DeclarativeProperty nullability fixes

src/AppCommon/FileTime.App.ContainerSizeScanner/SizeScanContainer.cs

@@ -88,7 +88,7 @@ public record SizeScanContainer : ISizeScanContainer
         return Task.CompletedTask;
     }
 
-    public required IDeclarativeProperty<string> Status { get; init; } = new DeclarativeProperty<string>();
+    public required IDeclarativeProperty<string> Status { get; init; } = new DeclarativeProperty<string>(string.Empty);
 
     public required SizeScanTask SizeScanTask { get; init; }
 

src/AppCommon/FileTime.App.ContainerSizeScanner/SizeScanTask.cs

@@ -17,7 +17,7 @@ public class SizeScanTask : ISizeScanTask
     private readonly ILogger<SizeScanTask> _logger;
     private Thread? _sizeScanThread;
     private static int _searchId = 1;
-    private readonly DeclarativeProperty<string> _containerStatus = new();
+    private readonly DeclarativeProperty<string> _containerStatus = new(string.Empty);
     private readonly IDeclarativeProperty<string> _containerStatusDebounced;
     public ISizeScanContainer SizeSizeScanContainer { get; private set; } = null!;
     public bool IsRunning { get; private set; }

src/AppCommon/FileTime.App.Core/ViewModels/AppStateBase.cs

@@ -13,7 +13,7 @@ namespace FileTime.App.Core.ViewModels;
 public abstract partial class AppStateBase : IAppState
 {
     private readonly BehaviorSubject<string?> _searchText = new(null);
-    private readonly DeclarativeProperty<ITabViewModel?> _selectedTab = new();
+    private readonly DeclarativeProperty<ITabViewModel?> _selectedTab = new(null);
     private readonly DeclarativeProperty<ViewMode> _viewMode = new(Models.Enums.ViewMode.Default);
     private readonly ObservableCollection<ITabViewModel> _tabs = new();
     private readonly DeclarativeProperty<string?> _rapidTravelText;

src/Core/FileTime.Core.Command/Copy/CopyCommand.cs

@@ -15,11 +15,11 @@ public class CopyCommand : CommandBase, ITransportationCommand
     private readonly CancellationTokenSource _cancellationTokenSource = new();
 
     private readonly List<OperationProgress> _operationProgresses = new();
-    private readonly DeclarativeProperty<OperationProgress?> _currentOperationProgress = new();
+    private readonly DeclarativeProperty<OperationProgress?> _currentOperationProgress = new(null);
 
     private long _recentTotalSum;
-    private readonly DeclarativeProperty<long> _recentTotalProcessed = new();
+    private readonly DeclarativeProperty<long> _recentTotalProcessed = new(0);
-    private readonly DeclarativeProperty<DateTime> _recentStartTime = new();
+    private readonly DeclarativeProperty<DateTime> _recentStartTime = new(DateTime.Now);
 
     public IReadOnlyList<FullName> Sources { get; }
 

src/Core/FileTime.Core.Services/Tab.cs

@@ -19,9 +19,9 @@ public class Tab : ITab
 
     private readonly ITimelessContentProvider _timelessContentProvider;
     private readonly ITabEvents _tabEvents;
-    private readonly DeclarativeProperty<IContainer?> _currentLocation = new();
+    private readonly DeclarativeProperty<IContainer?> _currentLocation = new(null);
-    private readonly DeclarativeProperty<IContainer?> _currentLocationForced = new();
+    private readonly DeclarativeProperty<IContainer?> _currentLocationForced = new(null);
-    private readonly DeclarativeProperty<AbsolutePath?> _currentRequestItem = new();
+    private readonly DeclarativeProperty<AbsolutePath?> _currentRequestItem = new(null);
     private readonly ObservableCollection<ItemFilter> _itemFilters = new();
     private readonly CircularBuffer<FullName> _history = new(20);
     private readonly CircularBuffer<FullName> _future = new(20);
@@ -64,28 +64,26 @@ public class Tab : ITab
             return Task.CompletedTask;
         });
 
-        CurrentItems = DeclarativePropertyHelpers.CombineLatest(
+        IDeclarativeProperty<ObservableCollection<IItem>?> asd1 = DeclarativePropertyHelpers.CombineLatest(
             CurrentLocation,
             itemFiltersProperty,
-            (container, filters) =>
+            Task<ObservableCollection<IItem>?> (container, filters) =>
             {
                 ObservableCollection<IItem>? items = null;
 
-                if (container is not null)
+                if (container is null) return Task.FromResult(items);
-                {
+
                 items = container
                     .Items
                     .Selecting<AbsolutePath, IItem>(i => MapItem(i));
 
-                    if (filters is not null)
-                    {
                 items = items.Filtering(i => filters.All(f => f.Filter(i)));
-                    }
-                }
 
-                return Task.FromResult(items);
+                return Task.FromResult(items)!;
             }
-        ).CombineLatest(
+        );
+
+        var asd2 = asd1.CombineLatest(
             Ordering.Map(ordering =>
             {
                 var (itemComparer, order) = ordering switch
@@ -103,7 +101,7 @@ public class Tab : ITab
 
                 return (itemComparer, order, ordering is not ItemOrdering.Name and not ItemOrdering.NameDesc);
             }),
-            (items, ordering) =>
+            Task<ObservableCollection<IItem>?> (items, ordering) =>
             {
                 if (items is null) return Task.FromResult<ObservableCollection<IItem>?>(null);
 
