Question

I have a requirement to focus on a specific textbox when a new view is loaded.

The solution was to add this line of code to the OnLoaded event for the view:

Dispatcher.BeginInvoke(() => { NameTextBox.Focus(); });

So this worked for one view, but not another. I spent some time debugging the problem and realized that the new view I was working on had a BusyIndicator that takes focus away from all controls since the BusyIndicator being set to true and false was occuring after the OnLoaded event.

So the solution is to call focus to the NameTextBox after my BusyIndicator has been set to false. My idea was to create a reusable BusyIndicator control that handles this extra work. However, I am having trouble doing this in MVVM.

I started by making a simple extension of the toolkit:BusyIndicator:

public class EnhancedBusyIndicator : BusyIndicator
{
    public UserControl ControlToFocusOn { get; set; }

    private bool _remoteFocusIsEnabled = false;
    public bool RemoteFocusIsEnabled
    {
        get
        {
            return _remoteFocusIsEnabled;
        }
        set
        {
            if (value == true)
                EnableRemoteFocus();
        }
    }

    private void EnableRemoteFocus()
    {
        if (ControlToFocusOn.IsNotNull())
            Dispatcher.BeginInvoke(() => { ControlToFocusOn.Focus(); });
        else
            throw new InvalidOperationException("ControlToFocusOn has not been set.");
    }

I added the control to my XAML file with no problem:

<my:EnhancedBusyIndicator
    ControlToFocusOn="{Binding ElementName=NameTextBox}"
    RemoteFocusIsEnabled="{Binding IsRemoteFocusEnabled}"
    IsBusy="{Binding IsDetailsBusyIndicatorActive}"
...
>    
...
    <my:myTextBox (this extends TextBox)
        x:Name="NameTextBox"
    ...
    />
...
</my:EnhancedBusyIndicator>

So the idea is when IsRemoteFocusEnabled is set to true in my ViewModel (which I do after I’ve set IsBusy to false in the ViewModel), focus will be set to NameTextBox. And if it works, others could use the EnhancedBusyIndicator and just bind to a different control and enable the focus appropriately in their own ViewModels, assuming their views have an intial BusyIndicator active.

However, I get this exception when the view is loaded:

Set property ‘foo.Controls.EnhancedBusyIndicator.ControlToFocusOn’ threw an exception. [Line: 45 Position: 26]

Will this solution I am attempting work? If so, what is wrong with what I have thus far (cannot set the ControlToFocusOn property)?

---

Update 1

I installed Visual Studio 10 Tools for Silverlight 5 and got a better error message when navigating to the new view. Now I gete this error message:

“System.ArgumentException: Object of type System.Windows.Data.Binding cannot be converted to type System.Windows.Controls.UserControl”

Also, I think I need to change the DataContext for this control. In the code-behind constructor, DataContext is set to my ViewModel. I tried adding a DataContext property to the EnhancedBusyIndicator, but that did not work:

<my:EnhancedBusyIndicator
    DataContext="{Binding RelativeSource={RelativeSource Self}}"
    ControlToFocusOn="{Binding ElementName=NameTextBox}"
    RemoteFocusIsEnabled="{Binding IsRemoteFocusEnabled}"
    IsBusy="{Binding IsDetailsBusyIndicatorActive}"
...
>

---

Update 2

I need to change UserControl to Control since I will be wanting to set focus to TextBox objects (which implement Control). However, this does not solve the issue.

Answer 1

Without a BusyIndicator present in the view, the common solution to solve the focus problem is to add the code

Dispatcher.BeginInvoke(() => { ControlToFocusOn.Focus(); });

to the Loaded event of the view. This actually works even with the BusyIndicator present; however, the BusyIndicator immediately takes focus away from the rest of the Silverlight controls. The solution is to invoke the Focus() method of the control after the BusyIndicator is not busy.

I was able to solve it by making a control like this:

public class EnhancedBusyIndicator : BusyIndicator
{
    public EnhancedBusyIndicator()
    {
        Loaded += new RoutedEventHandler(EnhancedBusyIndicator_Loaded);
    }

    void EnhancedBusyIndicator_Loaded(object sender, RoutedEventArgs e)
    {
        AllowedToFocus = true;
    }

    private readonly DependencyProperty AllowedToFocusProperty = DependencyProperty.Register("AllowedToFocus", typeof(bool), typeof(EnhancedBusyIndicator), new PropertyMetadata(true));

    public bool AllowedToFocus
    {
        get { return (bool)GetValue(AllowedToFocusProperty); }
        set { SetValue(AllowedToFocusProperty, value); }
    }

    public readonly DependencyProperty ControlToFocusOnProperty = DependencyProperty.Register("ControlToFocusOn", typeof(Control), typeof(EnhancedBusyIndicator), null);

    public Control ControlToFocusOn
    {
        get { return (Control)GetValue(ControlToFocusOnProperty); }
        set { SetValue(ControlToFocusOnProperty, value); }
    }

    protected override void OnIsBusyChanged(DependencyPropertyChangedEventArgs e)
    {
        base.OnIsBusyChanged(e);
        if (AllowedToFocus && !IsBusy)
        {
            Dispatcher.BeginInvoke(() => { ControlToFocusOn.Focus(); });
            AllowedToFocus = false;
        }
    }
}

To use it, replace the BusyIndicator tags in your xaml with the new EnhancedBusyIndicator and add the appropriate namespace.

Add a new property, ControlToFocusOn inside the element, and bind it to an existing element in the view that you want focus to be on after the EnhancedBusyIndicator disappears:

<my:EnhancedBusyIndicator
    ControlToFocusOn="{Binding ElementName=NameTextBox}"
    ...
>
    ...
</my:EnhancedBusyIndicator>

In this case, I focused to a textbox called NameTextBox.

That’s it. This control will get focus every time we navigate to the page. While we are on the page, if the EnhancedBusyIndicator becomes busy and not busy agiain, focus will not go to the control; this only happens on initial load.

If you want to allow the EnhancedBusyIndicator to focus to the ControlToFocusOn another time, add another property, AllowedToFocus:

<my:EnhancedBusyIndicator
    ControlToFocusOn="{Binding ElementName=NameTextBox}"
    AllowedToFocus="{Binding IsAllowedToFocus}"
    ...
>
    ...
</my:EnhancedBusyIndicator>

When AllowedToFocus is set to true, the next time EnhancedBusyIndicator switches from busy to not busy, focus will go to ControlToFocusOn.

AllowedToFocus can also be set to false when loading the view, to prevent focus from going to a control. If you bind AllowedToFocus to a ViewModel property, you may need to change the BindingMode. By default, it is OneTime.