Allow multiple logging::LogMessage{Handler,Listener}s

base/logging.h

 // Copyright (c) 2012 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 BASE_LOGGING_H_
 #define BASE_LOGGING_H_
 
 #include <stddef.h>
 
 #include <cassert>
@@ skipping 262 matching lines @@
 // Dialogs are not shown by default.
 BASE_EXPORT void SetShowErrorDialogs(bool enable_dialogs);
 
 // Sets the Log Assert Handler that will be used to notify of check failures.
 // The default handler shows a dialog box and then terminate the process,
 // however clients can use this function to override with their own handling
 // (e.g. a silent one for Unit Tests)
 typedef void (*LogAssertHandlerFunction)(const std::string& str);
 BASE_EXPORT void SetLogAssertHandler(LogAssertHandlerFunction handler);
 
-// Sets the Log Message Handler that gets passed every log message before
-// it's sent to other log destinations (if any).
-// Returns true to signal that it handled the message and the message
-// should not be sent to other log destinations.
-typedef bool (*LogMessageHandlerFunction)(int severity,
-    const char* file, int line, size_t message_start, const std::string& str);
-BASE_EXPORT void SetLogMessageHandler(LogMessageHandlerFunction handler);
-BASE_EXPORT LogMessageHandlerFunction GetLogMessageHandler();
+// LogMessageHandler is the callback interface for log message handling.
+// OnMessage() is called for every log message before it's sent to other log

[grt (UTC plus 2), 2017/01/04 09:12:19]

nit: i think it's more clear to say messages are sent here after all listeners
and before other destinations. i also think it's more clear to define listeners
first since they are recommended (as per comment in LogMessageListener) and
because they see messages before handlers.

+// destinations (if any), but after LogMessageListener.
+// Return true in OnMessage() to signal that it handled the message and the
+// message should not be sent to other log destinations, including other
+// handlers added before the current one.
+class BASE_EXPORT LogMessageHandler {
+ public:
+  virtual ~LogMessageHandler();
+  virtual bool OnMessage(int severity,
+                         const char* file,
+                         int line,
+                         size_t message_start,
+                         const std::string& str) = 0;
+
+ protected:
+  LogMessageHandler();
+};
+
+BASE_EXPORT size_t LogMessageHandlerCountForTesting();
+
+// LogMessageListener is similar to LogMessageHandler except that this
+// interface doesn't support message hijacking, and is preferred over
+// LogMessageHandler if hijacking is not needed.
+class BASE_EXPORT LogMessageListener {
+ public:
+  virtual ~LogMessageListener();
+  virtual void OnMessage(int severity,
+                         const char* file,
+                         int line,
+                         size_t message_start,
+                         const std::string& str) = 0;
+
+ protected:
+  LogMessageListener();
+};
+
+BASE_EXPORT size_t LogMessageListenerCountForTesting();
 
 typedef int LogSeverity;
 const LogSeverity LOG_VERBOSE = -1;  // This is level 1 verbosity
 // Note: the log severities are used to index into the array of names,
 // see log_severity_names.
 const LogSeverity LOG_INFO = 0;
 const LogSeverity LOG_WARNING = 1;
 const LogSeverity LOG_ERROR = 2;
 const LogSeverity LOG_FATAL = 3;
 const LogSeverity LOG_NUM_SEVERITIES = 4;
@@ skipping 717 matching lines @@
 #elif NOTIMPLEMENTED_POLICY == 5
 #define NOTIMPLEMENTED() do {\
   static bool logged_once = false;\
   LOG_IF(ERROR, !logged_once) << NOTIMPLEMENTED_MSG;\
   logged_once = true;\
 } while(0);\
 EAT_STREAM_PARAMETERS
 #endif
 
 #endif  // BASE_LOGGING_H_