@@ -117,14 +115,16 @@ public class Tab : ITab
                     ? primaryOrderedItems.ThenOrdering(i => i.DisplayName)
                     : primaryOrderedItems;
 
-                return Task.FromResult(orderedItems);
+                return Task.FromResult<ObservableCollection<IItem>?>(orderedItems);
             }
         );
 
+        CurrentItems = asd2;
+
         CurrentSelectedItem = DeclarativePropertyHelpers.CombineLatest(
-            CurrentItems.Watch<ObservableCollection<IItem>, IItem>(),
+            CurrentItems.Watch<ObservableCollection<IItem>?, IItem>(),
             _currentRequestItem.DistinctUntilChanged(),
-            (items, selected) =>
+            Task<AbsolutePath?> (items, selected) =>
             {
                 var itemSelectingContext = new LastItemSelectingContext(CurrentLocation.Value);
                 var lastItemSelectingContext = _lastItemSelectingContext;
@@ -143,7 +143,7 @@ public class Tab : ITab
                     var candidate = _selectedItemCandidates.FirstOrDefault(c => items.Any(i => i.FullName?.Path == c.Path.Path));
                     if (candidate != null)
                     {
-                        return Task.FromResult(candidate);
+                        return Task.FromResult<AbsolutePath?>(candidate);
                     }
                 }
 
@@ -160,7 +160,7 @@ public class Tab : ITab
         DeclarativePropertyHelpers.CombineLatest(
                 CurrentItems,
                 CurrentSelectedItem,
-                (items, selected) =>
+                Task<IEnumerable<AbsolutePath>?> (items, selected) =>
                 {
                     if (items is null || selected is null) return Task.FromResult<IEnumerable<AbsolutePath>?>(null);
                     var primaryCandidates = items.SkipWhile(i => i.FullName is {Path: var p} && p != selected.Path.Path).Skip(1);
@@ -169,7 +169,7 @@ public class Tab : ITab
                         .Concat(secondaryCandidates)
                         .Select(c => new AbsolutePath(_timelessContentProvider, c));
 
-                    return Task.FromResult(candidates);
+                    return Task.FromResult<IEnumerable<AbsolutePath>?>(candidates);
                 })
             .Subscribe(candidates =>
             {

src/GuiApp/Avalonia/FileTime.GuiApp.App/ViewModels/IMainWindowViewModelBase.cs

@@ -8,6 +8,6 @@ public interface IMainWindowViewModelBase
     DeclarativeProperty<string> Title { get; }
     bool Loading { get; }
     IObservable<string?> MainFont { get; } 
-    DeclarativeProperty<string> FatalError { get; }
+    DeclarativeProperty<string?> FatalError { get; }
     IReadOnlyList<WindowTransparencyLevel> TransparencyLevelHint { get; }
 }

src/GuiApp/Avalonia/FileTime.GuiApp.App/ViewModels/MainWindowLoadingViewModel.cs

@@ -9,6 +9,6 @@ public class MainWindowLoadingViewModel : IMainWindowViewModelBase
     public bool Loading => true;
     public IObservable<string?> MainFont { get; } = new BehaviorSubject<string?>("");
     public DeclarativeProperty<string> Title { get; } = new("Loading...");
-    public DeclarativeProperty<string> FatalError { get; } = new();
+    public DeclarativeProperty<string?> FatalError { get; } = new(null);
     public IReadOnlyList<WindowTransparencyLevel> TransparencyLevelHint { get; } = new[] {WindowTransparencyLevel.AcrylicBlur};
 }

src/GuiApp/Avalonia/FileTime.GuiApp.App/ViewModels/MainWindowViewModel.cs

@@ -53,10 +53,10 @@ public partial class MainWindowViewModel : IMainWindowViewModel
     private readonly OcConsumer _rootDriveInfosConsumer = new();
     public bool Loading => false;
     public IObservable<string?> MainFont => _fontService.MainFont.Select(x => x ?? "");
-    public DeclarativeProperty<string> FatalError { get; } = new();
+    public DeclarativeProperty<string?> FatalError { get; } = new(null);
     public IReadOnlyList<WindowTransparencyLevel> TransparencyLevelHint { get; } = new[] {WindowTransparencyLevel.Blur};
     public IGuiAppState AppState => _appState;
-    public DeclarativeProperty<string> Title { get; } = new();
+    public DeclarativeProperty<string> Title { get; } = new(string.Empty);
     public Thickness IconStatusPanelMargin { get; private set; } = new(20, 10, 10, 10);
     public Action? FocusDefaultElement { get; set; }
     public Action? ShowWindow { get; set; }

src/Library/DeclarativeProperty/CollectionRepeaterProperty.cs

@@ -3,11 +3,11 @@
 namespace DeclarativeProperty;
 
 public class CollectionRepeaterProperty<TCollection, TItem> : DeclarativePropertyBase<TCollection>
-    where TCollection : IList<TItem>, INotifyCollectionChanged
+    where TCollection : IList<TItem>?, INotifyCollectionChanged?
 {
     private TCollection? _currentCollection;
 
