To set the focus on a UI element, one must write code in the code-behind. With the Focus behavior, this is no longer necessary. This behavior sets the focus only once on the first time the associated object is loaded.

Add the following XML namespaces:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
xmlns:catel="http://catel.codeplex.com"

Focus when the control is loaded

The easiest and default method is to set the focus when the associated control is loaded. In WPF, this is immediately when the control is focused. In Silverlight, there is a delay of 400 milliseconds by default, otherwise the focus is not set correctly.

<ListBox ItemsSource="{Binding PersonCollection}" SelectedItem="{Binding SelectedPerson}">
    <i:Interaction.Behaviors>
        <catel:Focus />
    </i:Interaction.Behaviors>
</ListBox>

In Silverlight, simply calling Focus() on the associated object is not enough. Therefore, the focus is set with a timer with a default delay of 500 milliseconds. This is customizable via the FocusDelay property

Focus when an event occurs

It is possible to set the focus when a specific event occurs. For example, when the layout root gets a MouseEnter event, the focus must be set on a specific control. This can be done via the following code:

<ListBox ItemsSource="{Binding PersonCollection}" SelectedItem="{Binding SelectedPerson}">
    <i:Interaction.Behaviors>
        <catel:Focus FocusMoment="Event" Source="{Binding ElementName=layoutRoot}" EventName="MouseEnter" />
    </i:Interaction.Behaviors>
</ListBox>

If you are using Silverlight, and you don't want the default delay of 500 milliseconds, make sure to explicitly set it to 0

Focus when a property changes

It is possible to set the focus when a specific property changes. For example, when a value is set, the focus must move on to a new control. This can be done via the following code:

<ListBox ItemsSource="{Binding PersonCollection}" SelectedItem="{Binding SelectedPerson}">
    <i:Interaction.Behaviors>
        <catel:Focus FocusMoment="PropertyChanged" Source="{Binding }" PropertyName="MyProperty" />
    </i:Interaction.Behaviors>
</ListBox>

If you are using Silverlight, and you don't want the default delay of 500 milliseconds, make sure to explicitly set it to 0