Added a function to add callbacks that are called when a logger is registered (#2883)

* Added a function to add callbacks that are called when a logger is registered

* Fix non captured registration 2 not being properly tested for

* Replace std::list by std::vector

* Remove const refs to shared pointers

* Fix missing header

include/spdlog/details/registry-inl.h

@@ -259,6 +259,22 @@ SPDLOG_INLINE void registry::register_logger_(std::shared_ptr<logger> new_logger
     auto logger_name = new_logger->name();
     throw_if_exists_(logger_name);
     loggers_[logger_name] = std::move(new_logger);
+
+    const auto& logger = loggers_[logger_name];
+    for (const auto& on_registration_callback : on_registration_callbacks_) {
+        on_registration_callback(logger);
+    }
+}
+
+SPDLOG_INLINE void registry::add_on_registration_callback(
+    const std::function<void(std::shared_ptr<logger>)>& callback) {
+    std::lock_guard<std::mutex> lock(logger_map_mutex_);
+    on_registration_callbacks_.push_back(callback);
+}
+
+SPDLOG_INLINE void registry::drop_all_on_registration_callbacks() {
+    std::lock_guard<std::mutex> lock(logger_map_mutex_);
+    on_registration_callbacks_.clear();
 }
 
 }  // namespace details

include/spdlog/details/registry.h

@@ -17,6 +17,7 @@
 #include <mutex>
 #include <string>
 #include <unordered_map>
+#include <vector>
 
 namespace spdlog {
 class logger;
@@ -92,6 +93,10 @@ public:
 
     void apply_logger_env_levels(std::shared_ptr<logger> new_logger);
 
+    void add_on_registration_callback(const std::function<void(std::shared_ptr<logger>)>& callback);
+
+    void drop_all_on_registration_callbacks();
+
 private:
     registry();
     ~registry();
@@ -112,6 +117,7 @@ private:
     std::shared_ptr<logger> default_logger_;
     bool automatic_registration_ = true;
     size_t backtrace_n_messages_ = 0;
+    std::vector<std::function<void(std::shared_ptr<logger>)>> on_registration_callbacks_;
 };
 
 }  // namespace details

include/spdlog/spdlog-inl.h

@@ -89,4 +89,12 @@ SPDLOG_INLINE void apply_logger_env_levels(std::shared_ptr<logger> logger) {
     details::registry::instance().apply_logger_env_levels(std::move(logger));
 }
 
+SPDLOG_INLINE void add_on_registration_callback(const std::function<void(const std::shared_ptr<logger>&)>& callback) {
+    details::registry::instance().add_on_registration_callback(callback);
+}
+
+SPDLOG_INLINE void drop_all_on_registration_callbacks() {
+    details::registry::instance().drop_all_on_registration_callbacks();
+}
+
 }  // namespace spdlog

include/spdlog/spdlog.h

@@ -140,6 +140,21 @@ SPDLOG_API void set_default_logger(std::shared_ptr<spdlog::logger> default_logge
 //   spdlog::apply_logger_env_levels(mylogger);
 SPDLOG_API void apply_logger_env_levels(std::shared_ptr<logger> logger);
 
+// Add a callback that is called whenever a logger is registered
+//
+// Useful for intercepting loggers that are dynamically created during the application's lifetime
+// (e.g. from dynamically loaded libraries, etc.)
+// Example:
+// void my_on_registration_callback(const std::shared_ptr<spdlog::logger>& logger) {
+//     logger->do_stuff();
+// }
+// ...
+// spdlog::add_on_registration_callback(my_on_registration_callback);
+SPDLOG_API void add_on_registration_callback(const std::function<void(const std::shared_ptr<logger>&)>& callback);
+
+// Clear all callbacks that were added to intercept registrations (see "add_on_registration_callback")
+SPDLOG_API void drop_all_on_registration_callbacks();
+
 template <typename... Args>
 inline void log(source_loc source,
                 level::level_enum lvl,