Add a trivial callback sink (#2610)

Add a trivial callback sink

README.md

@@ -232,6 +232,28 @@ void multi_sink_example()
 }
 ```
 
+---
+#### Logger with a custom callback function that receives the logs
+```c++
+
+// create logger with a lambda function callback, the callback will be called
+// each time something is logged to the logger
+void callback_example()
+{
+    auto callback_sink = std::make_shared<spdlog::sinks::callback_sink_mt>([](const spdlog::details::log_msg &msg) {
+         // for example you can be notified by sending an email to yourself
+    });
+    callback_sink->set_level(spdlog::level::err);
+
+    auto console_sink = std::make_shared<spdlog::sinks::stdout_color_sink_mt>();
+    spdlog::logger logger("custom_callback_logger", {console_sink, callback_sink});
+
+    logger.info("some info log");
+    logger.debug("some debug log");
+    logger.error("critical issue"); // will notify you
+}
+```
+
 ---
 #### Asynchronous logging
 ```c++

example/example.cpp

@@ -12,6 +12,7 @@ void stdout_logger_example();
 void basic_example();
 void rotating_example();
 void daily_example();
+void callback_example();
 void async_example();
 void binary_example();
 void vector_example();
@@ -72,6 +73,7 @@ int main(int, char *[])
         basic_example();
         rotating_example();
         daily_example();
+        callback_example();
         async_example();
         binary_example();
         vector_example();
@@ -136,6 +138,15 @@ void daily_example()
     auto daily_logger = spdlog::daily_logger_mt("daily_logger", "logs/daily.txt", 2, 30);
 }
 
+#include "spdlog/sinks/callback_sink.h"
+void callback_example()
+{
+    // Create the logger
+    auto logger = spdlog::callback_logger_mt("custom_callback_logger", [](const spdlog::details::log_msg &/*msg*/) {
+        // do what you need to do with msg
+    });
+}
+
 #include "spdlog/cfg/env.h"
 void load_levels_example()
 {

include/spdlog/sinks/callback_sink.h

@@ -0,0 +1,61 @@
+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
+#pragma once
+
+#include <spdlog/details/null_mutex.h>
+#include <spdlog/sinks/base_sink.h>
+#include <spdlog/details/synchronous_factory.h>
+
+#include <mutex>
+#include <string>
+
+namespace spdlog {
+
+// callbacks type
+typedef std::function<void(const details::log_msg &msg)> custom_log_callback;
+
+namespace sinks {
+/*
+ * Trivial callback sink, gets a callback function and calls it on each log
+ */
+template<typename Mutex>
+class callback_sink final : public base_sink<Mutex>
+{
+public:
+    explicit callback_sink(const custom_log_callback &callback)
+        : callback_{callback}
+    {}
+
+protected:
+    void sink_it_(const details::log_msg &msg) override
+    {
+        callback_(msg);
+    }
+    void flush_() override{};
+
+private:
+    custom_log_callback callback_;
+};
+
+using callback_sink_mt = callback_sink<std::mutex>;
+using callback_sink_st = callback_sink<details::null_mutex>;
+
+} // namespace sinks
+
+//
+// factory functions
+//
+template<typename Factory = spdlog::synchronous_factory>
+inline std::shared_ptr<logger> callback_logger_mt(const std::string &logger_name, const custom_log_callback &callback)
+{
+    return Factory::template create<sinks::callback_sink_mt>(logger_name, callback);
+}
+
+template<typename Factory = spdlog::synchronous_factory>
+inline std::shared_ptr<logger> callback_logger_st(const std::string &logger_name, const custom_log_callback &callback)
+{
+    return Factory::template create<sinks::callback_sink_st>(logger_name, callback);
+}
+
+} // namespace spdlog

tests/CMakeLists.txt

@@ -31,6 +31,7 @@ set(SPDLOG_UTESTS_SOURCES
     test_stdout_api.cpp
     test_backtrace.cpp
     test_create_dir.cpp
+    test_custom_callbacks.cpp
     test_cfg.cpp
     test_time_point.cpp
     test_stopwatch.cpp)

tests/test_custom_callbacks.cpp

@@ -0,0 +1,34 @@
+/*
+ * This content is released under the MIT License as specified in https://raw.githubusercontent.com/gabime/spdlog/master/LICENSE
+ */
+#include "includes.h"
+#include "test_sink.h"
+#include "spdlog/sinks/callback_sink.h"
+#include "spdlog/async.h"
+#include "spdlog/common.h"
+
+TEST_CASE("custom_callback_logger", "[custom_callback_logger]]")
+{
+    std::vector<std::string> lines;
+    spdlog::pattern_formatter formatter;
+    auto callback_logger = std::make_shared<spdlog::sinks::callback_sink_st>([&](const spdlog::details::log_msg &msg) {
+        spdlog::memory_buf_t formatted;
+        formatter.format(msg, formatted);
+        auto eol_len = strlen(spdlog::details::os::default_eol);
+        lines.emplace_back(formatted.begin(), formatted.end() - eol_len);
+    });
+    std::shared_ptr<spdlog::sinks::test_sink_st> test_sink(new spdlog::sinks::test_sink_st);
+
+    spdlog::logger logger("test-callback", {callback_logger, test_sink});
+
+    logger.info("test message 1");
+    logger.info("test message 2");
+    logger.info("test message 3");
+
+    std::vector<std::string> ref_lines = test_sink->lines();
+
+    REQUIRE(lines[0] == ref_lines[0]);
+    REQUIRE(lines[1] == ref_lines[1]);
+    REQUIRE(lines[2] == ref_lines[2]);
+    spdlog::drop_all();
+}