[css-grid] Implementation of Baseline Self-Alignment

third_party/WebKit/Source/core/layout/BaselineAlignment.h

+// Copyright (c) 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef BaselineAlignment_h
+#define BaselineAlignment_h
+
+#include "core/layout/LayoutBox.h"
+
+namespace blink {
+
+// These classes are used to implement the Baseline Alignment logica, as
+// described in the CSS Box Alignment specification.
+// https://drafts.csswg.org/css-align/#baseline-terms
+
+// A baseline-sharing group is composed of boxes that participate in
+// baseline alignment together. This is possible only if they:
+//
+//   * Share an alignment context along an axis perpendicular to their
+//   baseline alignment axis.
+//   * Have compatible baseline alignment preferences (i.e., the baselines
+//   that want to align are on the same side of the alignment context).
+class BaselineGroup {
+ public:
+  void update(const LayoutBox&, LayoutUnit ascent, LayoutUnit descent);
+  LayoutUnit maxAscent() const { return m_maxAscent; }
+  LayoutUnit maxDescent() const { return m_maxDescent; }
+  int size() const { return m_items.size(); }
+
+ private:
+  friend class BaselineContext;
+  BaselineGroup(WritingMode blockFlow, ItemPosition childPreference);
+  bool isCompatible(WritingMode, ItemPosition) const;
+  bool isOppositeBlockFlow(WritingMode blockFlow) const;
+  bool isOrthogonalBlockFlow(WritingMode blockFlow) const;
+
+  WritingMode m_blockFlow;
+  ItemPosition m_preference;
+  LayoutUnit m_maxAscent;
+  LayoutUnit m_maxDescent;
+  HashSet<const LayoutBox*> m_items;
+};
+
+// Boxes share an alignment context along a particular axis when they
+// are:
+//
+//  * table cells in the same row, along the table's row (inline) axis
+//  * table cells in the same column, along the table's column (block)
+//  axis
+//  * grid items in the same row, along the grid's row (inline) axis
+//  * grid items in the same column, along the grid's colum (block) axis
+//  * flex items in the same flex line, along the flex container's main
+//  axis
+class BaselineContext {
+ public:
+  BaselineContext(const LayoutBox& child,
+                  ItemPosition preference,
+                  LayoutUnit ascent,
+                  LayoutUnit descent);
+  Vector<BaselineGroup>& sharedGroups() { return m_sharedGroups; }
+  const BaselineGroup& getSharedGroup(const LayoutBox& child,
+                                      ItemPosition preference) const;
+  void updateSharedGroup(const LayoutBox& child,
+                         ItemPosition preference,
+                         LayoutUnit ascent,
+                         LayoutUnit descent);
+
+ private:
+  // TODO Properly implement baseline-group compatibility
+  // See https://github.com/w3c/csswg-drafts/issues/721
+  BaselineGroup& findCompatibleSharedGroup(const LayoutBox& child,
+                                           ItemPosition preference);
+
+  Vector<BaselineGroup> m_sharedGroups;
+};
+
+static inline bool isBaselinePosition(ItemPosition position) {
+  return position == ItemPositionBaseline ||
+         position == ItemPositionLastBaseline;
+}
+
+}  // namespace blink
+
+#endif  // BaselineContext_h