Add containerCreateContainerFromModel to workaround issue when updati…

…ng offset during merge

ChangeLog.md

@@ -4,13 +4,11 @@
 
 - Add `customModelBuilder` in Composite, for custom models support. These can consume information
   from the parent model.
-- Add `OverlayWrapper` widget.
-- Add `ColorPopup` widget. Besides being useful on its own, it serves as an `OverlayWrapper` example.
 
 ### Fixed
 
 - Keep old Composite root if model has not changed. This did not affect previous code,
-  it was only relevant with new features.
+  it is only relevant with new features.
 
 ### Changed
 
@@ -19,6 +17,7 @@
 - ZStack's `onlyTopActive` now follows the same pattern as other boolean combinators.
 - Shortened labels for `ColorPicker`.
 - Changed `_weFindByPath` to `_weFindBranchByPath`, now returning the complete branch up to the given path.
+- Add `containerCreateContainerFromModel` to workaround issue when updating offset during merge.
 
 ## 1.1.1.0
 

src/Monomer/Widgets/Container.hs

@@ -98,6 +98,32 @@ type ContainerGetCurrentStyle s e
   -> WidgetNode s e    -- ^ The widget node.
   -> StyleState        -- ^ The active style for the node.
 
+{-|
+Returns an updated version of the Container instance. Currently only used
+during merge. Not needed in general, except for widgets that modify offset
+or layout direction.
+
+An example can be found in "Monomer.Widgets.Containers.Scroll".
+
+Note:
+
+Some widgets, scroll for example, provide offset/layout direction to the
+Container instance; this information is passed down to children widgets in all
+the lifecycle methods. During merge a problem arises: the new node has not yet
+processed the old state and, since it is the Container whose merge function is
+being invoked, it is not able to provide the correct offset/layout direction.
+It is also not possible to directly extract this information from the old node,
+because we have a Widget instance and not its Container instance. This function
+provides a workaround for this.
+
+This is a hacky solution; a more flexible dispatcher for Composite could avoid it.
+-}
+type ContainerCreateContainerFromModel s e a
+  = WidgetEnv s e            -- ^ The widget environment.
+  -> WidgetNode s e          -- ^ The widget node.
+  -> a                       -- ^ The previous model.
+  -> Maybe (Container s e a) -- ^ An updated Container instance.
+
 {-|
 Updates the widget environment before passing it down to children. This function
 is called during the execution of all the widget functions. Useful for
@@ -353,6 +379,8 @@ data Container s e a = Container {
   containerUseScissor :: Bool,
   -- | Returns the base style for this type of widget.
   containerGetBaseStyle :: ContainerGetBaseStyle s e,
+  -- | Returns an updated version of the Container instance.
+  containerCreateContainerFromModel :: ContainerCreateContainerFromModel s e a,
   -- | Returns the current style, depending on the status of the widget.
   containerGetCurrentStyle :: ContainerGetCurrentStyle s e,
   -- | Updates the widget environment before passing it down to children.
@@ -403,6 +431,7 @@ instance Default (Container s e a) where
     containerUseScissor = False,
     containerGetBaseStyle = defaultGetBaseStyle,
     containerGetCurrentStyle = defaultGetCurrentStyle,
+    containerCreateContainerFromModel = defaultCreateContainerFromModel,
     containerUpdateCWenv = defaultUpdateCWenv,
     containerInit = defaultInit,
     containerInitPost = defaultInitPost,
@@ -461,6 +490,9 @@ defaultGetBaseStyle wenv node = Nothing
 defaultGetCurrentStyle :: ContainerGetCurrentStyle s e
 defaultGetCurrentStyle wenv node = currentStyle wenv node
 
+defaultCreateContainerFromModel :: ContainerCreateContainerFromModel s e a
+defaultCreateContainerFromModel wenv node state = Nothing
+
 defaultUpdateCWenv :: ContainerUpdateCWenvHandler s e
 defaultUpdateCWenv wenv node cnode cidx = wenv
 
@@ -571,7 +603,8 @@ mergeWrapper
   -> WidgetResult s e
 mergeWrapper container wenv newNode oldNode = newResult where
   getBaseStyle = containerGetBaseStyle container
-  updateCWenv = getUpdateCWenv container
+  createContainerFromModel = containerCreateContainerFromModel container
+
   mergeRequiredHandler = containerMergeChildrenReq container
   mergeHandler = containerMerge container
   mergePostHandler = containerMergePost container
@@ -582,8 +615,14 @@ mergeWrapper container wenv newNode oldNode = newResult where
     Nothing -> True
 
   styledNode = initNodeStyle getBaseStyle wenv newNode
+
+  -- Check if an updated container can be used for offset/layout direction.
+  pNode = pResult ^. L.node
+  updateCWenv = case useState oldState >>= createContainerFromModel wenv pNode of
+    Just newContainer -> getUpdateCWenv newContainer
+    _ -> getUpdateCWenv container
   cWenvHelper idx child = cwenv where
-    cwenv = updateCWenv wenv (pResult ^. L.node) child idx
+    cwenv = updateCWenv wenv pNode child idx
 
   pResult = mergeParent mergeHandler wenv styledNode oldNode oldState
   cResult = mergeChildren cWenvHelper wenv newNode oldNode pResult

src/Monomer/Widgets/Containers/Scroll.hs

@@ -344,12 +344,13 @@ makeNode widget managedWidget = defaultWidgetNode "scroll" widget
 
 makeScroll :: ScrollCfg s e -> ScrollState -> Widget s e
 makeScroll config state = widget where
-  widget = createContainer state def {
+  container = def {
     containerChildrenOffset = Just offset,
     containerChildrenScissor = Just (_sstScissor state),
     containerLayoutDirection = layoutDirection,
     containerGetBaseStyle = getBaseStyle,
     containerGetCurrentStyle = scrollCurrentStyle,
+    containerCreateContainerFromModel = createContainerFromModel,
     containerUpdateCWenv = updateCWenv,
     containerInit = init,
     containerMerge = merge,
@@ -360,6 +361,7 @@ makeScroll config state = widget where
     containerResize = resize,
     containerRenderAfter = renderAfter
   }
+  widget = createContainer state container
 
   ScrollState dragging dx dy _ _ _ = state
   Size childWidth childHeight = _sstChildSize state
@@ -386,6 +388,12 @@ makeScroll config state = widget where
         & L.children . ix 0 . L.info . L.style .~ childStyle
       | otherwise = node
 
+  createContainerFromModel wenv node state = Just newContainer where
+    offset = Point (_sstDeltaX state) (_sstDeltaX state)
+    newContainer = container {
+      containerChildrenOffset = Just offset
+    }
+
   -- This is overriden to account for space used by scroll bars
   updateCWenv wenv node cnode cidx = newWenv where
     theme = currentTheme wenv node