How do you handle multiple windows with different threads?

Question

In an empty WPF Project first I've deleted everything except AssemblyInfo.cs, then added App.cs, where I start the app, and four other windows. RootWindow acts as the positioner/resizer of three other windows, TopWindow, MiddleWindow and BottomWindow. These three windows are identical, all they have is a constructor with these:

class MiddleWindow : Window  
{  
    public MiddleWindow() {  
        Title = "Middle";  
        ResizeMode = ResizeMode.NoResize;  
        AllowsTransparency = true;  
        WindowStyle = WindowStyle.None;  
        ShowInTaskbar = false;  
    }  
}  

in the RootWindow, I've three Rectangle, topRect, midRect, botRect, and in its ArrangeOverride I set the Top, Left, Width and Height of those three Windows based on the position of those Rectangles. Here's what happens when I run the app:

initially there's always a gap between the MiddleWindow and BottomWindow, As I resize the RootWindow those windows resizes sometimes with the gap and other times without gap. If I maximize and restore the RootWindow, those three also resize accordingly and when I move the RootWindow, they also move along. The problem arises when I click on any of those three windows or minimize the RootWindow. If I click on any of those three, all three starts blinking and all of them disappears eventually!

In the OnDeactivated of the RootWindow, I hide those so they become invisible and in the OnActivated of the RootWindow, I set Visibility of those to Visible BUT only the last one appears when I restore the RootWindow by clicking on taskbar!

Here's the RootWindow.cs:

class RootWindow : Window  
{  
    double radius = 5;  
    Border windowBorder, titleBar;  
    Grid contentGrid, titlebarIconGrid, rectGrid;  
    ActionButton close, minimize, maxRestore;  
    Rectangle topRect, midRect, botRect;  
    public TopWindow TopWin { get; set; }  
    public MiddleWindow MidWin { get; set; }  
    public BottomWindow BotWin { get; set; }  
    public RootWindow() {  
        Height = 800;  
        Width = 1200;  
        WindowStartupLocation = WindowStartupLocation.CenterScreen;  
        WindowStyle = WindowStyle.None;  
        AllowsTransparency = true;  
        WindowChrome.SetWindowChrome(this, new WindowChrome() {  
            ResizeBorderThickness = new Thickness(0, 0, 5, 5),  
            CaptionHeight = 0  
        });  
        addTitleIcons();  
        titleBar = new Border() {  
            CornerRadius = new CornerRadius(radius, radius, 0, 0),  
            Background = Brushes.LightGray,  
            Height = 32,  
            Effect = new DropShadowEffect() { BlurRadius = 5, Opacity = 0.5, Direction = -90 },  
            Child = titlebarIconGrid  
        };  
        initRects();  
        Grid.SetRow(rectGrid, 1);  
        contentGrid = new Grid() {  
            RowDefinitions = {  
                new RowDefinition() { Height = GridLength.Auto },  
                new RowDefinition()  
            },  
            Children = { titleBar, rectGrid }  
        };  
        windowBorder = new Border() {  
            Background = Brushes.Transparent,  
            CornerRadius = new CornerRadius(radius),  
            BorderThickness = new Thickness(1),  
            BorderBrush = Brushes.LightBlue,  
            Child = contentGrid  
        };  
        AddVisualChild(windowBorder);  
        titleBar.MouseLeftButtonDown += handleResize;  
        titleBar.MouseMove += move;  
        Loaded += onLoaded;  
    }  
    void onLoaded(object sender, RoutedEventArgs e) {  
        TopWin.Dispatcher.Invoke(TopWin.Show);  
        MidWin.Dispatcher.Invoke(MidWin.Show);  
        BotWin.Dispatcher.Invoke(BotWin.Show);  
    }  
    void initRects() {  
        topRect = new Rectangle()/* { IsHitTestVisible = false }*/;  
        midRect = new Rectangle()/* { IsHitTestVisible = false }*/;  
        botRect = new Rectangle()/* { IsHitTestVisible = false }*/;  
        Grid.SetRow(midRect, 1);  
        Grid.SetRow(botRect, 2);  
        rectGrid = new Grid() {  
            IsHitTestVisible = false,  
            RowDefinitions = {  
                new RowDefinition(),  
                new RowDefinition(),  
                new RowDefinition()  
            },  
            Children = { topRect, midRect, botRect }  
        };  
    }  
    void move(object sender, MouseEventArgs e) {  
        if (e.LeftButton == MouseButtonState.Pressed) {  
            DragMove();  
            InvalidateArrange();  
        }  
    }  
    void handleResize(object sender, MouseButtonEventArgs e) {  
        if (e.ClickCount == 2) resize();  
    }  
    void addTitleIcons() {  
        close = new ActionButton() {  
            Width = 24,  
            Height = 24,  
            ToolTip = "Close",  
            Margin = new Thickness(0, 0, 5, 0),  
            Icon = Icons.CloseCircle,  
            Command = Application.Current.Shutdown  
        };  
        maxRestore = new ActionButton() {  
            Width = 18,  
            Height = 18,  
            ToolTip = "Maximize",  
            Margin = new Thickness(0, 0, 5, 0),  
            Icon = Icons.Maximize,  
            Command = resize  
        };  
        minimize = new ActionButton() {  
            Width = 18,  
            Height = 18,  
            ToolTip = "Minimize",  
            Margin = new Thickness(0, 0, 5, 0),  
            Icon = Icons.Minimize,  
            Command = () => WindowState = WindowState.Minimized  
        };  
        Grid.SetColumn(close, 3);  
        Grid.SetColumn(maxRestore, 2);  
        Grid.SetColumn(minimize, 1);  
        titlebarIconGrid = new Grid() {  
            ColumnDefinitions = {  
                new ColumnDefinition(),  
                new ColumnDefinition(){ Width = GridLength.Auto },  
                new ColumnDefinition(){ Width = GridLength.Auto },  
                new ColumnDefinition(){ Width = GridLength.Auto }  
            },  
            Children = { close, maxRestore, minimize }  
        };  
    }  
    void resize() {  
        if (WindowState == WindowState.Maximized) {  
            ResizeMode = ResizeMode.CanResizeWithGrip;  
            WindowState = WindowState.Normal;  
            maxRestore.Icon = Icons.Maximize;  
            maxRestore.ToolTip = "Maximize";  
        }  
        else {  
            ResizeMode = ResizeMode.NoResize;  
            WindowState = WindowState.Maximized;  
            maxRestore.Icon = Icons.Restore;  
            maxRestore.ToolTip = "Restore";  
        }  
    }  

