Binding Visibility to Text.Length

Question

I implemented a small visual indicator (just a textblock with a border), that should be hidden if there is no text to be shown at the moment. The text is bound to the Indicator property, the data context seems to be set correctly.

What I got so far is this (indicator text appears, hide/show doesn't work):

<Border>
    <Border.Style>
        <Style TargetType="{x:Type Border}">
            <Setter Property="Visibility" Value="Visible"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding Path=Indicator.Length}" Value="0">
                    <Setter Property="Visibility" Value="Hidden"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </Border.Style>
    <TextBlock Text="{Binding Indicator}" />
</Border>

My problem is that the element is not hidden if the text length is zero.

Do you spot my mistake?

Indicator is part of the corresponding viewmodel:

public string Indicator
{ get; set;}

---

UPDATE

It works if I change the property above to this:

public const string IndicatorPropertyName = "Indicator";
private string _indicator = "";
public string Indicator
{
    get
    { return _indicator;}

    set
    {
        if (_indicator == value) { return;}

        RaisePropertyChanged(IndicatorPropertyName);
    }
}

Why does it only work, if I raise PropertyChanged event?

Answer 1

I think as the name implies a Trigger only executes or checks its state when an event occurs. In case of a DataTrigger it is the PropertyChanged-Event of the interface INotifyPropertyChanged.

Wihout raising the Event the DataTrigger doesn´t know that he has to check the binding and if the value meets the trigger condition.