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 261 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
+// Derive the LogMessageHandler 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();
+class BASE_EXPORT LogMessageHandler {
+ public:
+  explicit LogMessageHandler();

[grt (UTC plus 2), 2016/07/27 20:38:21]

nit: omit "explicit" and make the ctor protected, moving it down below the
public section as per
https://google.github.io/styleguide/cppguide.html#Declaration_Order.

[wychen, 2016/08/01 16:12:26]

Done.

+  // Returns true 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.
+  virtual bool OnMessage(int severity, const char* file, int line,
+      size_t message_start, const std::string& str) = 0;
+  virtual ~LogMessageHandler();
+};
+
+BASE_EXPORT unsigned LogMessageHandlerCountForTesting();
+
+// Derive the LogMessageListener that gets passed every log message before
+// it's sent to LogMessageHandler. If hijacking message is not
+// needed, use this one instead of LogMessageHandler.
+class BASE_EXPORT LogMessageListener {
+ public:
+  explicit LogMessageListener();
+  virtual void OnMessage(int severity, const char* file, int line,

[grt (UTC plus 2), 2016/07/27 20:38:21]

maybe it's simpler to replace this single-method interface with a CallbackList,
and have consumers of this API just add/remove their callback.

[grt (UTC plus 2), 2016/07/28 06:28:48]

I see that this is counter to Brett's suggestion, so nevermind. Please clean up
these class definitions so that they comply with the style guide.

[wychen, 2016/08/01 16:12:25]

CallbackList handles thread safety nicely, and I think it is faster than using a
lock. However, its type is only good for listeners, but not handlers, so we'll
have to keep LogMessageHandler anyway.

[wychen, 2016/08/01 16:12:25]

Done.

+      size_t message_start, const std::string& str) = 0;
+  virtual ~LogMessageListener();
+};
+
+BASE_EXPORT unsigned 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 694 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_