Custom log handler setup

Summary: Add custom log handler setup so people can use their own log infra instead of "printf".

Reviewed By: Neil-Clifford-FB

Differential Revision: D46590343

fbshipit-source-id: ded9cf6caa580d477a71a9155cad9db295151c84

xplat/Flipper/Log.cpp

@@ -14,7 +14,26 @@
 namespace facebook {
 namespace flipper {
 
+namespace {
+static LogHandlerFunc* getHandle() {
+  static LogHandlerFunc sHandler = defaultLogHandler;
+  return &sHandler;
+}
+} // namespace
+
 void log(const std::string& message) {
+  return (*getHandle())(message);
+}
+
+void setLogHandler(LogHandlerFunc handler) {
+  *getHandle() = handler;
+}
+
+LogHandlerFunc getLogHandler() {
+  return *getHandle();
+}
+
+void defaultLogHandler(const std::string& message) {
 #ifdef __ANDROID__
   __android_log_print(
       ANDROID_LOG_INFO, "flipper", "flipper: %s", message.c_str());

xplat/Flipper/Log.h

@@ -14,5 +14,11 @@ namespace flipper {
 
 void log(const std::string& message);
 
+using LogHandlerFunc = void (*)(const std::string& message);
+
+void setLogHandler(LogHandlerFunc handler);
+LogHandlerFunc getLogHandler();
+void defaultLogHandler(const std::string& message);
+
 } // namespace flipper
 } // namespace facebook