of the Label control itself! The fact that it still works is due to the default inheritance of the DataContext value from the nearest parent.
But if you have this label wrapped in a custom control, and you don't want to bind your data to the DataContext property of that control, you would more likely love to have:
<Controls:SearchSettings Settings="{Binding Path=Settings}" />
And here you are. Now you need to set Settings as the DataContext for the SearchSettings control, for Label inside to bind against, but you can't, because that will trigger re-binding of Settings property.
I can't see the point in mixing binding properties using different sources: DataContext, by ElementName, etc.
So why would I ever use DataContext?
When you write
<Label name="myLabel" Content="{Binding Path=Name}" />
you are binding to myLabel.DataContext.Name, and not myLabel.Name.
The XAML in WPF is just a pretty user interface to display and interact with the actual data, otherwise known as the DataContext. The purpose of other binding sources (RelativeSource, ElementName, etc) is to point to another property that doesn't exist in the current control's DataContext
So suppose you have a Window. Without setting the DataContext, the window still displays but there is no data behind it.
Now suppose to set myWindow.DataContext = new ClassA();. Now the data that the window is displaying is ClassA. If ClassA has a property called Name, I could write a label and bind it to Name (such as your example), and whatever value is stored in ClassA.Name would get displayed.
Now, suppose ClassA has a property of ClassB and both classes have a property called Name. Here is a block of XAML which illustrates the purpose of the DataContext, and an example of how a control would refer to a property not in it's own DataContext
<Window x:Name="myWindow"> <!-- DataContext is set to ClassA -->
<StackPanel> <!-- DataContext is set to ClassA -->
<!-- DataContext is set to ClassA, so will display ClassA.Name -->
<Label Content="{Binding Name}" />
<!-- DataContext is still ClassA, however we are setting it to ClassA.ClassB -->
<StackPanel DataContext="{Binding ClassB}">
<!-- DataContext is set to ClassB, so will display ClassB.Name -->
<Label Content="{Binding Name}" />
<!-- DataContext is still ClassB, but we are binding to the Window's DataContext.Name which is ClassA.Name -->
<Label Content="{Binding ElementName=myWindow, Path=DataContext.Name}" />
</StackPanel>
</StackPanel>
</Window>
As you can see, the DataContext is based on whatever data is behind the UI object.
Update: I see this question so often from new WPF users that I expanded this answer into a post on my blog: What is this “DataContext” you speak of?
From CodeProject by kishore Gaddam:
DataContext is one of the most fundamental concepts in Data Binding. The Binding object needs to get its data from somewhere, and there are a few ways to specify the source of the data like using Source property directly in the Binding, inheriting a DataContext from the nearest element when traversing up in the tree, setting the ElementName and RelativeSource properties in the Binding object.
Detailed example on CodeProject: http://www.codeproject.com/Articles/321899/DataContext-in-WPF
In that particular case, you could do:
<Controls:SearchSettings DataContext="{Binding Path=Settings}" Settings="{Binding}" />
Assuming you want everything that may be content of the SearchSettings to use Settings as it's data context. Basically, the DataContext affects the element itself an any descendants that don't explicitly override it.
In most cases you do want to bind to the DataContext, in some templates on ItemsControls it is the only way to bind to the currently templated item for example. Further bindings to the DataContext are nice to write and read as they are concise.
In your example you can still set the DataContext, you only need to modify the binding on the Settings respectively:
<Controls:SearchSettings DataContext="{Binding Settings}" Settings="{Binding}"/>
I have a sub control embedded inside my main control, it allows the user to edit an address. Because this is reused all over the place (sometimes in multiple places on one control) I bind it like so
<Controls:EditAddressUserControl DataContext="{Binding Path=HomeAddress}"/>
<Controls:EditAddressUserControl DataContext="{Binding Path=WorkAddress}"/>
But the EditAddressUserControl needs access to the main control's list of CountrySummary objects so it can choose which country the address belongs to.
I have added a Countries DependencyProperty to EditAddressUserControl and added
Countries="{Binding Countries}"
So far all is going well, the EditAddressUserControl.Countries property has the correct countries in it. However, how do I databind my Combobox.ItemsSource to that in XAML?
I still want everything on my EditAddressUserControl to bind to its DataContext, but the ComboBoxCountries.ItemsSource needs to bind to "this.Countries".
How do I do that?
I've tried this
<ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Controls:EditAddressUserControl}}, Path=Countries}" />
I saw no binding errors in the output box, but I also saw no items in the combobox.
You can accomplish this by using a RelativeSource for the binding source, instead of the DataContext.
This would most likely look something like:
<ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Controls:EditAddressUserControl}}, Path=Countries}" />
The way to do it was to stop using DataContext completely. Instead I added a DependencyProperty to my control
public static DependencyProperty AddressProperty = DependencyProperty.Register("Address", typeof(EditPostalAddressDto), typeof(EditPostalAddressControl));
Then in the parent control instead of setting DataContext="..." I set Address="..." - The XAML for the control is then changed to include an ElementName on the binding
<UserControl ..... x:Name="MainControl">
<TextBox Text="{Binding ElementName=MainControl,Path=Address.Region}"/>
Now I can specifically bind to the Address property, but also bind to properties on the main data context.
I have a XAML code that should load my UserControl inside the TabControl.
If I put this XAML code:
<DataTemplate x:Key="WorkspacesTemplate">
<TabControl
IsSynchronizedWithCurrentItem="True"
ItemsSource="{Binding Path=Gui}"
ItemTemplate="{StaticResource ClosableTabItemTemplate}"
Margin="4"
/>
</DataTemplate>
I have absolutly nothing appear in the windows (Gui property is inside the ViewModel class and return a UserControl).
But if I put his XAML code instead of the previous one:
<DataTemplate x:Key="WorkspacesTemplate">
<TabControl
IsSynchronizedWithCurrentItem="True"
ItemsSource="{Binding}"
ItemTemplate="{StaticResource ClosableTabItemTemplate}"
Margin="4"
/>
</DataTemplate>
I have the ViewModel Object loading:
(source: clip2net.com)
Here is a piece of code of the TextBoxInputViewModel that has the property Gui that should be binded to be able to get the Visual (usercontrol):
private UserControl gui;
public UserControl Gui
{
get
{
if (this.gui == null)
{
this.gui = new SimpleTextBoxInputControl();//Xaml User Control
this.gui.DataContext = this;//Bind the Visual and ViewModel
}
return this.gui;
}
}
Any idea how that I can get the UserControl instead of this object reference text?
The problem is that ItemSource is a collection, where as you're binding it to a property that is just one value. The error in the Output window that you're seeing is likely related to this.
Instead of returning a UserControl directly from your View Model, it would be better to return another View Model that represents the contents of the tab, and use templates to display that content. If you need it to be more dynamic than choosing the template based on the Type of the View Model, look into setting TabControl.ContentTemplateSelector. This needs to be set to a class that derives from DataTemplateSelector. You can use this class to decide which template to load based on the object bound to that tab.
you should create a template for your viewmodel in your app.xaml file like this
<DataTemplate DataType="{x:Type simpleModel:TextBoxInputViewModel}">
<myView:TextBoxInputControl />
</DataTemplate>
where simpleModel is the namespace of TextBoxInputViewModel, and TextBoxInputControl is the user control you want to show and myView is the namespace of that user control.