Николай Мохов

<Window x:Class="Wpf_Task.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow"
        Width="525"
        Height="350"
        WindowStartupLocation="CenterScreen">
   <Window.Resources>
      <Storyboard x:Key="OnLoaded1"
                  RepeatBehavior="Forever">
         <DoubleAnimationUsingKeyFrames Storyboard.TargetName="border"
                                        Storyboard.TargetProperty="(UIElement.RenderTransform).(TransformGroup.Children)[2].(RotateTransform.Angle)">
            <EasingDoubleKeyFrame KeyTime="0"
                                  Value="0" />
            <EasingDoubleKeyFrame KeyTime="0:0:1"
                                  Value="360" />
         </DoubleAnimationUsingKeyFrames>
      </Storyboard>
   </Window.Resources>

   <Window.Triggers>
      <EventTrigger RoutedEvent="FrameworkElement.Loaded">
         <BeginStoryboard Storyboard="{StaticResource OnLoaded1}" />
      </EventTrigger>
   </Window.Triggers>

   <Grid>

      <Button x:Name="executeButton"
              Width="75"
              Margin="0,216,0,0"
              HorizontalAlignment="Center"
              VerticalAlignment="Top"
              Click="Button_Click"
              Content="Выполнить"
              TextOptions.TextFormattingMode="Display" />

      <Rectangle x:Name="border"
                 Width="100"
                 Height="100"
                 Margin="208,57,208,161"
                 HorizontalAlignment="Center"
                 VerticalAlignment="Center"
                 RenderTransformOrigin="0.5,0.5">
         <Rectangle.Fill>
            <LinearGradientBrush StartPoint="0.5,0" EndPoint="0.5,1">
               <GradientStop Offset="0" Color="#FFD1BD69" />
               <GradientStop Offset="1" Color="#FFFFF5C9" />
            </LinearGradientBrush>
         </Rectangle.Fill>
         <Rectangle.RenderTransform>
            <TransformGroup>
               <ScaleTransform />
               <SkewTransform />
               <RotateTransform />
               <TranslateTransform />
            </TransformGroup>
         </Rectangle.RenderTransform>
      </Rectangle>

      <TextBlock x:Name="_result"
                 Margin="0,187,0,0"
                 HorizontalAlignment="Center"
                 VerticalAlignment="Top"
                 Text="..."
                 TextOptions.TextFormattingMode="Display"
                 TextWrapping="Wrap" />

   </Grid>
</Window>

using System.Threading;
using System.Threading.Tasks;
using System.Windows;

namespace Wpf_Task
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            executeButton.IsEnabled = false;
            Task<int>.Factory.StartNew(() =>
            {
               return Auth.Login();
            })
            .ContinueWith(task => //Выполнить код в основном потоке (TaskScheduler.FromCurrentSynchronizationContext())
            {
                executeButton.IsEnabled = true;
                _result.Text = task.Result.ToString();
            }, TaskScheduler.FromCurrentSynchronizationContext());
        }
    }

    public static class Auth
    {
        private static int _counter;

        public static int Login()
        {
            Thread.Sleep(1000);
            return ++_counter;
        }
    }
}