[LayoutNG] Introduce block child iterator.

third_party/WebKit/Source/core/layout/ng/ng_block_break_token.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 NGBlockBreakToken_h
 #define NGBlockBreakToken_h
 
+#include "core/CoreExport.h"
 #include "core/layout/ng/ng_break_token.h"
 #include "platform/LayoutUnit.h"
 
 namespace blink {
 
 class NGBlockNode;
 
 // Represents a break token for a block node.
-class NGBlockBreakToken : public NGBreakToken {
+class CORE_EXPORT NGBlockBreakToken : public NGBreakToken {
  public:
   // Creates a break token for a node which did fragment, and can potentially
   // produce more fragments.
+  //
+  // The NGBlockBreakToken takes ownership of child_break_tokens, leaving it
+  // empty for the caller.
   NGBlockBreakToken(NGBlockNode* node,
                     LayoutUnit used_block_size,
-                    HeapVector<Member<NGBreakToken>>& child_break_tokens)
-      : NGBreakToken(kBlockBreakToken, /* is_finished */ false, node),
-        used_block_size_(used_block_size) {
-    child_break_tokens_.swap(child_break_tokens);
-  }
+                    HeapVector<Member<NGBreakToken>>& child_break_tokens);
 
   // Creates a break token for a node which cannot produce any more fragments.
-  explicit NGBlockBreakToken(NGBlockNode* node)
-      : NGBreakToken(kBlockBreakToken, /* is_finished */ true, node) {}
+  explicit NGBlockBreakToken(NGBlockNode* node);
 
   // TODO(ikilpatrick): Remove this constructor and break_offset once we've
   // switched to new multi-col approach.
-  NGBlockBreakToken(NGBlockNode* node, LayoutUnit break_offset)
-      : NGBreakToken(kBlockBreakToken, false, node),
-        break_offset_(break_offset) {}
+  NGBlockBreakToken(NGBlockNode* node, LayoutUnit break_offset);
 
   // Represents the amount of block size used in previous fragments.
   //
   // E.g. if the layout block specifies a block size of 200px, and the previous
   // fragments of this block used 150px (used block size), the next fragment
   // should have a size of 50px (assuming no additional fragmentation).
   LayoutUnit UsedBlockSize() const { return used_block_size_; }
 
   // The break tokens for children of the layout node.
   //
-  // This is used to resume layout of any children which fragmented, it may
-  // contain "finished" break tokens so we know which children to skip for the
-  // next fragment.
+  // Each child we have visited previously in the block-flow layout algorithm
+  // has an associated break token. This may be either finished (we should skip
+  // this child) or unfinished (we should try and produce the next fragment for
+  // this child).
+  //
+  // A child which we haven't visited yet doesn't have a break token here.
   const HeapVector<Member<NGBreakToken>>& ChildBreakTokens() const {
     return child_break_tokens_;
   }
 
   // TODO(ikilpatrick): Remove this accessor.
   LayoutUnit BreakOffset() const { return break_offset_; }
 
   DEFINE_INLINE_VIRTUAL_TRACE() {
     NGBreakToken::trace(visitor);
     visitor->trace(child_break_tokens_);
   }
 
  private:
   LayoutUnit break_offset_;
   LayoutUnit used_block_size_;
   HeapVector<Member<NGBreakToken>> child_break_tokens_;
 };
 
 DEFINE_TYPE_CASTS(NGBlockBreakToken,
                   NGBreakToken,
                   token,
                   token->Type() == NGBreakToken::kBlockBreakToken,
                   token.Type() == NGBreakToken::kBlockBreakToken);
 
 }  // namespace blink
 
 #endif  // NGBlockBreakToken_h