[logger] Replace LogListener virtual interface with LogCallback struct

Replace the LogListener abstract class (single pure virtual method) with
a lightweight LogCallback struct containing a function pointer + instance
pointer. This eliminates a vtable sub-table and thunk from every class
that previously inherited LogListener.

Savings per former LogListener implementer:
- 12 bytes vtable (sub-table header + thunk slot)
- 4 bytes vtable (on_log in primary table)
- ~23 bytes thunk code
- ~39 bytes total per class

Affected classes: APIServer, WebServer, MQTTClientComponent, Syslog,
BLENUS, LoggerMessageTrigger (6 classes, ~234 bytes total).

The non-capturing lambdas used at registration sites decay to plain
function pointers at compile time -- zero closure/std::function overhead.

esphome/components/api/api_server.cpp

@@ -92,7 +92,10 @@ void APIServer::setup() {
 
 #ifdef USE_LOGGER
   if (logger::global_logger != nullptr) {
-    logger::global_logger->add_log_listener(this);
+    logger::global_logger->add_log_callback(
+        this, [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+          static_cast<APIServer *>(self)->on_log(level, tag, message, message_len);
+        });
   }
 #endif
 

esphome/components/api/api_server.h

@@ -37,10 +37,6 @@ struct SavedNoisePsk {
 
 class APIServer : public Component,
                   public Controller
-#ifdef USE_LOGGER
-    ,
-                  public logger::LogListener
-#endif
 #ifdef USE_CAMERA
     ,
                   public camera::CameraListener
@@ -56,7 +52,7 @@ class APIServer : public Component,
   void on_shutdown() override;
   bool teardown() override;
 #ifdef USE_LOGGER
-  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override;
+  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len);
 #endif
 #ifdef USE_CAMERA
   void on_camera_image(const std::shared_ptr<camera::CameraImage> &image) override;

esphome/components/ble_nus/ble_nus.cpp

@@ -87,7 +87,10 @@ void BLENUS::setup() {
   global_ble_nus = this;
 #ifdef USE_LOGGER
   if (logger::global_logger != nullptr && this->expose_log_) {
-    logger::global_logger->add_log_listener(this);
+    logger::global_logger->add_log_callback(
+        this, [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+          static_cast<BLENUS *>(self)->on_log(level, tag, message, message_len);
+        });
   }
 #endif
 }

esphome/components/ble_nus/ble_nus.h

@@ -10,12 +10,7 @@
 
 namespace esphome::ble_nus {
 
-class BLENUS : public Component
-#ifdef USE_LOGGER
-    ,
-               public logger::LogListener
-#endif
-{
+class BLENUS : public Component {
   enum TxStatus {
     TX_DISABLED,
     TX_ENABLED,
@@ -29,7 +24,7 @@ class BLENUS : public Component
   size_t write_array(const uint8_t *data, size_t len);
   void set_expose_log(bool expose_log) { this->expose_log_ = expose_log; }
 #ifdef USE_LOGGER
-  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override;
+  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len);
 #endif
 
  protected:

esphome/components/logger/logger.h

@@ -40,26 +40,25 @@ struct device;
 
 namespace esphome::logger {
 
-/** Interface for receiving log messages without std::function overhead.
+/** Lightweight callback for receiving log messages without virtual dispatch overhead.
  *
- * Components can implement this interface instead of using lambdas with std::function
- * to reduce flash usage from std::function type erasure machinery.
+ * Replaces the former LogListener virtual interface to eliminate per-implementer
+ * vtable sub-tables and thunk code (~39 bytes saved per class that used LogListener).
  *
  * Usage:
- *   class MyComponent : public Component, public LogListener {
- *    public:
- *     void setup() override {
- *       if (logger::global_logger != nullptr)
- *         logger::global_logger->add_log_listener(this);
- *     }
- *     void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override {
- *       // Handle log message
- *     }
- *   };
+ *   // In your component's setup():
+ *   if (logger::global_logger != nullptr)
+ *     logger::global_logger->add_log_callback(
+ *         this, [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+ *           static_cast<MyComponent *>(self)->on_log(level, tag, message, message_len);
+ *         });
  */
-class LogListener {
- public:
-  virtual void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) = 0;
+struct LogCallback {
+  void *instance;
+  void (*fn)(void *, uint8_t, const char *, const char *, size_t);
+  void invoke(uint8_t level, const char *tag, const char *message, size_t message_len) const {
+    this->fn(this->instance, level, tag, message, message_len);
+  }
 };
 
