How to set IsOpen property in wpf popup window to True?

I have this wpf style:

<Style x:Key="RequiredControlTemplate">
    <Style.Setters>
        <Setter Property="Validation.ErrorTemplate">
            <Setter.Value>
                <ControlTemplate>
                    <StackPanel>
                        <Popup IsOpen="{Binding ElementName=Adorner, Path=IsMouseOver}">
                            <TextBlock Text="{Binding ElementName=Adorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}" />
                        </Popup>
                开发者_如何学运维        <Border Grid.Column="1" BorderBrush="Red" BorderThickness="1" HorizontalAlignment="Stretch">
                            <AdornedElementPlaceholder Name="Adorner"/>
                        </Border>
                    </StackPanel>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
    </Style.Setters>
</Style>

I want that if mouse is over Adorner then Popup is open, but IsOpen="{Binding ElementName=Adorner, Path=IsMouseOver} not work...

I made few changes in your style. added couple of story boards to play with pop up open... and trigger storyboard on adorner element

<Style x:Key="RequiredControlTemplate">
            <Style.Setters>
                <Setter Property="Validation.ErrorTemplate">
                    <Setter.Value>
                        <ControlTemplate>
                            <StackPanel>
                                <StackPanel.Resources>
                                    <Storyboard x:Key="OnMouseEnter1">
                                        <BooleanAimationUsingKeyFrames Storyboard.TargetProperty="(Popup.IsOpen)" Storyboard.TargetName="popup">
                                            <DiscreteBooleanKeyFrame KeyTime="0" Value="True"/>
                                        </BooleanAnimationUsingKeyFrames>
                                    </Storyboard>
                                    <Storyboard x:Key="OnMouseLeave1">
                                        <BooleanAnimationUsingKeyFrames Storyboard.TargetProperty="(Popup.IsOpen)" Storyboard.TargetName="popup">
                                            <DiscreteBooleanKeyFrame KeyTime="0" Value="True"/>
                                            <DiscreteBooleanKeyFrame KeyTime="0:0:0.1" Value="False"/>
                                        </BooleanAnimationUsingKeyFrames>
                                    </Storyboard>
                                </StackPanel.Resources>
                                <Popup  Name="popup">
                                    <TextBlock Text="{Binding ElementName=Adorner, Path=AdornedElement.(Validation.Errors)[0].ErrorContent}" />
                                </Popup>
                                <Border Grid.Column="1" BorderBrush="Red" BorderThickness="1" HorizontalAlignment="Stretch">
                                    <AdornedElementPlaceholder Name="Adorner">
                                        <AdornedElementPlaceholder.Triggers>
                                            <EventTrigger RoutedEvent="Mouse.MouseLeave" SourceName="Adorner">
                                                <BeginStoryboard x:Name="OnMouseLeave1_BeginStoryboard" Storyboard="{StaticResource OnMouseLeave1}"/>
                                            </EventTrigger>
                                            <EventTrigger RoutedEvent="Mouse.MouseEnter" SourceName="Adorner">
                                                <BeginStoryboard Storyboard="{StaticResource OnMouseEnter1}"/>
                                            </EventTrigger>
                                        </AdornedElementPlaceholder.Triggers>
                                    </AdornedElementPlaceholder>
                                </Border>
                            </StackPanel>
                        </ControlTemplate>
                    </Setter.Value>
                </Setter>
            </Style.Setters>
        </Style>

Add Mode=OneWay to binding. IsOpen is TwoWay by default and read-only properties doesn't allow TwoWay binding.

If you don't follow by MVVM pattern, you can specify x:code in your xaml, where you can handle MouseOver Event, and use EventSetter to attach it.