Enable ViewAdapter to attach to an existing view

src/Fabulous.Tests/APISketchTests/TestUI.Widgets.fs

@@ -47,7 +47,8 @@ module TestUI_Widgets =
                         let oldWidget: Widget voption = ValueNone
 
                         Reconciler.update context.CanReuseView oldWidget widget viewNode
-                        struct (viewNode :> IViewNode, box view) }
+                        struct (viewNode :> IViewNode, box view)
+                  AttachView = fun (_widget, _context, _parentNode, _view) -> failwith "not implemented" }
 
             WidgetDefinitionStore.set key definition
             key

src/Fabulous/Memo.fs

@@ -88,6 +88,7 @@ module Memo =
                 // store widget that was used to produce this node
                 // to pass it to reconciler later on
                 node.MemoizedWidget <- Some memoizedWidget
-                struct (node, view) }
+                struct (node, view)
+          AttachView = fun (_widget, _context, _parentNode, _view) -> failwith "Memo widget cannot be attached" }
 
     WidgetDefinitionStore.set MemoWidgetKey widgetDefinition

src/Fabulous/Runners.fs

@@ -36,6 +36,8 @@ type IViewAdapter =
     inherit IDisposable
     /// Instantiates a new view using the current state associated with this ViewAdapter
     abstract CreateView: unit -> obj
+    /// Attaches to the existing view and updates it with the current state associated with this ViewAdapter
+    abstract Attach: obj -> unit
 
 module RunnerStore =
     let private _runners = Dictionary<StateKey, IRunner>()
@@ -183,6 +185,24 @@ module ViewAdapters =
             _root <- root
             _root
 
+        member _.Attach(root) =
+            let state = unbox(StateStore.get stateKey)
+            let widget = (view state).Compile()
+            _widget <- widget
+
+            let treeContext =
+                { CanReuseView = canReuseView
+                  GetViewNode = getViewNode
+                  Logger = logger
+                  Dispatch = this.Dispatch }
+
+            let definition = WidgetDefinitionStore.get widget.Key
+
+            let _node =
+                definition.AttachView(widget, treeContext, ValueNone, root)
+
+            _root <- root
+
         /// Listens for StateStore changes and updates the view if necessary
         member _.OnStateChanged(args) =
             try
@@ -211,6 +231,7 @@ module ViewAdapters =
         interface IViewAdapter with
             member x.Dispose() = x.Dispose()
             member x.CreateView() = x.CreateView()
+            member x.Attach(root) = x.Attach(root)
 
     /// Create a new ViewAdapter for the component
     let create<'arg, 'model, 'msg, 'marker>

src/Fabulous/WidgetDefinitions.fs

@@ -9,7 +9,8 @@ type WidgetDefinition =
     { Key: WidgetKey
       Name: string
       TargetType: Type
-      CreateView: Widget * ViewTreeContext * IViewNode voption -> struct (IViewNode * obj) }
+      CreateView: Widget * ViewTreeContext * IViewNode voption -> struct (IViewNode * obj)
+      AttachView: Widget * ViewTreeContext * IViewNode voption * obj -> IViewNode }
 
 module WidgetDefinitionStore =
     let private _widgets = ResizeArray<WidgetDefinition>()