misc: extract VMs.

demo/Semi.Avalonia.Demo/ViewModels/AboutUsViewModel.cs

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+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+using System.Windows.Input;
+using Avalonia.Platform.Storage;
+using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
+
+namespace Semi.Avalonia.Demo.ViewModels;
+
+public partial class AboutUsViewModel : ObservableObject
+{
+    public ICommand NavigateCommand { get; set; }
+
+    internal ILauncher? Launcher { get; set; }
+
+    public AboutUsViewModel()
+    {
+        NavigateCommand = new AsyncRelayCommand<string>(OnNavigateAsync);
+    }
+
+    private static readonly IReadOnlyDictionary<string, string> _keyToUrlMapping = new Dictionary<string, string>()
+    {
+        ["semi"] = "https://github.com/irihitech/Semi.Avalonia",
+        ["ursa"] = "https://github.com/irihitech/Ursa.Avalonia",
+        ["mantra"] = "https://www.bilibili.com/video/BV15pfKYbEEQ",
+        ["huska"] = "https://www.bilibili.com/video/BV1knj1zWE4A",
+    };
+
+    private async Task OnNavigateAsync(string? arg)
+    {
+        if (Launcher is not null && arg is not null && _keyToUrlMapping.TryGetValue(arg.ToLower(), out var uri))
+        {
+            await Launcher.LaunchUriAsync(new Uri(uri));
+        }
+    }
+}

demo/Semi.Avalonia.Demo/ViewModels/AutoCompleteBoxDemoViewModel.cs

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+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Semi.Avalonia.Demo.ViewModels;
+
+public class AutoCompleteBoxDemoViewModel : ObservableObject
+{
+    public ObservableCollection<StateData> States { get; set; } =
+    [
+        new("Alabama", "AL", "Montgomery"),
+        new("Alaska", "AK", "Juneau"),
+        new("Arizona", "AZ", "Phoenix"),
+        new("Arkansas", "AR", "Little Rock"),
+        new("California", "CA", "Sacramento"),
+        new("Colorado", "CO", "Denver"),
+        new("Connecticut", "CT", "Hartford"),
+        new("Delaware", "DE", "Dover"),
+        new("Florida", "FL", "Tallahassee"),
+        new("Georgia", "GA", "Atlanta"),
+        new("Hawaii", "HI", "Honolulu"),
+        new("Idaho", "ID", "Boise"),
+        new("Illinois", "IL", "Springfield"),
+        new("Indiana", "IN", "Indianapolis"),
+        new("Iowa", "IA", "Des Moines"),
+        new("Kansas", "KS", "Topeka"),
+        new("Kentucky", "KY", "Frankfort"),
+        new("Louisiana", "LA", "Baton Rouge"),
+        new("Maine", "ME", "Augusta"),
+        new("Maryland", "MD", "Annapolis"),
+        new("Massachusetts", "MA", "Boston"),
+        new("Michigan", "MI", "Lansing"),
+        new("Minnesota", "MN", "St. Paul"),
+        new("Mississippi", "MS", "Jackson"),
+        new("Missouri", "MO", "Jefferson City"),
+        new("Montana", "MT", "Helena"),
+        new("Nebraska", "NE", "Lincoln"),
+        new("Nevada", "NV", "Carson City"),
+        new("New Hampshire", "NH", "Concord"),
+        new("New Jersey", "NJ", "Trenton"),
+        new("New Mexico", "NM", "Santa Fe"),
+        new("New York", "NY", "Albany"),
+        new("North Carolina", "NC", "Raleigh"),
+        new("North Dakota", "ND", "Bismarck"),
+        new("Ohio", "OH", "Columbus"),
+        new("Oklahoma", "OK", "Oklahoma City"),
+        new("Oregon", "OR", "Salem"),
+        new("Pennsylvania", "PA", "Harrisburg"),
+        new("Rhode Island", "RI", "Providence"),
+        new("South Carolina", "SC", "Columbia"),
+        new("South Dakota", "SD", "Pierre"),
+        new("Tennessee", "TN", "Nashville"),
+        new("Texas", "TX", "Austin"),
+        new("Utah", "UT", "Salt Lake City"),
+        new("Vermont", "VT", "Montpelier"),
+        new("Virginia", "VA", "Richmond"),
+        new("Washington", "WA", "Olympia"),
+        new("West Virginia", "WV", "Charleston"),
+        new("Wisconsin", "WI", "Madison"),
+        new("Wyoming", "WY", "Cheyenne")
+    ];
+}
+
+public class StateData(string name, string abbreviation, string capital)
+{
+    public string Name { get; private set; } = name;
+    public string Abbreviation { get; private set; } = abbreviation;
+    public string Capital { get; private set; } = capital;
+}

demo/Semi.Avalonia.Demo/ViewModels/ComboBoxDemoViewModel.cs

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+using System.Collections.ObjectModel;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Semi.Avalonia.Demo.ViewModels;
+
+public class ComboBoxDemoViewModel : ObservableObject
+{
+    public ObservableCollection<string> Items { get; set; } = ["Ding", "Otter", "Husky", "Mr.17", "Cass"];
+}

demo/Semi.Avalonia.Demo/ViewModels/RefreshContainerDemoViewModel.cs

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+using System.Collections.ObjectModel;
+using System.Linq;
+using System.Threading.Tasks;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Semi.Avalonia.Demo.ViewModels;
+
+public class RefreshContainerDemoViewModel : ObservableObject
+{
+    public ObservableCollection<string> Items { get; }
+
+    public RefreshContainerDemoViewModel()
+    {
+        Items = new ObservableCollection<string>(Enumerable.Range(1, 200).Select(i => $"Item {i}"));
+    }
+
+    public async Task AddToTop()
+    {
+        await Task.Delay(1000);
+        Items.Insert(0, $"Item {200 - Items.Count}");
+    }
+}

demo/Semi.Avalonia.Demo/ViewModels/SplitViewDemoViewModel.cs

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+using System.Collections.ObjectModel;
+using Avalonia.Controls;
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace Semi.Avalonia.Demo.ViewModels;
+
+public class SplitViewDemoViewModel : ObservableObject
+{
+    public ObservableCollection<string> Songs { get; set; } =
+    [
+        "320万年前",
+        "隐德来希",
+        "孔明",
+        "锦鲤卟噜噜",
+        "指鹿为马",
+        "热带季风Remix",
+        "加州梦境",
+        "渐近自由",
+        "世界所有的烂漫",
+    ];
+
+    public static ObservableCollection<SplitViewDisplayMode> DisplayModes { get; set; } =
+    [
+        SplitViewDisplayMode.Inline,
+        SplitViewDisplayMode.CompactInline,
+        SplitViewDisplayMode.Overlay,
+        SplitViewDisplayMode.CompactOverlay,
+    ];
+}