-    public CollectionRepeaterProperty(IDeclarativeProperty<TCollection?> from) : base(from.Value)
+    public CollectionRepeaterProperty(IDeclarativeProperty<TCollection> from) : base(from.Value)
     {
         _currentCollection = from.Value;
         if (from.Value is { } value)
@@ -18,7 +18,7 @@ public class CollectionRepeaterProperty<TCollection, TItem> : DeclarativePropert
         AddDisposable(from.Subscribe(Handle));
     }
 
-    public CollectionRepeaterProperty(TCollection? collection) : base(collection)
+    public CollectionRepeaterProperty(TCollection collection) : base(collection)
     {
         ArgumentNullException.ThrowIfNull(collection);
 
@@ -32,7 +32,7 @@ public class CollectionRepeaterProperty<TCollection, TItem> : DeclarativePropert
         t.Wait();
     }
 
-    private async Task Handle(TCollection? collection, CancellationToken cancellationToken = default)
+    private async Task Handle(TCollection collection, CancellationToken cancellationToken = default)
     {
         if (_currentCollection is { } currentCollection)
         {

src/Library/DeclarativeProperty/CombineAllProperty.cs

@@ -3,18 +3,18 @@
 public class CombineAllProperty<T, TResult> : DeclarativePropertyBase<TResult>
 {
     private readonly List<IDeclarativeProperty<T>> _sources;
-    private readonly Func<IEnumerable<T>, Task<TResult?>> _combiner;
+    private readonly Func<IEnumerable<T>, Task<TResult>> _combiner;
 
     public CombineAllProperty(
         IEnumerable<IDeclarativeProperty<T>> sources,
-        Func<IEnumerable<T>, Task<TResult?>> combiner,
+        Func<IEnumerable<T>, Task<TResult>> combiner,
-        Action<TResult?>? setValueHook = null) : base(setValueHook)
+        Action<TResult>? setValueHook = null) : base(default!, setValueHook)
     {
         var sourcesList = sources.ToList();
         _sources = sourcesList;
         _combiner = combiner;
 
-        var initialValueTask = _combiner(sourcesList.Select(p => p.Value)!);
+        var initialValueTask = Task.Run(async () => await _combiner(sourcesList.Select(p => p.Value)));
         initialValueTask.Wait();
         SetNewValueSync(initialValueTask.Result);
 
@@ -24,12 +24,12 @@ public class CombineAllProperty<T, TResult> : DeclarativePropertyBase<TResult>
         }
     }
 
-    private async Task OnSourceChanged(T? arg1, CancellationToken arg2) => await Update();
+    private async Task OnSourceChanged(T arg1, CancellationToken arg2) => await Update();
 
     private async Task Update()
     {
         var values = _sources.Select(p => p.Value);
-        var result = await _combiner(values!);
+        var result = await _combiner(values);
 
         await SetNewValueAsync(result);
     }

src/Library/DeclarativeProperty/CombineLatestProperty.cs

@@ -2,23 +2,27 @@
 
 public sealed class CombineLatestProperty<T1, T2, TResult> : DeclarativePropertyBase<TResult>
 {
-    private readonly Func<T1?, T2?, Task<TResult>> _func;
+    private readonly Func<T1, T2, Task<TResult>> _func;
-    private T1? _value1;
+    private T1 _value1;
-    private T2? _value2;
+    private T2 _value2;
 
     public CombineLatestProperty(
         IDeclarativeProperty<T1> prop1,
         IDeclarativeProperty<T2> prop2,
-        Func<T1?, T2?, Task<TResult>> func,
+        Func<T1, T2, Task<TResult>> func,
-        Action<TResult?>? setValueHook = null) : base(setValueHook)
+        Action<TResult>? setValueHook = null) : base(default!, setValueHook)
     {
         ArgumentNullException.ThrowIfNull(prop1);
         ArgumentNullException.ThrowIfNull(prop2);
 
         _func = func;
 
-        _value1 = prop1.Value is null ? default : prop1.Value;
+        _value1 = prop1.Value;
-        _value2 = prop2.Value is null ? default : prop2.Value;
+        _value2 = prop2.Value;
+
+        var initialValueTask = Task.Run(async () => await _func(_value1, _value2));
+        initialValueTask.Wait();
+        SetNewValueSync(initialValueTask.Result);
 
         prop1.Subscribe(async (value1, token) =>
         {
@@ -51,7 +55,7 @@ public sealed class CombineLatestProperty<T1, T2, T3, TResult> : DeclarativeProp
         IDeclarativeProperty<T2> prop2,
         IDeclarativeProperty<T3> prop3,
         Func<T1?, T2?, T3?,Task<TResult>> func,
-        Action<TResult?>? setValueHook = null) : base(setValueHook)
+        Action<TResult>? setValueHook = null) : base(default!, setValueHook)
     {
         ArgumentNullException.ThrowIfNull(prop1);
         ArgumentNullException.ThrowIfNull(prop2);
@@ -59,9 +63,13 @@ public sealed class CombineLatestProperty<T1, T2, T3, TResult> : DeclarativeProp
 
         _func = func;
 
