import FIREFOX_52_6_0esr_RELEASE from mozilla-esr52 hg repo

dom/base/nsStubDocumentObserver.h

@@ -0,0 +1,35 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * nsStubDocumentObserver is an implementation of the nsIDocumentObserver
+ * interface (except for the methods on nsISupports) that is intended to be
+ * used as a base class within the content/layout library.  All methods do
+ * nothing.
+ */
+
+#ifndef nsStubDocumentObserver_h_
+#define nsStubDocumentObserver_h_
+
+#include "nsIDocumentObserver.h"
+
+/**
+ * There are two advantages to inheriting from nsStubDocumentObserver
+ * rather than directly from nsIDocumentObserver:
+ *  1. smaller compiled code size (since there's no need for the code
+ *     for the empty virtual function implementations for every
+ *     nsIDocumentObserver implementation)
+ *  2. the performance of document's loop over observers benefits from
+ *     the fact that more of the functions called are the same (which
+ *     can reduce instruction cache misses and perhaps improve branch
+ *     prediction)
+ */
+class nsStubDocumentObserver : public nsIDocumentObserver {
+public:
+  NS_DECL_NSIDOCUMENTOBSERVER
+};
+
+#endif /* !defined(nsStubDocumentObserver_h_) */