README.md

@@ -254,7 +254,24 @@ Then use the following attributes wherever you need a design-time VM:
 
 When targeting .NET Framework, “Project code” must be enabled in the XAML designer for this to work.
 
-When targeting .NET Core 3, a bug in the XAML designer causes design-time data to not be displayed through `DataContext` bindings.  See [this issue](https://developercommunity.visualstudio.com/content/problem/1133390/design-time-data-in-datacontext-binding-not-displa.html) for more details.
+##### .NET Core 3 workaround
+
+When targeting .NET Core 3, a bug in the XAML designer causes design-time data to not be displayed through `DataContext` bindings. See [this issue](https://developercommunity.visualstudio.com/content/problem/1133390/design-time-data-in-datacontext-binding-not-displa.html) for details. One workaround is to add a `d:DataContext` binding alongside your `DataContext` binding. Change from this:
+
+```xaml
+<local:MyControl DataContext="{Binding Child}" />
+```
+
+To this:
+
+```xaml
+<local:MyControl
+  DataContext="{Binding Child}"
+  d:DataContext="{Binding DataContext.Child,
+                          RelativeSource={RelativeSource AncestorType=T}}" />
+```
+
+Where `T` is the type of the parent object that contains `local:MyControl` (or a more distant ancestor, though there are issues with using `Window` as the type).
 
 #### Can I open new windows/dialogs?
 

src/Elmish.WPF.Tests/Elmish.WPF.Tests.fsproj

@@ -1,7 +1,8 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk.WindowsDesktop">
 
   <PropertyGroup>
     <TargetFrameworks>netcoreapp3.0;net461</TargetFrameworks>
+    <UseWpf>true</UseWpf>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <!--Turn on warnings for unused values (arguments and let bindings) -->
     <OtherFlags>$(OtherFlags) --warnon:1182</OtherFlags>
@@ -32,10 +33,4 @@
     <ProjectReference Include="..\Elmish.WPF\Elmish.WPF.fsproj" />
   </ItemGroup>
 
-  <ItemGroup>
-    <Reference Include="PresentationCore" />
-    <Reference Include="PresentationFramework" />
-    <Reference Include="WindowsBase" />
-  </ItemGroup>
-
 </Project>

src/Elmish.WPF/ViewModel.fs

@@ -13,62 +13,6 @@ open Elmish
 [<AutoOpen>]
 module internal ViewModelHelpers =
 
-  let historicalMerge
-      logInvalidGetSourceId
-      logInvalidGetTargetId
-      getSourceId
-      getTargetId
-      create
-      update
-      (target: ObservableCollection<_>)
-      (source: _ array) =
-    let sourceIdxItemPairsById = Dictionary<_,_>(source.Length)
-    for (idx, s) in source |> Seq.indexed do
-      let id = getSourceId s
-      if sourceIdxItemPairsById.ContainsKey id
-      then logInvalidGetSourceId id (sourceIdxItemPairsById.[id]) s
-      else sourceIdxItemPairsById.Add(id, (idx, s))
-
-    let targetIdxItemPairsById = Dictionary<_,_>(target.Count)
-    for (idx, t) in target |> Seq.indexed do
-      let id = getTargetId t
-      if targetIdxItemPairsById.ContainsKey id
-      then logInvalidGetTargetId id (targetIdxItemPairsById.[id]) t
-      else targetIdxItemPairsById.Add(id, (idx, t))
-
-    if sourceIdxItemPairsById.Count = source.Length && targetIdxItemPairsById.Count = target.Count then
-      // Update target items
-      for Kvp (tId, (tIdx, t)) in targetIdxItemPairsById do
-        match sourceIdxItemPairsById.TryGetValue tId with
-        | true, (_, s) -> update t s tIdx
-        | _ -> ()
-
-      // Remove target items that no longer exist
-      if target.Count <> 0 && source.Length = 0
-      then target.Clear ()
-      else
-        for tIdx in target.Count - 1..-1..0 do
-          let tId = getTargetId target.[tIdx]
-          if tId |> sourceIdxItemPairsById.ContainsKey |> not then
-            let (tIdx2, _) = targetIdxItemPairsById.[tId] // tIdx = tIdx2, so this line is unnecessary
-            target.RemoveAt tIdx2
-
-      // Add target items that don't currently exist
-      let create (Kvp (sId, (_, s))) = create s sId
-      sourceIdxItemPairsById
-      |> Seq.filter (Kvp.key >> targetIdxItemPairsById.ContainsKey >> not)
-      |> Seq.map create
-      |> Seq.iter target.Add
-
-      // Reorder according to source items
-      for Kvp (sId, (sIdx, _)) in sourceIdxItemPairsById do
-        let tIdx =
-          target
-          |> Seq.indexed
-          |> Seq.find (fun (_, t) -> sId = getTargetId t)
-          |> fst
-        if tIdx <> sIdx then target.Move(tIdx, sIdx)
-
   let elmStyleMerge
         getSourceId
         getTargetId