-        _value1 = prop1.Value is null ? default : prop1.Value;
+        _value1 = prop1.Value;
-        _value2 = prop2.Value is null ? default : prop2.Value;
+        _value2 = prop2.Value;
-        _value3 = prop3.Value is null ? default : prop3.Value;
+        _value3 = prop3.Value;
+
+        var initialValueTask = Task.Run(async () => await _func(_value1, _value2, _value3));
+        initialValueTask.Wait();
+        SetNewValueSync(initialValueTask.Result);
 
         prop1.Subscribe(async (value1, token) =>
         {

src/Library/DeclarativeProperty/CombineProperty.cs

@@ -5,7 +5,7 @@ public sealed class CombineProperty<TFrom, TTo> : DeclarativePropertyBase<TTo>
     private readonly Func<IReadOnlyList<TFrom?>, Task<TTo>> _combiner;
     private readonly List<IDeclarativeProperty<TFrom>> _sourceProperties = new();
 
-    public CombineProperty(Func<IReadOnlyList<TFrom?>, Task<TTo>> combiner)
+    public CombineProperty(Func<IReadOnlyList<TFrom?>, Task<TTo>> combiner) : base(default!)
     {
         _combiner = combiner;
     }

src/Library/DeclarativeProperty/DebounceProperty.cs

@@ -3,9 +3,9 @@
 public sealed class DebounceProperty<T> : DeclarativePropertyBase<T>
 {
     private readonly object _lock = new();
-    private readonly Func<T?, TimeSpan> _interval;
+    private readonly Func<T, TimeSpan> _interval;
     private DateTime _startTime = DateTime.MinValue;
-    private T? _nextValue;
+    private T _nextValue = default!;
     private CancellationToken _nextCancellationToken;
     private bool _isThrottleTaskRunning;
     public bool ResetTimer { get; init; }
@@ -13,14 +13,14 @@ public sealed class DebounceProperty<T> : DeclarativePropertyBase<T>
 
     public DebounceProperty(
         IDeclarativeProperty<T> from,
-        Func<T?, TimeSpan> interval,
+        Func<T, TimeSpan> interval,
-        Action<T?>? setValueHook = null) : base(from.Value, setValueHook)
+        Action<T>? setValueHook = null) : base(from.Value, setValueHook)
     {
         _interval = interval;
         AddDisposable(from.Subscribe(SetValue));
     }
 
-    private Task SetValue(T? next, CancellationToken cancellationToken = default)
+    private Task SetValue(T next, CancellationToken cancellationToken = default)
     {
         lock (_lock)
         {
@@ -38,7 +38,7 @@ public sealed class DebounceProperty<T> : DeclarativePropertyBase<T>
 
             _startTime = DateTime.Now;
             _isThrottleTaskRunning = true;
-            Task.Run(async () => await StartDebounceTask());
+            Task.Run(StartDebounceTask);
         }
 
         return Task.CompletedTask;
@@ -51,7 +51,7 @@ public sealed class DebounceProperty<T> : DeclarativePropertyBase<T>
             await Task.Delay(WaitInterval);
         }
 
-        T? next;
+        T next;
         CancellationToken cancellationToken;
         lock (_lock)
         {

src/Library/DeclarativeProperty/DeclarativeProperty.cs

@@ -6,15 +6,15 @@ public static class DeclarativePropertyHelpers
         IDeclarativeProperty<T1> prop1,
         IDeclarativeProperty<T2> prop2,
         Func<T1, T2, Task<TResult>> func,
-        Action<TResult?>? setValueHook = null)
+        Action<TResult>? setValueHook = null)
-        => new(prop1, prop2, func!, setValueHook);
+        => new(prop1, prop2, func, setValueHook);
 
     public static CombineLatestProperty<T1, T2, T3, TResult> CombineLatest<T1, T2, T3, TResult>(
         IDeclarativeProperty<T1> prop1,
         IDeclarativeProperty<T2> prop2,
         IDeclarativeProperty<T3> prop3,
         Func<T1, T2, T3, Task<TResult>> func,
-        Action<TResult?>? setValueHook = null)
+        Action<TResult>? setValueHook = null)
         => new(prop1, prop2, prop3, func!, setValueHook);
     
     public static MergeProperty<T> Merge<T>(params IDeclarativeProperty<T>[] props)
@@ -23,11 +23,7 @@ public static class DeclarativePropertyHelpers
 
 public sealed class DeclarativeProperty<T> : DeclarativePropertyBase<T>
 {
-    public DeclarativeProperty(Action<T?>? setValueHook = null) : base(setValueHook)
+    public DeclarativeProperty(T initialValue, Action<T>? setValueHook = null) : base(initialValue, setValueHook)
-    {
-    }
-
-    public DeclarativeProperty(T initialValue, Action<T?>? setValueHook = null) : base(initialValue, setValueHook)
     {
     }
 

src/Library/DeclarativeProperty/DeclarativePropertyBase.cs

@@ -5,8 +5,8 @@ namespace DeclarativeProperty;
 
 public abstract class DeclarativePropertyBase<T> : IDeclarativeProperty<T>
 {
-    private readonly List<Func<T?, CancellationToken, Task>> _subscribers = new();
+    private readonly List<Func<T, CancellationToken, Task>> _subscribers = new();
-    private readonly Action<T?>? _setValueHook;
+    private readonly Action<T>? _setValueHook;
     private readonly List<IDisposable> _disposables = new();
     private readonly List<Func<IDeclarativeProperty<T>, T, IDisposable?>> _subscribeTriggers = new();
     private readonly List<Action<IDeclarativeProperty<T>, T>> _unsubscribeTriggers = new();
@@ -14,29 +14,24 @@ public abstract class DeclarativePropertyBase<T> : IDeclarativeProperty<T>
     private readonly object _triggerLock = new();
     private readonly object _subscriberLock = new();
 
