CODEWITHSUNDEEP
wpfdata-bindingbinding-mode
Gishu
Gishu

Reputation: 136613

Why does data binding break in OneWay mode?

Here's a little XAML fragment. You will see

<StackPanel>
     <TextBox x:Name="txtValue">250</TextBox>
     <Slider x:Name="slide" 
             Value="{Binding ElementName=txtValue, Path=Text, 
                             Mode=OneWay, UpdateSourceTrigger=PropertyChanged}" 
            Minimum="0" Maximum="500"></Slider>
</StackPanel>
  1. when you change the textbox value, the slider updates
  2. If you change the slider value explicitly, the previous behavior breaks a.k.a. stops working.

If I remove the Mode=OneWay set directive, (defaults to two-way) everything works perfectly.

Why is this happening?

Upvotes: 10

Views: 3969

Answers (3)

IAmJersh
IAmJersh

Reputation: 799

If you create a control which inherits from Slider, then override the Value property and use DependencyObject.SetCurrentValue when changing the value rather than DependencyOpbject.SetValue, then your databindings will be preserved when changing the values programmatically.

Sorry this isn't particularly exhaustive, will update this answer to include a basic implementation of this at a later date.

Alternatively, a UserControl which contains both the textbox and the slider would make for a very reusable implementation, in which you could bind them both to the same custom dependencyproperty, make the binding oneway, hijack the valuechanged event of the slider, set e.Handled = true, and call the SetCurrentValue function from that.

Upvotes: 0

h4444
h4444

Reputation: 131

Use mode=TwoWay and set the UpdateSourceTrigger=Explicit.

Upvotes: 13

Alex Janzik
Alex Janzik

Reputation: 3832

Your data binding is not broken but deactivated (http://en.wikipedia.org/wiki/Euphemism):

System.Windows.Data Warning: 75 : BindingExpression (hash=52697953): Deactivate
System.Windows.Data Warning: 99 : BindingExpression (hash=52697953): Replace item at level 0 with {NullDataItem}
System.Windows.Data Warning: 59 : BindingExpression (hash=52697953): Detach

Setting the trace level to high will produce this message in the VS output window in case you move the slider:

<Slider xmlns:trace="clr-namespace:System.Diagnostics;assembly=WindowsBase"
        Value="{Binding trace:PresentationTraceSources.TraceLevel=High,
            ElementName=txtValue, Path=Text, Mode=OneWay,
            UpdateSourceTrigger=PropertyChanged}"
        Minimum="0" Maximum="500"></Slider>

Upvotes: 2

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