Make positionAbsoluteChild the sole place that matters when determini…

…ng absolute node's position (facebook#41684)

Summary:
X-link: facebook/yoga#1481


Absolute nodes can be laid out by themselves and do not have to care about what is happening to their siblings. Because of this we can make `positionAbsoluteChild` the sole place where we handle this logic. Right now that is scattered around algorithm with many `if (child is absolute)` cases everywhere. This makes implementing position static a lot harder since we are relying on the CB to do all this work, not the parent.

With this change the only time we set position for an absolute node and it matter (i.e. not overwritten) is in `positionAbsoluteChild`

Reviewed By: NickGerleman

Differential Revision: D51290723

packages/react-native/ReactAndroid/src/main/java/com/facebook/yoga/YogaErrata.java

@@ -14,6 +14,7 @@ public enum YogaErrata {
   STRETCH_FLEX_BASIS(1),
   STARTING_ENDING_EDGE_FROM_FLEX_DIRECTION(2),
   POSITION_STATIC_BEHAVES_LIKE_RELATIVE(4),
+  ABSOLUTE_POSITIONING(8),
   ALL(2147483647),
   CLASSIC(2147483646);
 
@@ -33,6 +34,7 @@ public static YogaErrata fromInt(int value) {
       case 1: return STRETCH_FLEX_BASIS;
       case 2: return STARTING_ENDING_EDGE_FROM_FLEX_DIRECTION;
       case 4: return POSITION_STATIC_BEHAVES_LIKE_RELATIVE;
+      case 8: return ABSOLUTE_POSITIONING;
       case 2147483647: return ALL;
       case 2147483646: return CLASSIC;
       default: throw new IllegalArgumentException("Unknown enum value: " + value);

packages/react-native/ReactCommon/yoga/yoga/YGEnums.cpp

@@ -99,6 +99,8 @@ const char* YGErrataToString(const YGErrata value) {
       return "starting-ending-edge-from-flex-direction";
     case YGErrataPositionStaticBehavesLikeRelative:
       return "position-static-behaves-like-relative";
+    case YGErrataAbsolutePositioning:
+      return "absolute-positioning";
     case YGErrataAll:
       return "all";
     case YGErrataClassic:

packages/react-native/ReactCommon/yoga/yoga/YGEnums.h

@@ -58,6 +58,7 @@ YG_ENUM_DECL(
     YGErrataStretchFlexBasis = 1,
     YGErrataStartingEndingEdgeFromFlexDirection = 2,
     YGErrataPositionStaticBehavesLikeRelative = 4,
+    YGErrataAbsolutePositioning = 8,
     YGErrataAll = 2147483647,
     YGErrataClassic = 2147483646)
 YG_DEFINE_ENUM_FLAG_OPERATORS(YGErrata)