    protected override Size ArrangeOverride(Size arrangeBounds) {  
        windowBorder.Width = arrangeBounds.Width;  
        windowBorder.Height = arrangeBounds.Height;  
        windowBorder.Measure(arrangeBounds);  
        windowBorder.Arrange(new Rect(windowBorder.DesiredSize));  
        double width = topRect.ActualWidth;  
        double height = topRect.ActualHeight;  
        var topPoint = topRect.PointToScreen(new Point(0, 0));  
        var midPoint = midRect.PointToScreen(new Point(0, 0));  
        var botPoint = botRect.PointToScreen(new Point(0, 0));  
        TopWin.Dispatcher.Invoke(() => {  
            TopWin.Left = topPoint.X;  
            TopWin.Top = topPoint.Y;  
            TopWin.Width = width;  
            TopWin.Height = height;  
        });  
        MidWin.Dispatcher.Invoke(() => {  
            MidWin.Left = midPoint.X;  
            MidWin.Top = midPoint.Y;  
            MidWin.Width = width;  
            MidWin.Height = height;  
        });  
        BotWin.Dispatcher.Invoke(() => {  
            BotWin.Left = botPoint.X;  
            BotWin.Top = botPoint.Y;  
            BotWin.Width = width;  
            BotWin.Height = height;  
        });  
        return windowBorder.DesiredSize;  
    }  
    protected override void OnActivated(EventArgs e) {  
        TopWin.Dispatcher.Invoke(() => TopWin.Visibility = Visibility.Visible);  
        MidWin.Dispatcher.Invoke(() => MidWin.Visibility = Visibility.Visible);  
        BotWin.Dispatcher.Invoke(() => BotWin.Visibility = Visibility.Visible);  
        //TopWin.Dispatcher.Invoke(TopWin.Show);  
        //MidWin.Dispatcher.Invoke(MidWin.Show);  
        //BotWin.Dispatcher.Invoke(BotWin.Show);  
        //TopWin.Dispatcher.Invoke(() => TopWin.WindowState = WindowState.Normal);  
        //MidWin.Dispatcher.Invoke(() => MidWin.WindowState = WindowState.Normal);  
        //BotWin.Dispatcher.Invoke(() => BotWin.WindowState = WindowState.Normal);  
        //InvalidateArrange();  
        InvalidateVisual();  
    }  
    protected override void OnDeactivated(EventArgs e) {  
        TopWin.Dispatcher.Invoke(() => TopWin.Visibility = Visibility.Collapsed);  
        MidWin.Dispatcher.Invoke(() => MidWin.Visibility = Visibility.Collapsed);  
        BotWin.Dispatcher.Invoke(() => BotWin.Visibility = Visibility.Collapsed);  
        //TopWin.Dispatcher.Invoke(TopWin.Hide);  
        //MidWin.Dispatcher.Invoke(MidWin.Hide);  
        //BotWin.Dispatcher.Invoke(BotWin.Hide);  
        //TopWin.Dispatcher.Invoke(() => TopWin.WindowState = WindowState.Minimized);  
        //MidWin.Dispatcher.Invoke(() => MidWin.WindowState = WindowState.Minimized);  
        //BotWin.Dispatcher.Invoke(() => BotWin.WindowState = WindowState.Minimized);  
    }  
    protected override void OnClosed(EventArgs e) {  
        TopWin.Dispatcher.Invoke(TopWin.Close);  
        MidWin.Dispatcher.Invoke(MidWin.Close);  
        BotWin.Dispatcher.Invoke(BotWin.Close);  
        TopWin.Dispatcher.InvokeShutdown();  
        MidWin.Dispatcher.InvokeShutdown();  
        BotWin.Dispatcher.InvokeShutdown();  
        App.Current.Shutdown();  
    }  
    protected override Visual GetVisualChild(int index) => windowBorder;  
    protected override int VisualChildrenCount => 1;  
    protected override void OnClosing(CancelEventArgs e) {  
        titleBar.MouseLeftButtonDown -= handleResize;  
        titleBar.MouseMove -= move;  
    }  
}  

