fix #435

src/Avalonia.FuncUI.UnitTests/Avalonia.FuncUI.UnitTests.fsproj

@@ -23,6 +23,7 @@
 
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.10.0" />
+    <PackageReference Include="System.Reactive" Version="6.0.1" />
     <PackageReference Include="xunit" Version="2.9.0" />
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
       <PrivateAssets>all</PrivateAssets>

src/Avalonia.FuncUI.UnitTests/LibTests.fs

@@ -1,13 +1,14 @@
 namespace Avalonia.FuncUI.UnitTests
 
+open System.Reactive.Subjects
 open Avalonia.FuncUI.Library
 open Xunit
 
 module Library =
 
     [<Fact>]
     let ``function equality`` () =
-        
+
         let view () =
             let add = fun (count: int) -> count + 1
             let sub = fun (count: int) -> count - 1
@@ -19,4 +20,23 @@ module Library =
         Assert.Equal(add'.GetType(), add''.GetType())
         Assert.Equal(sub'.GetType(), sub''.GetType())
         Assert.NotEqual(add'.GetType(), sub'.GetType())
-        Assert.NotEqual(add''.GetType(), sub''.GetType())
+        Assert.NotEqual(add''.GetType(), sub''.GetType())
+
+
+    [<Fact>]
+    let ``IObservable<T>.SkipFirst`` () =
+        let observable = new Subject<int>()
+        let seenItems = ResizeArray<int>()
+
+        let _ =
+            observable
+                .SkipFirst()
+                .Subscribe(fun n -> seenItems.Add(n))
+
+        observable.OnNext(1)
+        observable.OnNext(2)
+        observable.OnNext(3)
+
+        Assert.Equal(2, seenItems.Count)
+        Assert.Equal(2, seenItems.[0])
+        Assert.Equal(3, seenItems.[1])

src/Avalonia.FuncUI/DSL/AttrBuilder.fs

@@ -235,6 +235,8 @@ type AttrBuilder<'view>() =
             let cts = new CancellationTokenSource()
             control
                 .GetObservable(property)
+                // GetObservable immediately emits the current value. We're not interested in that, so we skip the first value.
+                .SkipFirst()
                 .Subscribe(func, cts.Token)
             cts
 
@@ -261,6 +263,8 @@ type AttrBuilder<'view>() =
             let cts = new CancellationTokenSource()
             control
                 .GetObservable(property)
+                 // GetObservable immediately emits the current value. We're not interested in that, so we skip the first value.
+                .SkipFirst()
                 .Subscribe(func, cts.Token)
             cts
 

src/Avalonia.FuncUI/Library.fs

@@ -37,17 +37,40 @@ module internal Extensions =
             let disposable = Observable.subscribe callback this
             token.Register(fun () -> disposable.Dispose()) |> ignore
 
+        member this.SkipFirst() =
+            { new IObservable<'a> with
+                member __.Subscribe(observer: IObserver<'a>) =
+                    let skipFirstObserver =
+                        let mutable isFirst = true
+
+                        { new IObserver<'a> with
+                            member __.OnNext(value) =
+                                if isFirst then
+                                    isFirst <- false
+                                else
+                                    observer.OnNext(value)
+
+                            member __.OnError(error) =
+                                observer.OnError(error)
+
+                            member __.OnCompleted() =
+                                observer.OnCompleted()
+                        }
+
+                    this.Subscribe(skipFirstObserver)
+            }
+
     type Interactive with
         member this.GetObservable<'args when 'args :> RoutedEventArgs>(routedEvent: RoutedEvent<'args>) : IObservable<'args> =
             let sub = Func<IObserver<'args>, IDisposable>(fun observer ->
                 // push new update to subscribers
                 let handler = EventHandler<'args>(fun _ e ->
                     observer.OnNext e
                 )
-                
+
                 // subscribe to event changes so they can be pushed to subscribers
                 this.AddDisposableHandler(routedEvent, handler, routedEvent.RoutingStrategies)
             )
-            
+
             { new IObservable<'args>
               with member this.Subscribe(observer: IObserver<'args>) = sub.Invoke(observer) }