User control parameter in wpf mvvm

Question

I want to create a user control with a bool parameter that defines a dynamic behavior, using MVVM pattern, so I can use the user control in another view that way :

<local:MyUserControl BoolParam={Binding aBoolBinding} />

About the coding of the user control, the xaml should use the value of BoolParam to do something like this :

...
<TextBox>
    <TextBox.Style>
        <Style TargetType="TextBox">
            <Style.Trigger>
                <DataTrigger Binding="{referenceToBoolParam}" Value="False" >
                    <Setter Property="Visibility" Value="Collapsed" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>
...

Am I supposed to define a property

public bool BoolParam { get; set; }

in the code-behind of the user control, and to code the logic associated to this parameter in the ViewModel of the user control ? Actually I am a bit confused about it, what is the good practice ?

yes, if I understand well the property will be dynamically updated if I declare it as a DependencyProperty in the code behind. But I am not sure it respects the MVVM pattern if I use the getter/setter do the dynamic work for the User Control ? — Sylvain B.
@SylvainB.In MVVM the "aBoolBinding" is located in a class in the ViewModel and "BoolParam" is a DependencyProperty of MyUserControl. For this to work the DataContext of MyUserControl should be the instance of the ViewModel class that contains the "aBoolBinding" — nkoniishvt
@SylvainB. so use Attached property insead, and make your dynamic work with setterGetter of userControl in it's viewModel. — sTrenat
@sTrenat & nkoniishvt : Sorry, I realized my question was not clear. I have done edits... — Sylvain B.

mm8 mm8 · Accepted Answer · 2017-09-06T09:48:13

BoolParam should be a dependency property for you to be able to bind something to it. You define this in the code-behind of MyUserControl:

public bool BoolParam
{
    get { return (bool)this.GetValue(BoolParamProperty); }
    set { this.SetValue(BoolParamProperty, value); }
}

public static readonly DependencyProperty BoolParamProperty = DependencyProperty.Register(
  "BoolParam", typeof(bool), typeof(MyUserControl), new PropertyMetadata(false));

You could then set the DataContext of the UserControl to an instance of a view model that contains a public bool property called aBoolBinding and bind to this one as usual.

View Model:

private bool _b;
public bool aBoolBinding
{
    get
    {
        return _b;
    }

    set
    {
        _b = value;
        NotifyPropertyChanged();
    }
}

View:

<local:MyUserControl BoolParam="{Binding aBoolBinding}" />

This is how data binding works. A target property in the views is bound to a source property of a view model.

Edit:

This binds to the BoolParam property of the UserControl from a TextBox style defined in the UserControl:

<TextBox>
    <TextBox.Style>
        <Style TargetType="TextBox">
            <Style.Triggers>
                <DataTrigger Binding="{Binding BoolParam, RelativeSource={RelativeSource AncestorType=UserControl}}" Value="False" >
                    <Setter Property="Visibility" Value="Collapsed" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBox.Style>
</TextBox>

User control parameter in wpf mvvm

1 Answers