From 78e8f28e7207bb3cbd3337805c4820995cb540e1 Mon Sep 17 00:00:00 2001
From: Josua Jaeger <mail@jaggerjo.com>
Date: Thu, 5 Sep 2024 21:38:30 +0200
Subject: [PATCH] fix https://github.com/fsprojects/Avalonia.FuncUI/issues/435

---
 .../Avalonia.FuncUI.UnitTests.fsproj          |  1 +
 src/Avalonia.FuncUI.UnitTests/LibTests.fs     | 24 +++++++++++++++--
 src/Avalonia.FuncUI/DSL/AttrBuilder.fs        |  4 +++
 src/Avalonia.FuncUI/Library.fs                | 27 +++++++++++++++++--
 4 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/src/Avalonia.FuncUI.UnitTests/Avalonia.FuncUI.UnitTests.fsproj b/src/Avalonia.FuncUI.UnitTests/Avalonia.FuncUI.UnitTests.fsproj
index 2b830e9f..2e550c8e 100644
--- a/src/Avalonia.FuncUI.UnitTests/Avalonia.FuncUI.UnitTests.fsproj
+++ b/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>
diff --git a/src/Avalonia.FuncUI.UnitTests/LibTests.fs b/src/Avalonia.FuncUI.UnitTests/LibTests.fs
index fae51730..f9f7b9e1 100644
--- a/src/Avalonia.FuncUI.UnitTests/LibTests.fs
+++ b/src/Avalonia.FuncUI.UnitTests/LibTests.fs
@@ -1,5 +1,6 @@
 namespace Avalonia.FuncUI.UnitTests
 
+open System.Reactive.Subjects
 open Avalonia.FuncUI.Library
 open Xunit
 
@@ -7,7 +8,7 @@ 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())
\ No newline at end of file
+        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])
\ No newline at end of file
diff --git a/src/Avalonia.FuncUI/DSL/AttrBuilder.fs b/src/Avalonia.FuncUI/DSL/AttrBuilder.fs
index 434f8f94..fff01dd2 100644
--- a/src/Avalonia.FuncUI/DSL/AttrBuilder.fs
+++ b/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
 
diff --git a/src/Avalonia.FuncUI/Library.fs b/src/Avalonia.FuncUI/Library.fs
index a5d75a1e..13ab3e50 100644
--- a/src/Avalonia.FuncUI/Library.fs
+++ b/src/Avalonia.FuncUI/Library.fs
@@ -37,6 +37,29 @@ 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 ->
@@ -44,10 +67,10 @@ module internal Extensions =
                 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) }
\ No newline at end of file