I have created a UserControl, which has some values bound to its view model.
I want to create an instance of that UserControl, provide it with a view model, and save it to an image without actually showing it on screen anywhere.
The problem is that when I create an instance of the UserControl and give it its view model, the bindings don't update. So the image that I am saving is the control without any data in it.
How can I force all the bindings to update for a user control that I am creating in code, without actually displaying it on screen?
Here is a short example I wrote:
UserControl1.xaml:
<Grid>
<TextBlock Name="txt" Text="{Binding}" />
</Grid>
UserControl1.xaml.cs
public partial class UserControl1 : UserControl
{
public UserControl1()
{
InitializeComponent();
}
internal string GetText()
{
return txt.Text;
}
}
Test.cs
public void Test()
{
UserControl1 uc1 = new UserControl1();
uc1.DataContext = "Test";
Console.WriteLine("The text is " + uc1.GetText());
}
I am expecting it to print out "The text is Test", but it actually prints out "The text is".
I managed to get around the problem by setting the DataContext before calling InitializeComponent. In order to do this, I created a new constructor for my user control that looks like this:
public UserControl1(object viewModel)
{
DataContext = viewModel;
InitializeComponent();
}
If I use this constructor, the bindings will work even if I never show the control on screen.
Related
I am beginner to WPF and MVMM architecture. VI came across many links which explains about DataContext dependence property in WPF MVMM architecture,
i.e.
view.DataContext = new ViewModels.MainViewModel();
but they always made me confused. Although I have some basic idea about this DataContext like it is used to represent who's object we need in xaml file, but when blogs talks about tree structure inheritance of dataContext I gets confused. Can any one please help me with some very simple and clear example showing how this hierarchy of DataContext works?
Thanks in advanced.
The DataContext property specifies the default source for Data Binding. Consider the following example:
<TextBox Text="{Binding MyProperty}" />
What this Binding says: take the value of MyProperty from whatever object is inside the DataContext, convert it to a string and put it in the TextBox. So if we would set the DataContext of the TextBox to be an object of the following class:
public class Example {
int MyProperty { get { return 3; } }
}
Then, the Text of the TextBox would be set to 3.
What does it mean that the values Inherit? Consider a slightly more complex example:
<Window Name="MainWindow">
<StackPanel>
<TextBox Text="{Binding MyProperty}" />
...etc
If we would have 10 or more TextBox elements on our screen, it would be a lot of senseless work to assign the DataContext to each and every TextBox. To relieve this issue, the implementors of WPF decided that setting the DataContext on the MainWindow in our case would also apply it to everything inside that Window (all children, all nested elements) until the DataContext property is overwritten (i.e. we set the DataContext of the TextBox, then the TextBox and all its children would also receive this DataContext).
If you want to see this behavior in action, the same applies to the FontSize property, try setting the FontSize of your Window to 48 and see what happens to all the text in there!
The Datacontext property is the default source of all the binding of a View.
In MVVM, the Datacontext is used to link a ViewModel to a View.
As the Datacontext property is a dependence property, if you don't define it in a control, it will inherit from his father, etc.
Here is an exemple of MVVM implementation :
Parent of all ViewModel class (to implement INotifyPropertyChanged in all ViewModels) :
public abstract class ViewModelBase : INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
protected virtual void OnPropertyChanged(string propertyName)
{
if (this.PropertyChanged != null)
{
this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
}
Note : INotifyPropertyChanged allow your ViewModel to notify the View of a change (used for bindings).
Let's say I want a MainWindows (View) to be linked to a ViewModel :
public MainWindow()
{
InitializeComponent();
MainViewModel mainViewModel = new MainViewModel(this);
this.DataContext = mainViewModel;
}
With for ViewModel :
class MainViewModel : ViewModelBase
{
#region fields
private MainWindow mainWindow;
private string message = "Hello world !";
#endregion
#region properties
public MainWindow MainWindow
{
get
{
return this.mainWindow;
}
}
public string Message
{
get
{
return message;
}
set
{
this.message = value; OnPropertyChanged("Message");
}
}
// ...
#endregion
public MainViewModel(MainWindow mainWindow)
{
this.mainWindow = mainWindow;
}
}
So now if I want to bind a property of MainViewModel in my View (mainwindow), i just have to have a public property in my ViewModel and to create a binding in my XAML. I won't have to specify the source as the DataContext is the default source.
So MainWindow.xaml I can add :
<TextBox Text="{Binding Message}" />
I asked a similar question a while back here WPF MVVM User Control. I got some answers, but they were way off, so I guess I wasn't clear in what I want to do....
I am working on a WPF application using MVVM. The app is built using a component based approach, so I have some user controls that I've defined that will be used throughout the application. As an example, I have an Address control. I want to use it an multiple places throughout the application. Here's an example. See here:
http://sdrv.ms/1aD775H
The part with the green border is the Address control. The control has its own View Model.