and here's the App.cs:

class App : Application  
{  
    [STAThread]  
    static void Main() => new App().Run();  
    protected override void OnStartup(StartupEventArgs e) {  
        TopWindow top = null;  
        MiddleWindow mid = null;  
        BottomWindow bot = null;  
        var thread1 = new Thread(() => {   
            top = new TopWindow() {  
                Content = new TextBlock() {   
                    Text = "Top Window",  
                    HorizontalAlignment = HorizontalAlignment.Center,  
                    VerticalAlignment = VerticalAlignment.Center,  
                    FontSize = 36  
                }   
            };   
            Dispatcher.Run();   
        });  
        var thread2 = new Thread(() => {   
            mid = new MiddleWindow() {  
                Content = new TextBlock() {  
                    Text = "Middle Window",  
                    HorizontalAlignment = HorizontalAlignment.Center,  
                    VerticalAlignment = VerticalAlignment.Center,  
                    FontSize = 36  
                }  
            };   
            Dispatcher.Run();   
        });  
        var thread3 = new Thread(() => {   
            bot = new BottomWindow() {  
                Content = new TextBlock() {  
                    Text = "Bottom Window",  
                    HorizontalAlignment = HorizontalAlignment.Center,  
                    VerticalAlignment = VerticalAlignment.Center,  
                    FontSize = 36  
                }  
            };   
            Dispatcher.Run();   
        });  
        thread1.SetApartmentState(ApartmentState.STA);  
        thread2.SetApartmentState(ApartmentState.STA);  
        thread3.SetApartmentState(ApartmentState.STA);  
        thread1.Start();  
        thread2.Start();  
        thread3.Start();  
        App.Current.MainWindow = new RootWindow() {  
            TopWin = top,  
            MidWin = mid,  
            BotWin = bot  
        };  
        App.Current.ShutdownMode = ShutdownMode.OnExplicitShutdown;  
        App.Current.MainWindow.Show();  
    }  
}  

You could download the project from GitHub. How do you solve these issues?

Answer 1

No clue, from where did I get the idea of using ArrangeOcerride in subclass of Window. It works fine without that:

Still couldn't figure out why is there the gap between Middle and Bottom window! Tried SnapsToDevicePixels = true in several places BUT that didn't solve the issue! Other than that, looks like everything works as expected. Updated the project in GitHub.