How to center center a Window automatically?

[SuperJMN]

I'd like to center a Window automatically. My Window has SizeToContent, so it can grow. This breaks UX, because initially it in the center of the screen. I would like it to automatically center itself again.

I have made it work partially intercepting Window.LayoutUpdated, but it tries to center the Window even when the user resizes the window manually, that produces a very glitchy behavior.

How can I avoid re-centering when the user is resizing it manually? I haven't found any way to distinguish between manual and "programmatic" layout updates.

[LaurentInSeattle]

This boils down to resizing and moving a Window.
These are Desktop specific functions.
Your app will receive notifications for resizing but not for moving.

For these API:
Windows => MoveWindow (...)
Mac => setFrameOrigin(...)
Linux => ???
your app will not receive events.

[tobyfirth]

Are you saying that you only want to recentre the window if the content of the window causes the size to change and not change the position of the window if the user manually resizes the window? Or do you just want to give the user some time to change the size of the window before you recentre the window for them?

If you want the latter then maybe using something like the Throttle extension from System.Reactive.Linq so that the code to recentre only runs after the window size has not changed for a period of time.

[wieslawsoltes]

Try this:

using Avalonia;
using Avalonia.Controls;

namespace AvaloniaResizingDemo;

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        // initial centre
        WindowStartupLocation = WindowStartupLocation.CenterScreen;
        SizeToContent        = SizeToContent.WidthAndHeight;

        // keep it centred when *we* (or layout) resize it
        Resized += OnWindowResized;
    }

    private void OnWindowResized(object? sender, WindowResizedEventArgs e)
    {
        // Ignore live user-drags – they should keep control.
        if (e.Reason == WindowResizeReason.User)
            return;

        CenterInWorkingArea();
    }

    private void CenterInWorkingArea()
    {
        // Which screen is the window on?
        var screen  = Screens.ScreenFromWindow(this) ?? Screens.Primary;
        if (screen is null) return;

        var wa      = screen.WorkingArea;                    // PixelRect
        var newLeft = wa.X + (wa.Width  - (int)ClientSize.Width)  / 2;
        var newTop  = wa.Y + (wa.Height - (int)ClientSize.Height) / 2;

        Position = new PixelPoint(newLeft, newTop);
    }
}

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaResizingDemo.MainWindow"
        Title="AvaloniaResizingDemo">
        <StackPanel>
                <TextBlock Text="{Binding #TextBox.Text}" />
                <TextBox x:Name="TextBox" Text="Welcome to Avalonia!"
                         AcceptsReturn="True" AcceptsTab="True"/>
        </StackPanel>
</Window>

[SuperJMN]

Hi! Thanks for the response. The code looks fine. Unfortunately, it doesn't behave as expected, at least in Linux. I'm using Pop!_OS (Ubuntu/Debian) with Wayland, and I'm getting e.Reason = Unspecified when manually resizing the window.

[wieslawsoltes]

I only tested on macOS and got User reason when manually resizing, might be Linux specific issue.

[SuperJMN]

I only tested on macOS and got User reason when manually resizing, might be Linux specific issue.

Damn... I hate when this happens 😅