support flush_on(..) in async loggers too

example/example.cpp

@@ -102,6 +102,10 @@ void async_example()
     size_t q_size = 4096; //queue size must be power of 2
     spdlog::set_async_mode(q_size);
     auto async_file = spd::daily_logger_st("async_file_logger", "logs/async_log.txt");
+
+	// auto flush if the log severity is error or higher
+	async_file->flush_on(spd::level::err); 
+
     for (int i = 0; i < 100; ++i)
         async_file->info("Async message #{}", i);
 }

include/spdlog/details/async_logger_impl.h

@@ -54,7 +54,6 @@ inline spdlog::async_logger::async_logger(const std::string& logger_name,
 
 inline void spdlog::async_logger::flush()
 {
-
     _async_log_helper->flush();
 }
 
@@ -76,6 +75,8 @@ inline void spdlog::async_logger::_sink_it(details::log_msg& msg)
     try
     {
         _async_log_helper->log(msg);
+		if (_should_flush_on(msg))
+			flush();
     }
     catch (const std::exception &ex)
     {

include/spdlog/details/logger_impl.h

@@ -244,13 +244,11 @@ inline bool spdlog::logger::should_log(spdlog::level::level_enum msg_level) cons
 //
 inline void spdlog::logger::_sink_it(details::log_msg& msg)
 {
-
     _formatter->format(msg);
     for (auto &sink : _sinks)
         sink->log(msg);
 
-    const auto flush_level = _flush_level.load(std::memory_order_relaxed);
+    if(_should_flush_on(msg))
-    if (msg.level >= flush_level)
         flush();
 }
 
@@ -282,3 +280,9 @@ inline void spdlog::logger::_default_err_handler(const std::string &msg)
     sinks::stderr_sink_mt::instance()->log(err_msg);
     _last_err_time = now;
 }
+
+inline bool spdlog::logger::_should_flush_on(const details::log_msg &msg)
+{
+	const auto flush_level = _flush_level.load(std::memory_order_relaxed);
+	return (msg.level >= flush_level) && (msg.level != level::off);
+}

include/spdlog/logger.h

@@ -65,6 +65,7 @@ public:
 
     // automatically call flush() if message level >= log_level
     void flush_on(level::level_enum log_level);
+
     virtual void flush();
 
 protected:
@@ -75,6 +76,9 @@ protected:
     // default error handler: print the error to stderr with the max rate of 1 message/minute
     virtual void _default_err_handler(const std::string &msg);
 
+	// return true if the given message level should trigger a flush
+	bool _should_flush_on(const details::log_msg&);
+	
     const std::string _name;
     std::vector<sink_ptr> _sinks;
     formatter_ptr _formatter;