Dont measure single flex grow+shrink child

Reviewed By: gkassabli

Differential Revision: D4153133

fbshipit-source-id: 2333150a83857cc30078cc8d52761cbd00652830

React/CSSLayout/CSSLayout.c

@@ -286,7 +286,6 @@ static void _CSSNodeMarkDirty(const CSSNodeRef node) {
 }
 
 void CSSNodeSetMeasureFunc(const CSSNodeRef node, CSSMeasureFunc measureFunc) {
-  // You can always NULLify the measure function of a node.
   if (measureFunc == NULL) {
     node->measure = NULL;
   } else {
@@ -301,7 +300,8 @@ CSSMeasureFunc CSSNodeGetMeasureFunc(const CSSNodeRef node) {
 
 void CSSNodeInsertChild(const CSSNodeRef node, const CSSNodeRef child, const uint32_t index) {
   CSS_ASSERT(child->parent == NULL, "Child already has a parent, it must be removed first.");
-  CSS_ASSERT(node->measure == NULL, "Cannot add child: Nodes with measure functions cannot have children.");
+  CSS_ASSERT(node->measure == NULL,
+             "Cannot add child: Nodes with measure functions cannot have children.");
   CSSNodeListInsert(&node->children, child, index);
   child->parent = node;
   _CSSNodeMarkDirty(node);
@@ -1399,6 +1399,26 @@ static void layoutNodeImpl(const CSSNodeRef node,
   const float availableInnerMainDim = isMainAxisRow ? availableInnerWidth : availableInnerHeight;
   const float availableInnerCrossDim = isMainAxisRow ? availableInnerHeight : availableInnerWidth;
 
+  // If there is only one child with flexGrow + flexShrink it means we can set the
+  // computedFlexBasis to 0 instead of measuring and shrinking / flexing the child to exactly
+  // match the remaining space
+  CSSNodeRef singleFlexChild = NULL;
+  if ((isMainAxisRow && widthMeasureMode == CSSMeasureModeExactly) ||
+      (!isMainAxisRow && heightMeasureMode == CSSMeasureModeExactly)) {
+    for (uint32_t i = 0; i < childCount; i++) {
+      const CSSNodeRef child = CSSNodeGetChild(node, i);
+      if (singleFlexChild) {
+        if (isFlex(child)) {
+          // There is already a flexible child, abort.
+          singleFlexChild = NULL;
+          break;
+        }
+      } else if (CSSNodeStyleGetFlexGrow(child) > 0 && CSSNodeStyleGetFlexShrink(child) > 0) {
+        singleFlexChild = child;
+      }
+    }
+  }
+
   // STEP 3: DETERMINE FLEX BASIS FOR EACH ITEM
   for (uint32_t i = 0; i < childCount; i++) {
     const CSSNodeRef child = CSSNodeListGet(node->children, i);
@@ -1422,6 +1442,9 @@ static void layoutNodeImpl(const CSSNodeRef node,
       }
       currentAbsoluteChild = child;
       child->nextChild = NULL;
+    } else {
+      if (child == singleFlexChild) {
+        child->layout.computedFlexBasis = 0;
       } else {
         computeChildFlexBasis(node,
                               child,
@@ -1432,6 +1455,7 @@ static void layoutNodeImpl(const CSSNodeRef node,
                               direction);
       }
     }
+  }
 
   // STEP 4: COLLECT FLEX ITEMS INTO FLEX LINES
 
@@ -2193,19 +2217,16 @@ bool CSSNodeCanUseCachedMeasurement(const CSSMeasureMode widthMode,
       newMeasureSizeIsStricterAndStillValid(
           widthMode, width - marginRow, lastWidthMode, lastWidth, lastComputedWidth);
 
-  const bool heightIsCompatible = hasSameHeightSpec ||
-                                  newSizeIsExactAndMatchesOldMeasuredSize(heightMode,
+  const bool heightIsCompatible =
+      hasSameHeightSpec || newSizeIsExactAndMatchesOldMeasuredSize(heightMode,
                                                                    height - marginColumn,
                                                                    lastComputedHeight) ||
       oldSizeIsUnspecifiedAndStillFits(heightMode,
                                        height - marginColumn,
                                        lastHeightMode,
                                        lastComputedHeight) ||
-                                  newMeasureSizeIsStricterAndStillValid(heightMode,
-                                                                        height - marginColumn,
-                                                                        lastHeightMode,
-                                                                        lastHeight,
-                                                                        lastComputedHeight);
+      newMeasureSizeIsStricterAndStillValid(
+          heightMode, height - marginColumn, lastHeightMode, lastHeight, lastComputedHeight);
 
   return widthIsCompatible && heightIsCompatible;
 }