When I place it on a window or other control, I need to tell it the PK of the customer to load addresses for. So I created a Customer ID DependencyProperty:
public partial class AddressView : UserControl
{
public AddressView()
{
InitializeComponent();
}
public static DependencyProperty CustomerIdProperty = DependencyProperty.Register("CustomerId", typeof(int), typeof(AddressView),
new UIPropertyMetadata(0, AddressView.CustomerIdPropertyChangedCallback, AddressView.CustomerIdCoerce, true));
public int CustomerId
{
// THESE NEVER FIRE
get { return (int)GetValue(CustomerIdProperty); }
set { SetValue(CustomerIdProperty, value); }
}
private static void CustomerIdPropertyChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs args)
{
// THIS NEVER FIRES
AddressView instance = (AddressView)d;
instance.CustomerId = (int)args.NewValue;
}
enter code here
private static object CustomerIdCoerce(DependencyObject d, object value)
{
return value; // <=== THIS FIRES ON STARTUP
}
}
Then in the MainWindowView I have:
<vw:AddressView Grid.Row="1"
Grid.Column="0"
x:Name="AddressList"
CustomerId="{Binding ElementName=TheMainWindow, Path=SelectedCustomer.Id, Mode=TwoWay}"/>
Note my comments in the user control's CS. The Coerce fires on startup. The callback never fires, nor do the CustomerId getter or setter.
What I would like to happen seems simple, I just can't make it work....
When a customer is selected the CustomerId should be passed to the Address UserControl. Then in the VM for the Address UserControl should handle getting & saving the data.
So, again, 2 questions:
1) Anyone see what's wrong?
2) How does the UserControl DP send the PK to the ViewModel?
If anyone's interested, my sample project is here: http://sdrv.ms/136bj91
Thanks
Your CustomerId getter and setter will never fire in this situation. They are there simply as helper methods in case you want to access the CustomerIdProperty property from your code behind.
Your CustomerIdPropertyChangedCallback method will not fire because your binding expression is incorrect. You need to bind to the DataContext of MainWindow and not the window itself:
...
CustomerId="{Binding ElementName=TheMainWindow, Path=DataContext.SelectedCustomer.Id}"
...
Also, make sure that you are calling the INotifyPropertyChanged PropertyChanged event when the property bound to the ComboBox is changed.
Try this :
CustomerId="{Binding RelativeSource={RelativeSource FindAncestor,
AncestorType={x:Type Window}}, Path=DataContext.YourSelectedItem.TheProperty}"
I am not sure how to manage your seleted item in window, so please change yourSelectedItem.TheProperty accordingly.
I'm having some problems with data binding in WPF. Here's the scenario: I have made a user control which simulates a Dial Pad (i.e., an array of 12 buttons with the digits from '0' to '9' plus the '#' and 'Clear' keys). The control lives inside a class library and it's been implemented following the MVVM pattern, mainly because I need the components in the class library to be easily unit tested.
The view model for the control is quite simple, it basically updates a public "DialedNumber" string (which is internally connected to the model) every time the user presses a dial pad key button. The binding is working correctly and, by using the debugger, I can confirm that the "DialedNumber" variable inside the viewmodel is getting updated as I press button in the dial pad.
This DialPad control is used by a separate XAML file (Panel.xaml), which laids out several controls that belong to my custom class library.
Now, I'd like to add a TextBlock inside my Panel file in order to display the "DialedNumber" string held inside the DialPad. This is the code snippet in Panel.xaml:
<PanelControls:DialPad x:Name="MyDialPad" DialedNumber="55325"/>
<TextBlock Text="{Binding ElementName=MyDialPad, Path=DialedNumber}" />
The result I'm getting is that the textblock displays the correct number on start (i.e., "55325"), but its content doesn't get updated as I press the dial pad keys (even though the DialPad's viewmodel gets updated as I press new keys, as I've checked with the debugger).
