My application looks like the following
The black is my MainWindow, the red is a tab control and the yellow is a UserControl.
The UserControl has many Dependency Properties defined and they bind to the DataContext (Which is set in the MainWindow's code behind, using this.DataContext = this).
To bind my UserControl to the same DataContext as my MainWindow, in my UserControl xaml I have the following
DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorLevel=1,AncestorType=Window}}"
This works great, and when I interact with my UserControl, due to the two way binding, it updates the Properties of my MainWindow, which in turn updates my TabControl!
The issue is, my UserControl now has some extra functionality and as such, needs to bind to the UserControl's code behind (such as values for the GUI).
This is where I'm stuck. I can't bind from my UserControl to my code behind because I've already created a DataContext.
I know I could use the WinForms approach, and name each control with x:Name="MyControl" like
MyControl.Text = "This value";
or
MyControl.DataContext = this;
Yeuk I think!!
My question is, is this the only way to go, or can I still use binding.
First of all you don't need to manually set DataContext on UserControl. DataContext is an inheritable property so it will inherit DataContext from its parent unless you have explicitly set it.
Get rid of DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorLevel=1,AncestorType=Window}}" from your UserControl.
And now, in case you want to bind to code behind for some controls in your UserControl, you can bind using RelativeSource or can set DataContext on control:
DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=UserControl}}"
If controls can be clubbed together under one panel, set DataContext on parent panel say Grid and child controls will inherit from it:
<Grid DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=UserControl}}">
..Child Controls here will inherit DataContext
</Grid>
And to answer your question:
MyControl.DataContext = this; <-- Is this possible
Yes, it is possible like i mentioned above.
You can just use another RelativeSource Binding as you have for the MainWindow properties... to access the properties defined in the UserControl, try the following XAML in your UserControl:
<TextBlock Text="{Binding UserControlProperty, RelativeSource={RelativeSource
AncestorType={x:Type YourXmlNamespacePrefix:YourUserControl}}}" />
Obviously, you'll need to update YourXmlNamespacePrefix:YourUserControl to a valid XML Namespace and control type to get this to work.
I'm not saying either that you should set the DataContext anywhere, or change any properties. This is a RelativeSource Binding... you do not need to set any DataContext to make it work. I thought you would have known that seeing as you're already using one. Just try the example out.
Related
I have a simple data template in an UserControl composed of a TextBlock and two buttons. The DataContext is set to a list of objects. One button and the TextBlock are bound to properties of that object, but I need one button to be bound to a property of the UserControl.
Can I use RelativeSource to try and find the UC's class, and get the property that way? Something like this:
{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type MyUserControlClass}}, Path=SomeProperty}
At the top of the UserControl give it:
x:Name="MyUserControl"
Then to access the property you can simply use:
"{Binding ElementName=MyUserControl, Path=SomeProperty}"
I have a button in a UserControl that I want to call a method in another class (which happens to be my main window's view model).
<ToggleButton cal:Message.Attach="[Event Click] = [Action ToggleWatch]">
The user control's DataContext is tied to a DataTemplate. This makes everything beautiful, unless I run into cases like this where implementing the ToggleWatch method in the DataTemplate class doesn't make much sense, since the DataTemplate should only contain data.
Is the best way to get around this is by setting the DataContext of this control to the MainWindowViewModel? That workaround fails when I want to bind a value from the DataTemplate to the same button, since the DataContext modification will then make it look for the value in the MainWindowViewModel.
In case I've over complicated the question, I will summarise. I have a UserControl whose DataContext is a seperate DataTemplate. I want to call a method from a button when it is clicked, but I want the method seperated from the DataTemplate. I want the method in a completely different class. What is the best way to solve this?
Here is how I'm setting the DataTemplate:
<WrapPanel>
<ItemsControl ItemsSource="{Binding Devices}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<local:DeviceCleanerBox/>
...
UserControl must exist as child of your MainWindow and if i am right you must have set DataContext of MainWindow to be MainWindowViewModel.
So, what you can do is declare ICommand in MainWindowViewModel and bind to button's Command using RelativeSource to search for window's DataContext:
<ToggleButton Command="{Binding DataContext.CommandName,
RelativeSource={RelativeSource Mode=FindAncestor,
AncestorType=Window}}"/>
My UserControl requires binding to the ancestor (the ancestor being the MainWindow) and to itself (it's code behind).
To bind to the ancestor, I'm using
DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorLevel=1,AncestorType=Window}}">
To bind a control to the code behind (and thus using the 'local' DataContext), I'm using
<TextBlock Text ="{Binding MyUC3Property}" Name="MyName" />
and in the code behind, setting it like
this.MyName.DataContext = this;
The above works fine, where I can bind to the codebehind and to the ancestor.
Now, I still want to bind to the code behind and the ancestor but set the DataContext in the XAML only (if possible).