packages/react-native/ReactCommon/yoga/yoga/algorithm/AbsoluteLayout.cpp

@@ -13,23 +13,88 @@
 
 namespace facebook::yoga {
 
-/*
- * Absolutely positioned nodes do not participate in flex layout and thus their
- * positions can be determined independently from the rest of their siblings.
- * For each axis there are essentially two cases:
- *
- * 1) The node has insets defined. In this case we can just use these to
- *    determine the position of the node.
- * 2) The node does not have insets defined. In this case we look at the style
- *    of the parent to position the node. Things like justify content and
- *    align content will move absolute children around. If none of these
- *    special properties are defined, the child is positioned at the start
- *    (defined by flex direction) of the leading flex line.
- *
- * This function does that positioning for the given axis. The spec has more
- * information on this topic: https://www.w3.org/TR/css-flexbox-1/#abspos-items
- */
-static void positionAbsoluteChild(
+static void justifyAbsoluteChild(
+    const yoga::Node* const parent,
+    yoga::Node* child,
+    const Direction direction,
+    const FlexDirection mainAxis,
+    const float containingBlockWidth) {
+  const Justify parentJustifyContent = parent->getStyle().justifyContent();
+  switch (parentJustifyContent) {
+    case Justify::FlexStart:
+    case Justify::SpaceBetween:
+      child->setLayoutPosition(
+          child->getFlexStartMargin(mainAxis, direction, containingBlockWidth) +
+              parent->getFlexStartBorder(mainAxis, direction),
+          flexStartEdge(mainAxis));
+      break;
+    case Justify::FlexEnd:
+      child->setLayoutPosition(
+          (parent->getLayout().measuredDimension(dimension(mainAxis)) -
+           child->getLayout().measuredDimension(dimension(mainAxis))),
+          flexStartEdge(mainAxis));
+      break;
+    case Justify::Center:
+    case Justify::SpaceAround:
+    case Justify::SpaceEvenly:
+      child->setLayoutPosition(
+          (parent->getLayout().measuredDimension(dimension(mainAxis)) -
+           child->getLayout().measuredDimension(dimension(mainAxis))) /
+              2.0f,
+          flexStartEdge(mainAxis));
+      break;
+  }
+}
+
+static void alignAbsoluteChild(
+    const yoga::Node* const parent,
+    yoga::Node* child,
+    const Direction direction,
+    const FlexDirection crossAxis,
+    const float containingBlockWidth) {
+  Align itemAlign = resolveChildAlignment(parent, child);
+  const Wrap parentWrap = parent->getStyle().flexWrap();
+  if (parentWrap == Wrap::WrapReverse) {
+    if (itemAlign == Align::FlexEnd) {
+      itemAlign = Align::FlexStart;
+    } else if (itemAlign != Align::Center) {
+      itemAlign = Align::FlexEnd;
+    }
+  }
+
+  switch (itemAlign) {
+    case Align::Auto:
+    case Align::FlexStart:
+    case Align::Baseline:
+    case Align::SpaceAround:
+    case Align::SpaceBetween:
+    case Align::Stretch:
+    case Align::SpaceEvenly:
+      child->setLayoutPosition(
+          parent->getFlexStartBorder(crossAxis, direction) +
+              child->getFlexStartMargin(
+                  crossAxis, direction, containingBlockWidth),
+          flexStartEdge(crossAxis));
+      break;
+    case Align::FlexEnd:
+      child->setLayoutPosition(
+          (parent->getLayout().measuredDimension(dimension(crossAxis)) -
+           child->getLayout().measuredDimension(dimension(crossAxis))),
+          flexStartEdge(crossAxis));
+      break;
+    case Align::Center:
+      child->setLayoutPosition(
+          (parent->getLayout().measuredDimension(dimension(crossAxis)) -
+           child->getLayout().measuredDimension(dimension(crossAxis))) /
+              2.0f,
+          flexStartEdge(crossAxis));
+      break;
+  }
+}
+
+// To ensure no breaking changes, we preserve the legacy way of positioning
+// absolute children and determine if we should use it using an errata.
+static void positionAbsoluteChildLegacy(
     const yoga::Node* const containingNode,
     const yoga::Node* const parent,
     yoga::Node* child,
@@ -93,6 +158,109 @@ static void positionAbsoluteChild(
   }
 }
 
+/*
+ * Absolutely positioned nodes do not participate in flex layout and thus their
+ * positions can be determined independently from the rest of their siblings.
+ * For each axis there are essentially two cases:
+ *
+ * 1) The node has insets defined. In this case we can just use these to
+ *    determine the position of the node.
+ * 2) The node does not have insets defined. In this case we look at the style
+ *    of the parent to position the node. Things like justify content and
+ *    align content will move absolute children around. If none of these
+ *    special properties are defined, the child is positioned at the start
+ *    (defined by flex direction) of the leading flex line.
+ *
+ * This function does that positioning for the given axis. The spec has more
+ * information on this topic: https://www.w3.org/TR/css-flexbox-1/#abspos-items
+ */
+static void positionAbsoluteChildImpl(
+    const yoga::Node* const containingNode,
+    const yoga::Node* const parent,
+    yoga::Node* child,
+    const Direction direction,
+    const FlexDirection axis,
+    const bool isMainAxis,
+    const float containingBlockWidth,
+    const float containingBlockHeight) {
+  const bool isAxisRow = isRow(axis);
+  const float containingBlockSize =
+      isAxisRow ? containingBlockWidth : containingBlockHeight;
+
+  // The inline-start position takes priority over the end position in the case
+  // that they are both set and the node has a fixed width. Thus we only have 2
+  // cases here: if inline-start is defined and if inline-end is defined.
+  //
+  // Despite checking inline-start to honor prioritization of insets, we write
+  // to the flex-start edge because this algorithm works by positioning on the
+  // flex-start edge and then filling in the flex-end direction at the end if
+  // necessary.
+  if (child->isInlineStartPositionDefined(axis, direction)) {
+    const float positionRelativeToInlineStart =
+        child->getInlineStartPosition(
+            axis,
+            direction,
+            containingNode->getLayout().measuredDimension(dimension(axis))) +
+        containingNode->getInlineStartBorder(axis, direction) +
+        child->getInlineStartMargin(axis, direction, containingBlockSize);
+    const float positionRelativeToFlexStart =
+        inlineStartEdge(axis, direction) != flexStartEdge(axis)
+        ? getPositionOfOppositeEdge(
+              positionRelativeToInlineStart, axis, containingNode, child)
+        : positionRelativeToInlineStart;
+
+    child->setLayoutPosition(positionRelativeToFlexStart, flexStartEdge(axis));
+  } else if (child->isInlineEndPositionDefined(axis, direction)) {
+    const float positionRelativeToInlineStart =
+        containingNode->getLayout().measuredDimension(dimension(axis)) -
+        child->getLayout().measuredDimension(dimension(axis)) -
+        containingNode->getInlineEndBorder(axis, direction) -
+        child->getInlineEndMargin(axis, direction, containingBlockSize) -
+        child->getInlineEndPosition(axis, direction, containingBlockSize);
+    const float positionRelativeToFlexStart =
+        inlineStartEdge(axis, direction) != flexStartEdge(axis)
+        ? getPositionOfOppositeEdge(
+              positionRelativeToInlineStart, axis, containingNode, child)
+        : positionRelativeToInlineStart;
+
+    child->setLayoutPosition(positionRelativeToFlexStart, flexStartEdge(axis));
+  } else {
+    isMainAxis ? justifyAbsoluteChild(
+                     parent, child, direction, axis, containingBlockWidth)
+               : alignAbsoluteChild(
+                     parent, child, direction, axis, containingBlockWidth);
+  }
+}
+
+static void positionAbsoluteChild(
+    const yoga::Node* const containingNode,
+    const yoga::Node* const parent,
+    yoga::Node* child,
+    const Direction direction,
+    const FlexDirection axis,
+    const bool isMainAxis,
+    const float containingBlockWidth,
+    const float containingBlockHeight) {
+  child->hasErrata(Errata::AbsolutePositioning) ? positionAbsoluteChildLegacy(
+                                                      containingNode,
+                                                      parent,
+                                                      child,
+                                                      direction,
+                                                      axis,
+                                                      isMainAxis,
+                                                      containingBlockWidth,
+                                                      containingBlockHeight)
+                                                : positionAbsoluteChildImpl(
+                                                      containingNode,
+                                                      parent,
+                                                      child,
+                                                      direction,
+                                                      axis,
+                                                      isMainAxis,
+                                                      containingBlockWidth,
+                                                      containingBlockHeight);
+}
+
 void layoutAbsoluteChild(
     const yoga::Node* const containingNode,
     const yoga::Node* const node,
@@ -155,8 +323,8 @@ void layoutAbsoluteChild(
                       .unwrap() +
         marginColumn;
   } else {
-    // If the child doesn't have a specified height, compute the height based on
-    // the top/bottom offsets if they're defined.
+    // If the child doesn't have a specified height, compute the height based
+    // on the top/bottom offsets if they're defined.
     if (child->isFlexStartPositionDefined(FlexDirection::Column, direction) &&
         child->isFlexEndPositionDefined(FlexDirection::Column, direction)) {
       childHeight =
@@ -203,9 +371,9 @@ void layoutAbsoluteChild(
         : SizingMode::StretchFit;
 