-    private T? _value;
+    private T _value;
 
-    public T? Value
+    public T Value
     {
         get => _value;
         set => _setValueHook?.Invoke(value);
     }
 
-    protected DeclarativePropertyBase(Action<T?>? setValueHook = null)
+    protected DeclarativePropertyBase(T initialValue, Action<T>? setValueHook = null)
-    {
-        _setValueHook = setValueHook;
-    }
-
-    protected DeclarativePropertyBase(T? initialValue, Action<T?>? setValueHook = null)
     {
         _setValueHook = setValueHook;
         _value = initialValue;
     }
 
 
-    protected async Task NotifySubscribersAsync(T? newValue, CancellationToken cancellationToken = default)
+    protected async Task NotifySubscribersAsync(T newValue, CancellationToken cancellationToken = default)
     {
-        List<Func<T?, CancellationToken, Task>> subscribers;
+        List<Func<T, CancellationToken, Task>> subscribers;
         lock (_subscriberLock)
         {
             subscribers = _subscribers.ToList();
@@ -54,7 +49,7 @@ public abstract class DeclarativePropertyBase<T> : IDeclarativeProperty<T>
     protected virtual void OnPropertyChanged([CallerMemberName] string? propertyName = null)
         => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
 
-    public IDisposable Subscribe(Func<T?, CancellationToken, Task> onChange)
+    public IDisposable Subscribe(Func<T, CancellationToken, Task> onChange)
     {
         lock (_subscriberLock)
         {
@@ -70,7 +65,7 @@ public abstract class DeclarativePropertyBase<T> : IDeclarativeProperty<T>
     {
         lock (_subscriberLock)
         {
-            _subscribers.Remove(onChange!);
+            _subscribers.Remove(onChange);
         }
     }
 
@@ -99,14 +94,14 @@ public abstract class DeclarativePropertyBase<T> : IDeclarativeProperty<T>
             return Task.CompletedTask;
         });
 
-    protected async Task SetNewValueAsync(T? newValue, CancellationToken cancellationToken = default)
+    protected async Task SetNewValueAsync(T newValue, CancellationToken cancellationToken = default)
     {
         SetNewValueSync(newValue, cancellationToken);
         if (cancellationToken.IsCancellationRequested) return;
         await NotifySubscribersAsync(newValue, cancellationToken);
     }
 
-    protected void SetNewValueSync(T? newValue, CancellationToken cancellationToken = default)
+    protected void SetNewValueSync(T newValue, CancellationToken cancellationToken = default)
     {
         if (!(Value?.Equals(newValue) ?? false))
         {

src/Library/DeclarativeProperty/DeclarativePropertyExtensions.cs

@@ -21,16 +21,16 @@ public static class DeclarativePropertyExtensions
     public static IDeclarativeProperty<T> DistinctUntilChanged<T>(this IDeclarativeProperty<T> from)
         => new DistinctUntilChangedProperty<T>(from);
 
-    public static IDeclarativeProperty<TTo> Map<TFrom, TTo>(this IDeclarativeProperty<TFrom?> from, Func<TFrom?, Task<TTo?>> mapper)
+    public static IDeclarativeProperty<TTo> Map<TFrom, TTo>(this IDeclarativeProperty<TFrom> from, Func<TFrom, Task<TTo>> mapper)
         => Map(from, async (v, _) => await mapper(v));
 
-    public static IDeclarativeProperty<TTo> Map<TFrom, TTo>(this IDeclarativeProperty<TFrom?> from, Func<TFrom?, CancellationToken, Task<TTo?>> mapper)
+    public static IDeclarativeProperty<TTo> Map<TFrom, TTo>(this IDeclarativeProperty<TFrom> from, Func<TFrom, CancellationToken, Task<TTo>> mapper)
-        => new MapProperty<TFrom?, TTo>(mapper, from);
+        => new MapProperty<TFrom, TTo>(mapper, from);
 
-    public static IDeclarativeProperty<TTo?> Map<TFrom, TTo>(this IDeclarativeProperty<TFrom?> from, Func<TFrom?, TTo?> mapper)
+    public static IDeclarativeProperty<TTo> Map<TFrom, TTo>(this IDeclarativeProperty<TFrom> from, Func<TFrom, TTo> mapper)
-        => new MapProperty<TFrom?, TTo?>((next, _) => Task.FromResult(mapper(next)), from);
+        => new MapProperty<TFrom, TTo>((next, _) => Task.FromResult(mapper(next)), from);
 
-    public static async Task<IDeclarativeProperty<TTo>> MapAsync<TFrom, TTo>(this IDeclarativeProperty<TFrom?> from, Func<TFrom?, CancellationToken, Task<TTo?>> mapper)
+    public static async Task<IDeclarativeProperty<TTo>> MapAsync<TFrom, TTo>(this IDeclarativeProperty<TFrom> from, Func<TFrom, CancellationToken, Task<TTo>> mapper)
         => await MapProperty<TFrom, TTo>.CreateAsync(mapper, from);
 
     public static async Task<IDeclarativeProperty<TTo?>> MapAsync<TFrom, TTo>(this IDeclarativeProperty<TFrom?> from, Func<TFrom?, TTo?> mapper)
@@ -43,39 +43,39 @@ public static class DeclarativePropertyExtensions
             return Task.CompletedTask;
         });
 
