Add sequence number to undo steps

third_party/WebKit/Source/core/editing/commands/UndoStack.cpp

 /*
  * Copyright (C) 2006, 2007 Apple, Inc.  All rights reserved.
  * Copyright (C) 2012 Google, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 20 matching lines @@
 #include "wtf/AutoReset.h"
 
 namespace blink {
 
 // Arbitrary depth limit for the undo stack, to keep it from using
 // unbounded memory. This is the maximum number of distinct undoable
 // actions -- unbroken stretches of typed characters are coalesced
 // into a single action.
 static const size_t maximumUndoStackDepth = 1000;
 
-UndoStack::UndoStack() : m_inRedo(false) {}
+UndoStack::UndoStack() : m_maxSequence(0), m_inRedo(false) {}
 
 UndoStack* UndoStack::create() {
   return new UndoStack();
 }
 
 void UndoStack::registerUndoStep(UndoStep* step) {
   if (m_undoStack.size() == maximumUndoStackDepth)
     m_undoStack.removeFirst();  // drop oldest item off the far end
-  if (!m_inRedo)
+  if (!m_inRedo) {
     m_redoStack.clear();
+    step->setSequence(++m_maxSequence);

[yosin_UTC9, 2017/01/17 08:13:29]

Usage of |m_maxSequence| isn't clear. Why increment here?

[Xiaocheng, 2017/01/17 09:04:02]

This is the place where a new undo step is pushed to the stack.
We don't want to set the sequence of an undo step if it's just popped from the
redo stack.

+  }
   m_undoStack.append(step);
 }
 
 void UndoStack::registerRedoStep(UndoStep* step) {
   m_redoStack.append(step);
 }
 
 bool UndoStack::canUndo() const {
   return !m_undoStack.isEmpty();
 }
@@ skipping 29 matching lines @@
   m_undoStack.clear();
   m_redoStack.clear();
 }
 
 DEFINE_TRACE(UndoStack) {
   visitor->trace(m_undoStack);
   visitor->trace(m_redoStack);
 }
 
 }  // namespace blink