Question

Force update of WPF data binding

Asked by: medi911

Is there an easy way to force an update of a data bound dependency property from the source? This problem seams to occur only if the context is set on a parent object. So far I found two possible solutions: Assign the source property of the bound control to the data object - or - use the BackgroundWorker class to force a thread switch. The first solution is very cumbersome if there are a lot of controls on the page and the second is - well - kinda overkill.
Please see the attached snippet as a case study.

// create text box with data binding
TextBox textBox = new TextBox();
Binding binding = new Binding();
binding.Path = new PropertyPath("Value");
binding.Mode = BindingMode.OneWay;
textBox.SetBinding(TextBox.TextProperty, binding);
 
// create panel and add text box
StackPanel panel = new StackPanel();
panel.DataContext = data;
panel.Children.Add(textBox);
 
// update binding
BindingExpression bindingExpression = textBox.GetBindingExpression(TextBox.TextProperty);
bindingExpression.UpdateTarget();
Console.WriteLine("1 > " + textBox.Text);   // --> NOT UPDATED !!!

                                  
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Asked On
2009-08-18 at 06:48:40ID24661355
Tags

WPF binding update

Topics

Visual Studio

,

WPF and Silverlight

,

.NET Framework 3.x versions

Participating Experts
1
Points
500
Comments
6

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Answers

 

by: ToddBeaulieuPosted on 2009-08-18 at 07:06:07ID: 25123312

Does it work if you set the datacontext of the parent after the control has been added to the panel?

Seems like a lot of work going on here to set up bindings. You can't use XAML? I have very few instances where I needed to establish bindings via code. I just looked at some of my code that does almost the same thing. I don't know if it makes a difference, but I'm setting the Source to the DC explicitly.
 

b = new Binding();
b.Source = this.DataContext;
b.Mode = BindingMode.OneWay;
b.Path = new PropertyPath(titleProperty);
b.Converter = new Converters.ViewRecordCountConverter();
b.ConverterParameter = status;
this.Title.SetBinding(Label.ContentProperty, b);
                                              
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by: medi911Posted on 2009-08-18 at 07:21:31ID: 25123517

It does not matter when I set the DataContext of the parent object.
The snippet is just an example to isolate the problem. My real code is In XAML and has 60+ data bound controls. That is why it is not practically to set the Source of each bound control.

 

by: ToddBeaulieuPosted on 2009-08-18 at 07:37:00ID: 25123685

I guess I don't see the issue. I've got tons of controls bound to properties that work just fine when I set the DC of *a* container to the DomainObject or ViewModel. That might be the Page, a Grid or a UserControl.

The code below is from a page that has it's DataContext set to itself:

DataContext="{Binding RelativeSource={RelativeSource Self}}"

And it registers the DP like this:

TitleProperty = DependencyProperty.Register("Title", typeof(string), typeof(ContentControlShim), metadata);

So ... the control simply binds to the property name and the DC handles the Source.  What happens when you set the underlying property? Does the binding THEN work? Is it just the initial value that's the problem?

<TextBlock Name="CategoryTitle" 
           FontSize="20" 
           FontWeight="Bold"
           Margin="5,0,100,0" 
           VerticalAlignment="Center"
           Text="{Binding Title}"
           />
                                              
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by: medi911Posted on 2009-08-18 at 08:05:10ID: 25123980

Here is the long story: In my case the application is for entering a contract. A lot of the input controls are optional. The controls work just fine as long as they are displayed on a page. The problem starts when I try to send the whole thing to the printer because in this case I have to remove (or collapse) sections of the contract that do not have any input (the customer is supposed to see only the options he signed up for). I have written a function to do this. The basic steps of printing are as followed:

1 - create user control (which contains all other controls)
2 - set data context of user control
3 - call function to reformat user control
4 - print

The function in step 3 does not work because the actual data binding is performed in step 4. All controls appear to be empty when the reformat function is running.

I was reading through some other posts and found out that WPF data binding does not work while the current thread is still active. I was able to make the code work by forcing a thread switch between steps 2 and 3. I just wonder if there is an easier solution to this.

 

by: ToddBeaulieuPosted on 2009-08-18 at 08:31:36ID: 25124281

Wow, that's an interesting solution you've got there!

I can see now that this isn't just a matter of binding. I wans't aware of this "restriction", and probably never would have been, had you not pointed it out.

Are you using the dispatcher, as referenced in this discussion?

http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/6fce9b7b-4a13-4c8d-8c3e-562667851baa

So how are you printing this, anyway? Just curious. Maybe an alternative approach could eliminate the need to recreate a reformatted UI?

 

by: medi911Posted on 2009-08-18 at 10:53:35ID: 25125759

Thanks for pointing out that discussion. I have seen this thread on MSDN earliear but did not dive into it since it is way over my head. However after digging through it I found that simply calling Invoke() from the Dispatcher class actually updates the bindings. I still have some research to do to make sure this is a stable solution but it looks promising.


delegate void DispatcherDelegate();
...
control.Dispatcher.Invoke(DispatcherPriority.DataBind,
                          new DispatchDelegate(delegate() { }));
                                              
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