I've tried
<TextBlock Text ="{Binding MyUC3Property}" DataContext="{Binding RelativeSource={RelativeSource Mode=Self}}" />
and ensured the constructor does not set the DataContext (since I want it all done in the XAML) - (although even if I do set this.DataContext = this; the error persists)
and the output window tells me there is a binding error.
System.Windows.Data Error: 40 : BindingExpression path error: 'MyUC3Property' property not found on 'object' ''TextBlock' (Name='')'. BindingExpression:Path=MyUC3Property; DataItem='TextBlock' (Name=''); target element is 'TextBlock' (Name=''); target property is 'Text' (type 'String')
I guess I'm missing something obvious, but I can't tell what.
You should be able to bind to the user control the same way as you do to the window:
DataContext="{Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorLevel=1,AncestorType=UserControl}}">
What you have tried was referring to the relative source Self from the TextBox. However, in that context, Self refers to the TextBox, not to the enclosing user control.
for usercontrols you should never set the datacontext to self. check to comment from H.B. from here
i use ElementName Binding
<UserControl x:Name="uc">
<TextBlock Text="{Binding ElementName=uc, Path=MyDependencyPropertyDefinedInMyUserControl}"/>
using the usercontrol:
<Window>
<MyUserControl MyDependencyPropertyDefinedInMyUserControl="{Binding Path=MyValueForTheTextBox}"/>
i try to explain it a little bit for your textbox case (ignor my poor english btw :))
if you want create a usercontrol with a textbox and this usercontrol/textbox should show the text from differrent viewmodels in different views - then you have a problem as far as the viewmodels have different propertynames. now the dependency property in your usercontrol come into the game. you create a DP where all your viewmodels can bind to and you bind your textbox within your usercontrol just to the DP from your usercontrol.
First thing is that you should probably push your parent DataContext to the lower levels. This will give you "God" ViewMode shared between all nested screens.
Second is that you should probably use something like MVVMLights Messanger to have cleaner separation.
Say I have a WPF application (exe) that has this in the MainWindow.xaml:
<Grid>
<extraControls:MyMVVMUserControl MyDependencyProperty="{Binding Something}"/>
<extraControls:MyUserControl MyDependencyProperty="{Binding Something}" />
</Grid>
and my MainWindow.xaml.cs looks like this:
public MainWindow()
{
DataContext = new MainWindowVM();
InitializeComponent();
}
And my MainWindowVM.cs has a property setup for Something that notifies on property changed.
The user controls are made in a separate dll. As you may guess, MyMVVMUserControl has the DataContext set to a view model.
public MyMVVMUserControl()
{
DataContext = new MyMVVMUserControlVM();
InitializeComponent();
}
MyUserControl does not have a DataContext set in the code behind.
So the interesting thing is that they both have MyDependencyProperty setup exactly the same.
But the MVVM version does not work.
After digging in a bit, I found that the {Binding Something} in MainWindow.xaml is using the View Model setup for the MyMVVMUserControl as the DataContext (rather than the DataContext set in MainWindow.cs (set to MainWindowVM)).
And my question is why?
Why would WPF look inside the user control and use it's DataContext for a binding that is in the actual application?
(NOTE: I know I can get around this by setting the source in the binding, but I want others to be able to use my user controls. But with this issue, I now have a built-in "gotcha" for anyone I want to use my user controls.)
I think I understand you problem, and I'm gonna to give a solution that works for me (I had this problem before). The think is that seams that you are setting the DataContext for you MyMVVMUserControl in code behind, and then it take the bindings from that.
The solution I found for this, is to set the datacontext in code behind, but not at the user control. Set the datacontext for the UserControl's child item. For instance, supose this is the Xaml of your UserControl:
<UserControl ... x:Name="userControl">
<Grid x:Name="rootContainer">
...
</Grid>
</UserControl>
Then in the code behind set the rootContainer's data context, in this way all visual children can access to the control data context, and also the user control datacontext is empty.
...
rootContainer.DataContext = new UserControlViewModel();
...
Hope this may helps you to solve your issues...
You really shouldn't ever set the DataContext of a UserControl from inside the UserControl. By doing so, you are preventing any other DataContext from getting passed to the UserControl, which defeats one of WPF's biggest advantages of having separate UI and data layers.
WPF objects only inherit their DataContext from the parent object if the DataContext is not set to anything else. When your MyMVVMUserControl is being created, you are setting the DataContext to a new MyMVVMUserControlVM, which prevents the DataContext from being inherited from MainWindow.
So its normal that your MVVMUserControl would have it's DataContext set to your MyMVVMUserControlVM, because you set it explicitly in the UserControl's constructor.
This is by-design. UI objects in WPF/MVVM are only meant to be visual representations of the data layer, so it wouldn't make much sense to set the data layer and then try to bind your properties to something that is not on the data layer.
