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 f06535dada056725121d0a31c0c745bca9c8f0f0..b4739039417538e62e31a9f50de92c883da9412a 100644
--- a/base/logging.h
+++ b/base/logging.h
@@ -279,14 +279,33 @@ 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
+// 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