OneWay binding for ToggleButton's IsChecked property in WPF

Question

I have a ToggleButton with its IsChecked property bound to a property using a OneWay binding.

Answer 1

I have similar problem.

It is not "the binding seems to get lost" (unless it's earlier frameworks problems). Binding continues working and can be easily proved by changing property outside of that command (e.g. in handler of another button click event/command).

The problem is what IsChecked can be changed in two ways: 1) binding (when value of SomeProperty is changed button will be updated 2) user (if user press button he will change IsChecked, but binding is OneWay so SomeProperty will not be updated).

So you may have desync occurs when SomeProperty == false but button IsChecked == true or vice-versa.

To optimize performance binding mechanism is checking if new value is different from current. So if desync occurs and you try to update SomeProperty with the value which it already have, then nothing will happens.

Workaround is simple: update property in 2 steps

SomeProperty = !set;
SomeProperty = set;

where set is the value you need (e.g. opposite to current SomeProperty).

Answer 2

There is a simple and elegant way to solve the original poster's problem - replacing IsChecked property of the ToggleButton with an attachable property that will set IsChecked of the button in its change handler:

namespace TBFix
{
  public class TBExtender
  {
    public static readonly DependencyProperty IsCheckedProperty =
      DependencyProperty.RegisterAttached("IsChecked", 
                                          typeof(bool),
                                          typeof(TBExtender),
                                          new PropertyMetadata(OnChanged));

    public static bool GetIsChecked(DependencyObject obj)
    {
      return (bool)obj.GetValue(IsCheckedProperty);
    }
    public static void SetIsChecked(DependencyObject obj, bool value)
    {
      obj.SetValue(IsCheckedProperty, value);
    }

    private static void OnChanged(DependencyObject o,
                                  DependencyPropertyChangedEventArgs args)
    {
      ToggleButton tb = o as ToggleButton;
      if (null != tb)
        tb.IsChecked = (bool)args.NewValue;
    }
  }
}

XAML then will look like this:

<ToggleButton Command="{Binding Path=SomeCommand}"
              TBFix:TBExtender.IsChecked="{Binding Path=SomeProperty,
                                                   Mode=OneWay}" />

EDIT: The OP solution does not work, because when the button is pressed the IsChecked property is set in the code (this is the way MS implemented ToggleButton control) - setting the property removes the binding from it and so it stops working.

By using attached property we can overcome this problem because it is never assigned a value in code and so the bindings stays intact.

Answer 3

This is by design when using oneway data binding. Add the attached property PresentationTraceSources.TraceLevel=High to your binding and you will see when it gets disconnected. This link describes the problem as well (without offering any solution): Debugging Data Binding in WPF

The way I normally solve it is to use a command for user interaction and code behind to change the control appearance because of some changed properties.