-    public static IDisposable Subscribe<T>(this IDeclarativeProperty<T> property, Action<T?> onChange)
+    public static IDisposable Subscribe<T>(this IDeclarativeProperty<T> property, Action<T> onChange)
         => property.Subscribe((value, _) =>
         {
             onChange(value);
             return Task.CompletedTask;
         });
 
-    public static IDeclarativeProperty<T> Extract<T>(
+    public static IDeclarativeProperty<TResult> Extract<T, TResult>(
-        this IDeclarativeProperty<ReadOnlyObservableCollection<T>> from,
+        this IDeclarativeProperty<ReadOnlyObservableCollection<T>?> from,
-        Func<IList<T>?, T?> extractor
+        Func<IList<T>?, TResult> extractor
     )
-        => new ExtractorProperty<T>(from, extractor);
+        => new ExtractorProperty<T, TResult>(from, extractor);
 
-    public static IDeclarativeProperty<T> Extract<T>(
+    public static IDeclarativeProperty<TResult> Extract<T, TResult>(
-        this IDeclarativeProperty<ObservableCollection<T>> from,
+        this IDeclarativeProperty<ObservableCollection<T>?> from,
-        Func<IList<T>?, T?> extractor
+        Func<IList<T>?, TResult> extractor
     )
-        => new ExtractorProperty<T>(from, extractor);
+        => new ExtractorProperty<T, TResult>(from, extractor);
 
-    public static IDeclarativeProperty<T> Extract<T>(
+    public static IDeclarativeProperty<TResult> Extract<T, TResult>(
-        this IDeclarativeProperty<CollectionComputing<T>> from,
+        this IDeclarativeProperty<CollectionComputing<T>?> from,
-        Func<IList<T>?, T?> extractor
+        Func<IList<T>?, TResult> extractor
     )
-        => new ExtractorProperty<T>(from, extractor);
+        => new ExtractorProperty<T, TResult>(from, extractor);
 
     public static IDeclarativeProperty<TCollection> Watch<TCollection, TItem>(
-        this IDeclarativeProperty<TCollection?> collection)
+        this IDeclarativeProperty<TCollection> collection)
-        where TCollection : IList<TItem>, INotifyCollectionChanged
+        where TCollection : IList<TItem>?, INotifyCollectionChanged?
         => new CollectionRepeaterProperty<TCollection, TItem>(collection);
 
     public static IDeclarativeProperty<TCollection> Watch<TCollection, TItem>(
         this TCollection collection)
-        where TCollection : IList<TItem>, INotifyCollectionChanged
+        where TCollection : IList<TItem>?, INotifyCollectionChanged?
         => new CollectionRepeaterProperty<TCollection, TItem>(collection);
 
     public static IDeclarativeProperty<ObservableCollection<TItem>> Watch<TItem>(
@@ -90,16 +90,16 @@ public static class DeclarativePropertyExtensions
     public static IDeclarativeProperty<TResult> CombineLatest<T1, T2, TResult>(
         this IDeclarativeProperty<T1> prop1,
         IDeclarativeProperty<T2> prop2,
-        Func<T1, T2, Task<TResult?>> func,
+        Func<T1, T2, Task<TResult>> func,
         Action<TResult?>? setValueHook = null)
-        => new CombineLatestProperty<T1,T2,TResult>(prop1, prop2, func!, setValueHook);
+        => new CombineLatestProperty<T1,T2,TResult>(prop1, prop2, func, setValueHook);
 
-    public static IDeclarativeProperty<T?> Switch<T>(this IDeclarativeProperty<IDeclarativeProperty<T?>?> from)
+    public static IDeclarativeProperty<T> Switch<T>(this IDeclarativeProperty<IDeclarativeProperty<T>?> from)
-        => new SwitchProperty<T?>(from);
+        => new SwitchProperty<T>(from);
     
     public static IDeclarativeProperty<TResult> CombineAll<T, TResult>(
         this IEnumerable<IDeclarativeProperty<T>> sources,
-        Func<IEnumerable<T>, Task<TResult?>> combiner,
+        Func<IEnumerable<T>, Task<TResult>> combiner,
         Action<TResult?>? setValueHook = null)
         => new CombineAllProperty<T,TResult>(sources, combiner, setValueHook);
 }

src/Library/DeclarativeProperty/DistinctUntilChangedProperty.cs

@@ -11,7 +11,7 @@ public sealed class DistinctUntilChangedProperty<T> : DeclarativePropertyBase<T>
         AddDisposable(from.Subscribe(Handle));
     }
 