For example, take this line of code:
<UserControl DataContext="{Binding ClassA}" Content="{Binding Name}" />
This would bind the Content property to UserControl.DataContext.Name, which is ClassA.Name. It wouldn't make much sense if this would result in binding to UserControl.Parent.DataContext.Name, as the binding should refer to to the current object's DataContext, and not the Parent's DataContext.
So the only time I ever set the DataContext of a UserControl from inside the UserControl itself is if the UserControl is its own separate object that is never meant to interact with data from the rest of the application. Which so far has been never :)
Typically my UserControls are almost always one of two things:
Either a visual representation of a ViewModel (or Model), such as a CustomerUserControl for a CustomerViewModel, in which case I pass them the DataContext they need when they get used
For example,
<local:CustomerUserControl DataContext="{Binding SelectedCustomer}" />
or
<DataTemplate DataType="{x:Type local:CustomerModel}">
<local:CustomerUserControl />
</DataTemplate>
Or a self-sustained UI object that receives any external data it needs via custom DependencyProperties, and does any additional logic in the code-behind the control, such as a DatePicker control that has a SelectedDate dependency property, or a CalculatorUserControl with dependency properties for Equation and Value
<local:DatePickerUserControl SelectedDate="{Binding SomeDate}" />
<local:CalculatorUserControl Equation="{Binding SomeString}"
Value="{Binding SomeDouble}" />
In your case, it sounds like you should be using the first case, and should be passing a ViewModel into your UserControl containing the data it needs.
<extraControls:MyMVVMUserControl DataContext="{Binding MyMVVMUserControlVM}"
MyDependencyProperty="{Binding Something}">
or
<extraControls:MyMVVMUserControl MyDependencyProperty="{Binding Something}">
<extraControls:MyMVVMUserControl.DataContext>
<viewModels:MyMVVMUserControlVM />
</extraControls:MyMVVMUserControl.DataContext>
<extraControls:MyMVVMUserControl />
I developed a user control in SilverLight that contains several child controls. Textboxes, ComboBoxes and so on.
The problem is, when I include that UserControl into a parent view and set the complete control to IsEnabled=False, the child controls in that specific UserControl are still enabled.
After all I found the problem.
Adding something like that, implies that the IsEnabled Binding is located in the UserControl binding, not as expected from myself in the DataContext of the parent.
<localControls:TeamEmployeeSelector Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2"
IsEnabled="{Binding CanModify}" DataContext="{Binding Confidentiality}"/>
QUESTION:
But there's still the question how I can bind the IsEnabled to the ViewModel of the Parent? Because it's not very elegant to copy the CanModify Property to the ViewModel of the Child Control.
Instead of modifying a binding in some way (for example you can make it dependent on other control name as it is proposed in other answer) I would move separate the control which will be disabled and control where DataContext will be changed. For example:
<ContentControl IsEnabled="{Binding CanModify}" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2">
<localControls:TeamEmployeeSelector DataContext="{Binding Confidentiality}"/>
</ContentControl>
Here is how I would do this.
Your TeamEmployeeSelector UserControl will contain a single root level element which by default is a Grid and is given the name "LayoutRoot".
Now you can bind the IsEnabled property of all the child elements to the UserControl like this:-
<TextBox IsEnabled="{Binding Parent.IsEnabled, ElementName=LayoutRoot}" ... />
By using element-to-element binding you do not need to copy the CanModify property in to child view models.
Some might suggest that you simply add an x:Name to your UserControl element and then bind directly to it rather than going via the Parent property of the root element as I do above. That'll work ok in Silverlight 4 but not in 3 or WP7. Personally I prefer the above.
This is a scoping issue. Generally, when creating a UserControl, you want to set itself as the DataContext for its sub-elements. This is most easily accomplished in the constructor:
UserControlExample() {
InitializeComponent();
RootElement.DataContext = this;
}
Where RootElement is the name you give to this first child (usually a Grid or panel) of your UserControl.
From here you can set natural bindings for your sub-elements like so:
<TextBox x:Name="MainTextBox" IsEnabled={Binding IsEnabled} />
This works, since TextBox inherits the DataContext of the parent layout panel.
Finally, if you want to have your UserControl's IsEnabled property to be related to its parent, this is best done at the point of declaration:
<Grid>
<UserControlExample IsEnabled={Binding CanModify} />
</Grid>
This way you keep your concerns separate. The sub-controls don't care what the UserControl is reflecting. They just need to know how to enable/disable when the control's IsEnabled property flips.
sub-controls IsEnabled bound to --> (UserControlExample is DataContext)
UserControlExample.IsEnabled bound to --> (VM is DataContext)
VM.CanModify
I don't know if it's possible in Silverlight, but in WPF I would use RelativeSource.
Have a look here.
Hope this help !
<localControls:TeamEmployeeSelector Grid.Row="1" Grid.Column="0"
Grid.ColumnSpan="2" IsEnabled="{Binding ElementName=SomeElementName_With_Parent_ViewModel, Path=DataContext.CanModify}" DataContext="{Binding Confidentiality}"/>