Question

I have a very simple UserControl that starts a progess bar animation when it is loaded:

<UserControl x:Class="WpfApplication2.UserControl1" 
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <ProgressBar Width="200"  Height="10" Maximum="{Binding Delay}" SmallChange="1">
        <ProgressBar.Triggers>
            <EventTrigger RoutedEvent="ProgressBar.Loaded">
                <BeginStoryboard>
                    <Storyboard>
                        <DoubleAnimation 
                        Storyboard.TargetProperty="Value" To="{Binding Delay}" Duration="00:00:10" />
                    </Storyboard>
                </BeginStoryboard>
            </EventTrigger>
        </ProgressBar.Triggers>
    </ProgressBar>
</UserControl>

The animation’s To value comes from the DataContext of UserControl:

class ViewModel
{
    public int Delay
    {
        get { return 10; }
    }
}

The UserControl is shown in a window, all put together like this:

var w = new Window();
var vm = new ViewModel();
var c = new UserControl1 {DataContext = vm};
w.Content = c;
w.Owner = this;
w.WindowStartupLocation = WindowStartupLocation.CenterOwner;
// to make the animation play, I have to remove the following line
w.SizeToContent = SizeToContent.WidthAndHeight;
w.ShowDialog();

The animation is not playing as long as I have the line

w.SizeToContent = SizeToContent.WidthAndHeight;

in my code.

Does anyone have an explanation for this very weird behavior and can come up with a solution?

The end goal is pretty simple. All I want is a progress bar animating for a period of time (speicified by viewmodel) right after the UserControl/Window was loaded. Preferably XAML-only.

Answer 1

Instead of

w.SizeToContent = SizeToContent.WidthAndHeight

use that code, its messy i agree but it works

   w.SourceInitialized += (s, a) =>
            {

                w.SizeToContent = SizeToContent.WidthAndHeight;

                w.UpdateLayout();
                w.Left = this.Left + (this.ActualWidth / 2.0 - w.ActualWidth / 2.0);
                w.Top = this.Top + (this.ActualHeight / 2.0 - w.ActualHeight / 2.0);
            };

i don’t have an explanation for that unfortunately. I only know that the way WPF works and the way how Windows are treated internally in windows itself, can conflict sometimes. I had the same problem with setting the left/top/Width/height of a window and then directly maximizing it. To solve it i just delayed it until the SourceInitialized event is fired. I tried the same with your code and it seems like a similar problem, because this did the trick.