Here's the code behind for the DialPad view:
public partial class DialPad : UserControl
{
public DialPad()
{
InitializeComponent();
DataContext = new DialPadViewModel();
}
public void DialedNumberChanged(object sender, EventArgs e)
{
return;
}
public DialPadViewModel DialPadViewModel
{
get { return DataContext as DialPadViewModel; }
}
public string DialedNumber
{
get
{
var dialPadViewModel = Resources["DialPadVM"] as DialPadViewModel;
return (dialPadViewModel != null) ? dialPadViewModel.DialedNumber : "";
}
set
{
var dialPadViewModel = Resources["DialPadVM"] as DialPadViewModel;
if (dialPadViewModel != null)
{
dialPadViewModel.DialedNumber = value;
}
}
}
}
Here's the DialPad view model:
public class DialPadViewModel : ObservableObject
{
public DialPadViewModel()
{
_dialPadModel = new DialPadModel();
}
#region Fields
private readonly DialPadModel _dialPadModel;
private ICommand _dialPadKeyPressed;
#endregion
#region Public Properties/Command
public DialPadModel DialPadModel
{
get { return _dialPadModel; }
}
public ICommand DialPadKeyPressedCommand
{
get
{
if (_dialPadKeyPressed == null)
{
_dialPadKeyPressed = new RelayCommand(DialPadKeyPressedCmd);
}
return _dialPadKeyPressed;
}
}
public string DialedNumber
{
get { return _dialPadModel.DialedNumber; }
set
{
_dialPadModel.DialedNumber = value;
RaisePropertyChanged("DialedNumber");
}
}
#endregion
#region Private Helpers
private void DialPadKeyPressedCmd(object parameter)
{
string keyPressedString = parameter.ToString();
if (keyPressedString.Length > 0)
{
if (char.IsDigit(keyPressedString[0]))
{
DialedNumber += keyPressedString[0].ToString(CultureInfo.InvariantCulture);
}
else if (keyPressedString == "C" || keyPressedString == "Clr" || keyPressedString == "Clear")
{
DialedNumber = "";
}
}
}
#endregion
}
Let me restate my problem: the textblock in Panel.xaml displays the correct number (55325) on start, but its value never gets updated as I press the DialPadButtons. I've placed a breakpoint inside DialPadKeyPressedCmd and I can confirm that the method gets executed everytime I press a key in the dial pad.
DependencyProperties are meant to point to some other property to get their value. So you can either point it to your DialPadViewModel.DialedNumber, or you can point it to some other string when the UserControl is used (either a binding or a hardcoded value like "551"), but you can't do both.
In your case, when someone binds to the DialedNumber dependency property, they are replacing the current value (the binding to DialPadViewModel.DialedNumber) with a new value.
Depending on how your code looks and what you want to do, there are a few ways around it.
First, you could insist that people who want to use your control also use your ViewModel, and don't make DialedNumber a public dependency property.
So instead of being allowed to create a custom class with a property of SomeOtherDialedNumber and binding
<DialPad DialedNumber="{Binding SomeOtherDialedNumber}">
they are forced to use the DialPadViewModel in their code anytime they want to use the DialPad control. For this to work, you would need to remove the this.DataContext = new DialPadViewModel in your code-behind the UserControl since the user will be providing the DialPadViewModel to your UserControl, and you can use an implicit DataTemplate to tell WPF to always draw DialPadViewModel with your DialPad UserControl.
<DataTemplate DataType="{x:Type DialPadViewModel}">
<local:DialPad />
</DataTemplate>
The other alternative I can think of is to synchronize your DependencyProperty with your ViewModel property with some PropertyChange notifications.
You would need to update DialPadViewModel.DialedNumber anytime the DialedNumber dependency property changes (You may need to use DependencyPropertyDescriptor.AddValueChanged for property change notification), and you would also have to write something to update the source of the DialedNumber dependency property anytime DialPadViewModel.DialedNumber changes.
Personally, if my UserControl has a ViewModel then I use the first option. If not, I get rid of the ViewModel entirely and build the logic for my UserControl in the code-behind, without a ViewModel.
The reason for this is that WPF works with two layers: a UI layer and a data layer. The DataContext is the data layer, and a ViewModel is typically part of the data layer. By setting the data layer (DataContext) explicitly in the UserControl's constructor, you are combining your data layer with your UI layer, which goes against one of the biggest reasons for using MVVM: separation of concerns. A UserControl should really just be a pretty shell only, and you should be able to place it on top of any data layer you want.
If you place your DialPad in your View, you can create a DialPadViewModel-Property (public+global) in your ViewViewModel:
public DialPadViewModel DialPadViewModel = new DialPadViewModel();
Now set the DataContext-Binding of your View to the ViewViewModel and bind the DialPads DataContext also to it, like
<local:DialPad DataContext="{Binding}"/>
Now you can bind to the properties in your DialPadViewModel:
<TextBox Text="{Binding Path=DialPadViewModel.DialedNumber}"/>
Thats how you can Access your DialPadViewModel from your View and your DialPad.
EDIT:
Now try changing your DialedNumber Property in your DialPad.xaml.cs like this:
public string DialedNumber
{
get
{
return DialPadViewModel.DialedNumber;
}
set
{
DialPadViewModel.DialedNumber = value;
}
}
EDIT 2: I found the Problem:
In your DialPad.xaml all your Commands were bound to the DialPadViewModel from the resources, while the TextBloc was bound to the DialPads DataContext, which is another instance of the DialPadViewModel.
So everytime you hit a DialPad-Button you changed the value of the DialedNumber from the resources' DPVM-instance not the DialedNumber from the DataContext's DPVM-instance.
