Add api for retrieving computed padding

Summary: Add API for retrieving the computed final padding of a node. Many frameworks such as React Native retrieve padding via `YGNodeStyleGetPadding` but given that we now support percentage values this is not correct anymore.

Differential Revision: D4376572

fbshipit-source-id: 3ffb66e77090fc1257511bec5c933f9b0c304b9f

tests/YGComputedPaddingTest.cpp

@@ -0,0 +1,30 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#include <yoga/Yoga.h>
+#include <gtest/gtest.h>
+
+TEST(YogaTest, computed_layout_padding) {
+  const YGNodeRef root = YGNodeNew();
+  YGNodeStyleSetWidth(root, 100);
+  YGNodeStyleSetHeight(root, 100);
+  YGNodeStyleSetPaddingPercent(root, YGEdgeStart, 10);
+
+  YGNodeCalculateLayout(root, 100, 100, YGDirectionLTR);
+
+  ASSERT_FLOAT_EQ(10, YGNodeLayoutGetPadding(root, YGEdgeLeft));
+  ASSERT_FLOAT_EQ(0, YGNodeLayoutGetPadding(root, YGEdgeRight));
+
+  YGNodeCalculateLayout(root, 100, 100, YGDirectionRTL);
+
+  ASSERT_FLOAT_EQ(0, YGNodeLayoutGetPadding(root, YGEdgeLeft));
+  ASSERT_FLOAT_EQ(10, YGNodeLayoutGetPadding(root, YGEdgeRight));
+
+  YGNodeFreeRecursive(root);
+}

yoga/Yoga.c

@@ -47,6 +47,7 @@ typedef struct YGCachedMeasurement {
 typedef struct YGLayout {
   float position[4];
   float dimensions[2];
+  float padding[6];
   YGDirection direction;
 
   uint32_t computedFlexBasisGeneration;
@@ -563,6 +564,28 @@ YG_NODE_LAYOUT_PROPERTY_IMPL(float, Width, dimensions[YGDimensionWidth]);
 YG_NODE_LAYOUT_PROPERTY_IMPL(float, Height, dimensions[YGDimensionHeight]);
 YG_NODE_LAYOUT_PROPERTY_IMPL(YGDirection, Direction, direction);
 
+float YGNodeLayoutGetPadding(const YGNodeRef node, const YGEdge edge) {
+  YG_ASSERT(edge <= YGEdgeEnd, "Cannot get layout paddings of multi-edge shorthands");
+
+  if (edge == YGEdgeLeft) {
+    if (node->layout.direction == YGDirectionRTL) {
+      return node->layout.padding[YGEdgeEnd];
+    } else {
+      return node->layout.padding[YGEdgeStart];
+    }
+  }
+
+  if (edge == YGEdgeRight) {
+    if (node->layout.direction == YGDirectionRTL) {
+      return node->layout.padding[YGEdgeStart];
+    } else {
+      return node->layout.padding[YGEdgeEnd];
+    }
+  }
+
+  return node->layout.padding[edge];
+}
+
 uint32_t gCurrentGenerationCount = 0;
 
 bool YGLayoutNodeInternal(const YGNodeRef node,
@@ -1657,6 +1680,11 @@ static void YGNodelayoutImpl(const YGNodeRef node,
   const YGDirection direction = YGNodeResolveDirection(node, parentDirection);
   node->layout.direction = direction;
 
+  node->layout.padding[YGEdgeStart] = YGNodeLeadingPadding(node, YGFlexDirectionResolve(YGFlexDirectionRow, direction), parentWidth);
+  node->layout.padding[YGEdgeEnd] = YGNodeTrailingPadding(node, YGFlexDirectionResolve(YGFlexDirectionRow, direction), parentWidth);
+  node->layout.padding[YGEdgeTop] = YGNodeLeadingPadding(node, YGFlexDirectionResolve(YGFlexDirectionColumn, direction), parentWidth);
+  node->layout.padding[YGEdgeBottom] = YGNodeTrailingPadding(node, YGFlexDirectionResolve(YGFlexDirectionColumn, direction), parentWidth);
+
   if (node->measure) {
     YGNodeWithMeasureFuncSetMeasuredDimensions(
         node, availableWidth, availableHeight, widthMeasureMode, heightMeasureMode);

yoga/Yoga.h

@@ -191,6 +191,12 @@ YG_NODE_LAYOUT_PROPERTY(float, Width);
 YG_NODE_LAYOUT_PROPERTY(float, Height);
 YG_NODE_LAYOUT_PROPERTY(YGDirection, Direction);
 
+// Get the computed padding for this node after performing layout. If padding was set using
+// pixel values then the returned value will be the same as YGNodeStyleGetPadding. However if
+// padding was set using a percentage value then the returned value is the computed value used
+// during layout.
+WIN_EXPORT float YGNodeLayoutGetPadding(const YGNodeRef node, const YGEdge edge);
+
 WIN_EXPORT void YGSetLogger(YGLogger logger);
 WIN_EXPORT void YGLog(YGLogLevel level, const char *message, ...);