-    async Task Handle(T? next, CancellationToken cancellationToken = default)
+    async Task Handle(T next, CancellationToken cancellationToken = default)
     {
         if (_comparer is { } comparer)
         {
@@ -20,8 +20,10 @@ public sealed class DistinctUntilChangedProperty<T> : DeclarativePropertyBase<T>
                 return;
             }
         }
-        else if ((next is null && Value is null && !_firstFire)
+        else if (
-                 || (Value?.Equals(next) ?? false))
+            (next is null && Value is null && !_firstFire)
+            || (Value?.Equals(next) ?? false)
+        )
         {
             return;
         }

src/Library/DeclarativeProperty/ExtractorProperty.cs

@@ -4,7 +4,7 @@ using ObservableComputations;
 
 namespace DeclarativeProperty;
 
-public sealed class ExtractorProperty<T> : DeclarativePropertyBase<T>
+public sealed class ExtractorProperty<T, TResult> : DeclarativePropertyBase<TResult>
 {
     private interface ICollectionWrapper : IDisposable
     {
@@ -108,12 +108,12 @@ public sealed class ExtractorProperty<T> : DeclarativePropertyBase<T>
         ) => new CollectionComputingWrapper(collection, handler);
     }
 
-    private readonly Func<IList<T>?, T?> _extractor;
+    private readonly Func<IList<T>?, TResult> _extractor;
     private ICollectionWrapper? _collectionWrapper;
 
     public ExtractorProperty(
-        IDeclarativeProperty<ObservableCollection<T>> from,
+        IDeclarativeProperty<ObservableCollection<T>?> from,
-        Func<IList<T>?, T?> extractor) : base(extractor(from.Value))
+        Func<IList<T>?, TResult> extractor) : base(extractor(from.Value))
     {
         _extractor = extractor;
         _collectionWrapper = from.Value is null
@@ -124,8 +124,8 @@ public sealed class ExtractorProperty<T> : DeclarativePropertyBase<T>
     }
 
     public ExtractorProperty(
-        IDeclarativeProperty<ReadOnlyObservableCollection<T>> from,
+        IDeclarativeProperty<ReadOnlyObservableCollection<T>?> from,
-        Func<IList<T>?, T?> extractor) : base(extractor(from.Value))
+        Func<IList<T>?, TResult> extractor) : base(extractor(from.Value))
     {
         _extractor = extractor;
         _collectionWrapper = from.Value is null
@@ -136,8 +136,8 @@ public sealed class ExtractorProperty<T> : DeclarativePropertyBase<T>
     }
 
     public ExtractorProperty(
-        IDeclarativeProperty<CollectionComputing<T>> from,
+        IDeclarativeProperty<CollectionComputing<T>?> from,
-        Func<IList<T>?, T?> extractor) : base(extractor(from.Value))
+        Func<IList<T>?, TResult> extractor) : base(extractor(from.Value))
     {
         _extractor = extractor;
         _collectionWrapper = from.Value is null
@@ -165,9 +165,7 @@ public sealed class ExtractorProperty<T> : DeclarativePropertyBase<T>
 
     private async Task Fire(IList<T>? items, CancellationToken cancellationToken = default)
     {
-        var newValue = items is null
+        var newValue = _extractor(items);
-            ? default
-            : _extractor(items);
 
         await SetNewValueAsync(newValue, cancellationToken);
     }

src/Library/DeclarativeProperty/FilterProperty.cs

@@ -7,13 +7,22 @@ public sealed class FilterProperty<T> : DeclarativePropertyBase<T>
     public FilterProperty(
         Func<T?, Task<bool>> filter,
         IDeclarativeProperty<T> from,
-        Action<T?>? setValueHook = null) : base(setValueHook)
+        Action<T>? setValueHook = null) : base(default!, setValueHook)
     {
         _filter = filter;
+
+        var initialValueTask = Task.Run(async () => await _filter(from.Value));
+        initialValueTask.Wait();
+        if (initialValueTask.Result)
+        {
+            SetNewValueSync(from.Value);
+        }
+        // Unfortunately we can't set a default value if the parent current value can not passed by the filter.
+
         AddDisposable(from.Subscribe(SetValue));
     }
 
-    private async Task SetValue(T? next, CancellationToken cancellationToken = default)
+    private async Task SetValue(T next, CancellationToken cancellationToken = default)
     {
         if (await _filter(next))
         {
@@ -24,7 +33,7 @@ public sealed class FilterProperty<T> : DeclarativePropertyBase<T>
     public static async Task<FilterProperty<T>> CreateAsync(
         Func<T?, Task<bool>> filter,
         IDeclarativeProperty<T> from,
-        Action<T?>? setValueHook = null)
+        Action<T>? setValueHook = null)
     {
         var prop = new FilterProperty<T>(filter, from, setValueHook);
         await prop.SetValue(from.Value);

src/Library/DeclarativeProperty/IDeclarativeProperty.cs

@@ -4,9 +4,9 @@ namespace DeclarativeProperty;
 
 public interface IDeclarativeProperty<T> : INotifyPropertyChanged, IDisposable, IObservable<T>
 {
-    T? Value { get; set; }
+    T Value { get; set; }
-    IDisposable Subscribe(Func<T?, CancellationToken, Task> onChange);
+    IDisposable Subscribe(Func<T, CancellationToken, Task> onChange);
-    void Unsubscribe(Func<T?, CancellationToken, Task> onChange);
+    void Unsubscribe(Func<T, CancellationToken, Task> onChange);
 
