Add a trivial callback sink (#2610)

Add a trivial callback sink

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