src/Samples/SubModel.Views/CounterWithClock.xaml

@@ -8,7 +8,7 @@
              mc:Ignorable="d"
              d:DataContext="{x:Static vm:Program.counterWithClockDesignVm}">
   <StackPanel>
-    <local:Counter DataContext="{Binding Counter}" HorizontalAlignment="Center" />
-    <local:Clock DataContext="{Binding Clock}" HorizontalAlignment="Center" />
+    <local:Counter DataContext="{Binding Counter}" d:DataContext="{Binding DataContext.Counter, RelativeSource={RelativeSource AncestorType=UserControl}}" HorizontalAlignment="Center" />
+    <local:Clock DataContext="{Binding Clock}" d:DataContext="{Binding DataContext.Clock, RelativeSource={RelativeSource AncestorType=UserControl}}" HorizontalAlignment="Center" />
   </StackPanel>
 </UserControl>

src/Samples/SubModel.Views/MainWindow.xaml

@@ -13,8 +13,8 @@
         d:DataContext="{x:Static vm:Program.mainDesignVm}">
   <StackPanel>
     <TextBlock Text="Counter with clock 1" FontSize="18" FontWeight="Bold" Margin="0,25,0,0" HorizontalAlignment="Center" />
-    <local:CounterWithClock DataContext="{Binding ClockCounter1}" />
+    <local:CounterWithClock DataContext="{Binding ClockCounter1}" d:DataContext="{Binding DataContext.ClockCounter1, RelativeSource={RelativeSource AncestorType=StackPanel}}"/>
     <TextBlock Text="Counter with clock 2" FontSize="18" FontWeight="Bold" Margin="0,25,0,0" HorizontalAlignment="Center" />
-    <local:CounterWithClock DataContext="{Binding ClockCounter2}" />
+    <local:CounterWithClock DataContext="{Binding ClockCounter2}" d:DataContext="{Binding DataContext.ClockCounter2, RelativeSource={RelativeSource AncestorType=StackPanel}}"/>
   </StackPanel>
 </Window>

src/Samples/SubModelOpt.Views/MainWindow.xaml

@@ -34,13 +34,15 @@
       <StackPanel Background="White">
         <local:Form1
             DataContext="{Binding Form1}"
+            d:DataContext="{Binding DataContext.Form1, RelativeSource={RelativeSource AncestorType=StackPanel}}"
             Visibility="{Binding DataContext.Form1Visible,
-                         RelativeSource={RelativeSource AncestorType=Window},
+                         RelativeSource={RelativeSource AncestorType=StackPanel},
                          Converter={StaticResource VisibilityConverter}}" />
         <local:Form2
             DataContext="{Binding Form2}"
+            d:DataContext="{Binding DataContext.Form2, RelativeSource={RelativeSource AncestorType=StackPanel}}"
             Visibility="{Binding DataContext.Form2Visible,
-                         RelativeSource={RelativeSource AncestorType=Window},
+                         RelativeSource={RelativeSource AncestorType=StackPanel},
                          Converter={StaticResource VisibilityConverter}}" />
       </StackPanel>
     </Border>