CODEWITHSUNDEEP
wpfunit-testingdata-binding
Snowbear
Snowbear

Reputation: 17274

Force binding in WPF

I'm writing tests which will check correctness of Binding elements specified in XAML. They work so far, the only issue is that I do not know how to correctly force databinding to happen. Surprisingly it is not enough to simply set something in DataContext, binding won't happen until you show your control/window. Please not that I'm writing 'unit'-tests and I'd like to avoid showing any windows.

Take a look at following code: 

// This is main class in console application where I have all WPF references added
public class Program
{
    [STAThread]
    public static void Main()
    {
        var view = new Window();
        BindingOperations.SetBinding(view, Window.TitleProperty, new Binding("Length"));
        view.DataContext = new int[5];
        //view.Show(); view.Close(); // <-- this is the code I'm trying not to write
        Console.WriteLine(view.Title);
    }
}

Here I'm creating a Window and putting an array as DataContext to that window. I'm binding Window.Title to Array.Length so I expect to see number 5 printed in console. But until I Show window (commented line) I will get empty string. If I uncomment that line then I will receive desired 5 in console output.

Is there any way I can make binding happen without showing a window? It is pretty annoying to look at ~20 windows while launching tests.

P.S.: I know I can make windows more transparent and etc, but I'm looking for more elegant solution.

UPDATE Code above is simplified version of what I really have. In real code I receive a View (some UIElement with bindings) and object ViewModel. I do not know which exactly binding there were set on View, but I still want all of them to be initialized.

UPDATE 2: Answering to the questions regarding what I test and I why. I do not intend to test that classes like Binding, BindingBase, etc are working as expected, I assume they are working. I'm trying to test that in all my XAML files I have written bindings correctly. Because bindings are stringly typed things, they are not verified during compilation and by default they cause only errors in output window, which I'm missing occasionally. So if we take my example from above and if we will made a typo there in binding: {Binding Lengthhh} then my tests will notify you that there is no property with name Lengthhh available for binding. So I have around 100 XAML files and for each XAML I have a test (3-5 lines of code) and after launching my tests I know for sure that there are no binding errors in my solution.

Upvotes: 20

Views: 3975

Answers (7)

Hans Karlsen
Hans Karlsen

Reputation: 2435

The bindings are updated by the dispatcher with the DispatcherPriority.DataBind - so if you wait for a dummy task with SystemIdle priority you are sure that any pending databinding is done. 

  try
  {
    this.Dispatcher.Invoke(DispatcherPriority.SystemIdle, new Action(() => { }));
  }
  catch
  {
    // Cannot perfom this while Dispatcher in suspended mode
  }

Upvotes: 4

Danny Varod
Danny Varod

Reputation: 18118

If you are trying to test correctness of your view, I suggest you test your view :-)

Why not run the UI from a unit test and write code that checks content of UI after changing data.

VS2010 does have GUI testing, or you could take a look at the code of tools such as Snoop.

Edit following comment:

If ALL you want to do is test a few simple bindings, try writing a static code test that runs as a post build event using reflection on view models and regular expressions on XAMLs. Add attributes on VM or use a config file so your test will know which view receives which View Model as DataContext. Compare property names and types in View Models with binding strings in View (automatically search XAML for these) and throw exception (thus failing build) if strings do not match.

If your bindings are more complex (converters, multibindings, ...) this may be a bit more complicated to implement.

Upvotes: 1

Oakcool
Oakcool

Reputation: 1496

Have you tryed to use the IsDataBound

http://msdn.microsoft.com/en-us/library/system.windows.data.bindingoperations.isdatabound.aspx

Also check this out:

System.Windows.Interop.WindowInteropHelper helper = new System.Windows.Interop.WindowInteropHelper(view).EnsureHandle();

http://msdn.microsoft.com/en-us/library/system.windows.interop.windowinterophelper.ensurehandle.aspx

My other question is why you trying to do a UNIT test on something that has been technically tested already? By the way I am not critising, just want to understand a little better.

Upvotes: 0

Vladimir Perevalov
Vladimir Perevalov

Reputation: 4157

I had the same problem, and from sixlettervariables gave me an idea. It's very simple. I am using WPF in WinForms application, so I use ElementHost control to host Wpf controls on WinForms control. To enforce WinForms control initialization you can just read value of Handle (which is actually Windows HWND) and this will force control to fully initialize itself including child ElementHost and all Wpf binding work. I didn`t try to perform the same thing for pure Wpf control. But you can easily use ElementHost to initialize your Wpf controls like this:

var el = new ElementHost();
var p = new TextBlock();
p.DataContext = new { Data = "1234" };
p.SetBinding(TextBlock.TextProperty, "Data");
el.Child = p;
var t = el.Handle;
Debug.Assert(p.Text == "1234");

PS: Found, that everything work better, if you first set DataContext and only then force a Handle to be created (just like my example). But, I think, this is already the case for you, so should not be a problem.

Upvotes: 0

user7116
user7116

Reputation: 64138

I don't believe the Window's bindings will run without calling Show or ShowDialog, because that is the only way it gets associated with the UI message loop/dispatcher.

Your best bet would be to set it to be as least visible as possible, potentially using an extension method to clean things up:

public static void PokeWindowDispatcher(this Window window)
{
    window.WindowState = WindowState.Minimized;
    window.ShowInTaskbar = false;
    window.Visibility = Visibility.None;

    using (var wait = new ManualResetEvent())
    {
        Action<object, RoutedEventArgs> loaded = (sender, e) => wait.Set();
        window.Loaded += loaded;
        try
        {
            window.Show();
            wait.WaitOne();
        }
        finally
        {
            window.Loaded -= loaded;
            window.Close();
        }
    }
}

Upvotes: 0

Gimno
Gimno

Reputation: 6606

I think you should first set the DataContext and then do the Binding, e.g.:

view.DataContext = new int[5];
BindingOperations.SetBinding(view, Window.TitleProperty, new Binding("Length"));

I'm not sure if this is real solution for your general problem, but it works in this case.

Upvotes: 0

mdm20
mdm20

Reputation: 4563

Not sure, but maybe something like this will work? 

view.GetBindingExpression(Window.TitleProperty).UpdateTarget();

Upvotes: -1

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