     // If the size of the owner is defined then try to constrain the absolute
-    // child to that size as well. This allows text within the absolute child to
-    // wrap to the size of its owner. This is the same behavior as many browsers
-    // implement.
+    // child to that size as well. This allows text within the absolute child
+    // to wrap to the size of its owner. This is the same behavior as many
+    // browsers implement.
     if (!isMainAxisRow && yoga::isUndefined(childWidth) &&
         widthMode != SizingMode::MaxContent &&
         yoga::isDefined(containingBlockWidth) && containingBlockWidth > 0) {

packages/react-native/ReactCommon/yoga/yoga/algorithm/CalculateLayout.h

@@ -35,14 +35,25 @@ bool calculateLayoutInternal(
     const uint32_t depth,
     const uint32_t generationCount);
 
+// Given an offset to an edge, returns the offset to the opposite edge on the
+// same axis. This assumes that the width/height of both nodes is determined at
+// this point.
+inline float getPositionOfOppositeEdge(
+    float position,
+    FlexDirection axis,
+    const yoga::Node* const containingNode,
+    const yoga::Node* const node) {
+  return containingNode->getLayout().measuredDimension(dimension(axis)) -
+      node->getLayout().measuredDimension(dimension(axis)) - position;
+}
+
 inline void setChildTrailingPosition(
     const yoga::Node* const node,
     yoga::Node* const child,
     const FlexDirection axis) {
-  const float size = child->getLayout().measuredDimension(dimension(axis));
   child->setLayoutPosition(
-      node->getLayout().measuredDimension(dimension(axis)) - size -
-          child->getLayout().position(flexStartEdge(axis)),
+      getPositionOfOppositeEdge(
+          child->getLayout().position(flexStartEdge(axis)), axis, node, child),
       flexEndEdge(axis));
 }
 

packages/react-native/ReactCommon/yoga/yoga/enums/Errata.h

@@ -20,6 +20,7 @@ enum class Errata : uint32_t {
   StretchFlexBasis = YGErrataStretchFlexBasis,
   StartingEndingEdgeFromFlexDirection = YGErrataStartingEndingEdgeFromFlexDirection,
   PositionStaticBehavesLikeRelative = YGErrataPositionStaticBehavesLikeRelative,
+  AbsolutePositioning = YGErrataAbsolutePositioning,
   All = YGErrataAll,
   Classic = YGErrataClassic,
 };