     IDeclarativeProperty<T> RegisterTrigger(
         Func<IDeclarativeProperty<T>, T, IDisposable?> triggerSubscribe,

src/Library/DeclarativeProperty/MapProperty.cs

@@ -6,14 +6,19 @@ public sealed class MapProperty<TFrom, TTo> : DeclarativePropertyBase<TTo>
 
     public MapProperty(
         Func<TFrom, CancellationToken, Task<TTo>> mapper, 
-        IDeclarativeProperty<TFrom?> from,
+        IDeclarativeProperty<TFrom> from,
-        Action<TTo?>? setValueHook = null) : base(setValueHook)
+        Action<TTo>? setValueHook = null) : base(default!, setValueHook)
     {
         _mapper = mapper;
+        
+        var initialValueTask = Task.Run(async () => await _mapper(from.Value, CancellationToken.None));
+        initialValueTask.Wait();
+        SetNewValueSync(initialValueTask.Result);
+
         AddDisposable(from.Subscribe(SetValue));
     }
 
-    private async Task SetValue(TFrom? next, CancellationToken cancellationToken = default)
+    private async Task SetValue(TFrom next, CancellationToken cancellationToken = default)
     {
         var newValue = await _mapper(next!, cancellationToken);
         await SetNewValueAsync(newValue, cancellationToken);
@@ -21,8 +26,8 @@ public sealed class MapProperty<TFrom, TTo> : DeclarativePropertyBase<TTo>
     
     public static async Task<MapProperty<TFrom, TTo>> CreateAsync(
         Func<TFrom, CancellationToken, Task<TTo>> mapper, 
-        IDeclarativeProperty<TFrom?> from,
+        IDeclarativeProperty<TFrom> from,
-        Action<TTo?>? setValueHook = null)
+        Action<TTo>? setValueHook = null)
     {
         var prop = new MapProperty<TFrom, TTo>(mapper, from, setValueHook);
         await prop.SetValue(from.Value);

src/Library/DeclarativeProperty/MergeProperty.cs

@@ -2,7 +2,7 @@
 
 public class MergeProperty<T> : DeclarativePropertyBase<T>
 {
-    public MergeProperty(params IDeclarativeProperty<T>[] props)
+    public MergeProperty(params IDeclarativeProperty<T>[] props) : base(default!)
     {
         ArgumentNullException.ThrowIfNull(props);
 
@@ -12,6 +12,6 @@ public class MergeProperty<T> : DeclarativePropertyBase<T>
         }
     }
 
-    private async Task UpdateAsync(T? newValue, CancellationToken token)
+    private async Task UpdateAsync(T newValue, CancellationToken token)
         => await SetNewValueAsync(newValue, token);
 }

src/Library/DeclarativeProperty/SwitchProperty.cs

@@ -4,7 +4,7 @@ public sealed class SwitchProperty<TItem> : DeclarativePropertyBase<TItem>
 {
     private IDisposable? _innerSubscription;
 
-    public SwitchProperty(IDeclarativeProperty<IDeclarativeProperty<TItem>?> from) : base(from.Value is null ? default : from.Value.Value)
+    public SwitchProperty(IDeclarativeProperty<IDeclarativeProperty<TItem>?> from) : base(from.Value is null ? default! : from.Value.Value)
     {
         AddDisposable(from.Subscribe(HandleStreamChange));
         _innerSubscription = from.Value?.Subscribe(HandleInnerValueChange);
@@ -15,10 +15,9 @@ public sealed class SwitchProperty<TItem> : DeclarativePropertyBase<TItem>
         _innerSubscription?.Dispose();
         _innerSubscription = next?.Subscribe(HandleInnerValueChange);
 
-
+        await SetNewValueAsync(next is null ? default! : next.Value, token);
-        await SetNewValueAsync(next is null ? default : next.Value, token);
     }
 
-    private async Task HandleInnerValueChange(TItem? next, CancellationToken token)
+    private async Task HandleInnerValueChange(TItem next, CancellationToken token)
         => await SetNewValueAsync(next, token);
 }

src/Library/DeclarativeProperty/ThrottleProperty.cs

@@ -9,13 +9,13 @@ public class ThrottleProperty<T> : DeclarativePropertyBase<T>
     public ThrottleProperty(
         IDeclarativeProperty<T> from,
         Func<TimeSpan> interval,
-        Action<T?>? setValueHook = null) : base(from.Value, setValueHook)
+        Action<T>? setValueHook = null) : base(from.Value, setValueHook)
     {
         _interval = interval;
         AddDisposable(from.Subscribe(SetValue));
     }
 
-    private async Task SetValue(T? next, CancellationToken cancellationToken = default)
+    private async Task SetValue(T next, CancellationToken cancellationToken = default)
     {
         lock (_lock)
         {

src/Library/DeclarativeProperty/Unsubscriber.cs

@@ -3,9 +3,9 @@
 internal sealed class Unsubscriber<T> : IDisposable
 {
     private readonly IDeclarativeProperty<T> _owner;
-    private readonly Func<T?, CancellationToken, Task> _onChange;
+    private readonly Func<T, CancellationToken, Task> _onChange;
 
-    public Unsubscriber(IDeclarativeProperty<T> owner, Func<T?, CancellationToken, Task> onChange)
+    public Unsubscriber(IDeclarativeProperty<T> owner, Func<T, CancellationToken, Task> onChange)
     {
         _owner = owner;
         _onChange = onChange;