Improve logger helper

So that we need a factory that checks the spdlog registry in its
own dll before creating a new logger. Or, if no factory is set we
check the KDUtils registry.

Call sites should not worry about checkign via spdlog::get()
themselves as this helper and any installed factory should now
do it instead.

This will ensure the same registry is checked as is used when
creating each logger.

Change-Id: I24399e7ca778ddd310d6074300e70143ca3d2d9f
Reviewed-on: https://codereview.kdab.com/c/kdab/kdutils/+/134715
Reviewed-by: Paul Lemire <[email protected]>

src/KDUtils/logging.cpp

@@ -18,13 +18,19 @@ std::shared_ptr<spdlog::logger> Logger::logger(const std::string &name)
 {
     std::shared_ptr<spdlog::logger> logger;
     if (ms_loggerFactory) {
+        // Use the factory set by the application which should check
+        // its own spdlog registry first before creating a new logger.
         logger = ms_loggerFactory(name);
     } else {
+        // No factory set, use the spdlog registry from KDUtils
+        logger = spdlog::get(name);
+        if (!logger) {
 #if defined(ANDROID)
-        logger = spdlog::android_logger_mt(name, name);
+            logger = spdlog::android_logger_mt(name, name);
 #else
-        logger = spdlog::stdout_color_mt(name);
+            logger = spdlog::stdout_color_mt(name);
 #endif
+        }
     }
     return logger;
 }