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

base/logging.h

Index: base/logging.h
diff --git a/base/logging.h b/base/logging.h
index 08189b6114bc020f10905d83e4165bdca67126a1..f76572f0f8633632f5ef8c5ca6112183422086f6 100644
--- a/base/logging.h
+++ b/base/logging.h
@@ -279,14 +279,36 @@ BASE_EXPORT void SetShowErrorDialogs(bool enable_dialogs);
 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
+// 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;

[grt (UTC plus 2), 2016/08/05 20:13:30]

please use "git cl format"

[wychen, 2016/08/12 21:32:39]

Done.

+ protected:

[grt (UTC plus 2), 2016/08/05 20:13:30]

nit: blank line before this (below, too)

[wychen, 2016/08/12 21:32:39]

Done.

+  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