[LayoutNG] Introduce block child iterator.

third_party/WebKit/Source/core/layout/ng/ng_block_child_iterator.h

+// Copyright 2017 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 NGBlockChildIterator_h
+#define NGBlockChildIterator_h
+
+#include "core/CoreExport.h"
+#include "platform/heap/Handle.h"
+
+namespace blink {
+
+class NGLayoutInputNode;
+class NGBreakToken;
+class NGBlockBreakToken;
+
+// A utility class for block-flow layout which given the first child and a
+// break token will iterate through unfinished children.
+//
+// This class does not handle modifications to its arguments after it has been
+// constructed.
+class CORE_EXPORT NGBlockChildIterator {
+ public:
+  NGBlockChildIterator(NGLayoutInputNode* first_child,
+                       NGBlockBreakToken* break_token);
+
+  // Returns the next input node which should be laid out, along with its
+  // respective break token.
+  struct Entry;
+  Entry NextChild();
+
+ private:
+  Persistent<NGLayoutInputNode> child_;
+  Persistent<NGBlockBreakToken> break_token_;
+
+  // An index into break_token_'s ChildBreakTokens() vector. Used for keeping
+  // track of the next child break token to inspect.
+  size_t child_token_idx_;
+};
+
+struct NGBlockChildIterator::Entry {
+  Entry(NGLayoutInputNode* node, NGBreakToken* token)
+      : node(node), token(token) {}
+
+  Persistent<NGLayoutInputNode> node;
+  Persistent<NGBreakToken> token;
+
+  bool operator==(const NGBlockChildIterator::Entry& other) const {
+    return node == other.node && token == other.token;
+  }
+};
+
+}  // namespace blink
+
+#endif  // NGBlockChildIterator_h