It sounds like you can add a TextBox to your view and bind it's Text property to your view-model's DialedNumber property.
<TextBox Text="{Binding Path=DialedNumber}"></TextBox>
Your view-model property can look something like this:
private string _dialedNumber;
[DefaultValue("551")]
public string DialedNumber
{
get { return _dialedNumber; }
set
{
if (value == _dialedNumber)
return;
_dialedNumber= value;
_yourModel.DialedNumber= _dialedNumber;
this.RaisePropertyChanged("DialedNumber");
}
}
Let me know if I misunderstood your question.
Background:
I have WPF application with a main window containing a user control. I want to pass a value from the main window to the user control.
In the Main window's constructor I have code:
public MainWindow()
{
InitializeComponent();
_vm = new MainWindowViewModel();
this.DataContext = _vm;
ucControl = new UserControl1("NameSet");
}
(ucControl is my user control)
User control has two constructors:
public UserControl1()
{
InitializeComponent();
this.ID = ID.GetNewID;
}
public UserControl1(string name)
{
InitializeComponent();
_vm = new UCViewModel(name);
this.DataContext = _vm;
this.ID = ID.GetNewID;
}
The problem is: although the second constructor (with parameter) is called, it is not loaded in the main window. I checked the ID (this.ID) in the user control's loaded event and I see the ID set in the default constructor and its DataContext is null. Because of this reason, I do not get the "name" string in my user control.
Any help please? Since I am using MVVM pattern I do not want to expose properties in user control (view) to be set from main window just for this.
You are instantiating the UserControl1 object twice:
Once within the XAML. The <uc:UserControl1> element instantiates a UserControl1 object, using the default constructor, and assigns it to the member ucControl.
You instantiate it again within the constructor of the MainWindow object
If you put a break point in the constructor of UserControl, you'll notice it is called twice. I assume WPF instantiate and initialize the XAML's UserControl (#1 from above) after you assign the dynamic UserControl (#2 from above), and this is why you see the former in the logical tree of MainWindow.
You should have only one instance. If you want to parameterized a user control, the canonical paradigm is what you mention that you don't want to do (why??). If you had such a property, you could set it in the XAML: <uc:UserControl1 x:Name="..." YourProperty="NameSet>
exposing such a property is a single line in the UserControl:
public YourProperty { get; set; }
If you insist of not having this line, you should do the following:
Remove the XAML's user control.
In main window, subscribe to the Loaded event
In the handler of the Loaded event, instantiate a new UserControl1 - with whatever constructor parameter that you want.
Manually add it to the Children array of the parent Grid element
Clearly this isn't my recommendation. In addition to the complexity, with the former method you'll also work very well with the Visual Studio designer.
I am learning Silverlight. In the process, I'm trying to build a custom user control. My ultimate goal is to be able to write the following statement in XAML:
<my:CustomControl>
<my:CustomControl.MainControl>
<Canvas><TextBlock Text="Hello!" /></Canvas>
</my:CustomControl.MainContent>
</my:CustomControl>
The content of the control will be wrapped in a custom border. Once again, this is just a learning exercise. To append my border, I have create the following UserControl:
<UserControl x:Class="CustomControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
<Grid x:Name="LayoutRoot">
<Border>
<!-- CustomControl Content -->
</Border>
</Grid>
</UserControl>
The code-behind for this file looks like the following:
public partial class CustomControl : UserControl
{
public UIElement MainContent
{
get { return (UIElement)GetValue(MainContentProperty); }
set { SetValue(MainContentProperty, value); }
}
public static readonly DependencyProperty MainContentProperty =
DependencyProperty.Register("MainContent", typeof(UIElement), typeof(CustomControl),
new PropertyMetadata(null));
public CustomControl()
{
InitializeComponent();
}
}
The thing I am having a problem with is getting the MainContent to appear in my CustomControl. I am confident that I am setting it properly, but I'm not sure how to display it. I really want it to be a DependencyProperty as well so I can take advantage of data binding and animations.
How do I get the MainContent to appear in the CustomControl? Thank you
First you need to wait until the rest of the control has been parsed so you need to hook the loaded event:-
public CustomControl()
{
InitializeComponent();
Loaded += new RoutedEventHandler(CustomControl_Loaded);
}
Then in the loaded event assign your MainControl property to the Child property of the border. To do that its best if you give your Border an x:Name which for now I'll simple call "border".
void CustomControl_Loaded(object sender, RoutedEventArgs e)
{
border.Child = MainControl;
}
That'll get you going. Of course you may need to deal with the MainControl property being changed dynamically so you need add a bool isLoaded field in your control and set that in the loaded event. When true your MainControl setter should assign the incoming value to the border.Child.
Things can start to get more complicated and in fact I don't recommend this approach to creating a custom control. For a better approach see Creating a New Control by Creating a ControlTemplate