 #ifdef USE_LOGGER_LEVEL_LISTENERS
@@ -187,11 +186,13 @@ class Logger : public Component {
   inline uint8_t level_for(const char *tag);
 
 #ifdef USE_LOG_LISTENERS
-  /// Register a log listener to receive log messages
-  void add_log_listener(LogListener *listener) { this->log_listeners_.push_back(listener); }
+  /// Register a log callback to receive log messages
+  void add_log_callback(void *instance, void (*fn)(void *, uint8_t, const char *, const char *, size_t)) {
+    this->log_callbacks_.push_back(LogCallback{instance, fn});
+  }
 #else
   /// No-op when log listeners are disabled
-  void add_log_listener(LogListener *listener) {}
+  void add_log_callback(void *instance, void (*fn)(void *, uint8_t, const char *, const char *, size_t)) {}
 #endif
 
 #ifdef USE_LOGGER_LEVEL_LISTENERS
@@ -253,11 +254,11 @@ class Logger : public Component {
   }
 #endif
 
-  // Helper to notify log listeners
+  // Helper to notify log callbacks
   inline void HOT notify_listeners_(uint8_t level, const char *tag, const LogBuffer &buf) {
 #ifdef USE_LOG_LISTENERS
-    for (auto *listener : this->log_listeners_)
-      listener->on_log(level, tag, buf.data, buf.pos);
+    for (auto &cb : this->log_callbacks_)
+      cb.invoke(level, tag, buf.data, buf.pos);
 #endif
   }
 
@@ -341,8 +342,8 @@ class Logger : public Component {
   std::map<const char *, uint8_t, CStrCompare> log_levels_{};
 #endif
 #ifdef USE_LOG_LISTENERS
-  StaticVector<LogListener *, ESPHOME_LOG_MAX_LISTENERS>
-      log_listeners_;  // Log message listeners (API, MQTT, syslog, etc.)
+  StaticVector<LogCallback, ESPHOME_LOG_MAX_LISTENERS>
+      log_callbacks_;  // Log message callbacks (API, MQTT, syslog, etc.)
 #endif
 #ifdef USE_LOGGER_LEVEL_LISTENERS
   std::vector<LoggerLevelListener *> level_listeners_;  // Log level change listeners
@@ -478,15 +479,16 @@ class Logger : public Component {
 };
 extern Logger *global_logger;  // NOLINT(cppcoreguidelines-avoid-non-const-global-variables)
 
-class LoggerMessageTrigger : public Trigger<uint8_t, const char *, const char *>, public LogListener {
+class LoggerMessageTrigger : public Trigger<uint8_t, const char *, const char *> {
  public:
-  explicit LoggerMessageTrigger(Logger *parent, uint8_t level) : level_(level) { parent->add_log_listener(this); }
-
-  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override {
-    (void) message_len;
-    if (level <= this->level_) {
-      this->trigger(level, tag, message);
-    }
+  explicit LoggerMessageTrigger(Logger *parent, uint8_t level) : level_(level) {
+    parent->add_log_callback(this,
+                             [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+                               auto *trigger = static_cast<LoggerMessageTrigger *>(self);
+                               if (level <= trigger->level_) {
+                                 trigger->trigger(level, tag, message);
+                               }
+                             });
   }
 
  protected:

esphome/components/mqtt/mqtt_client.cpp

@@ -64,7 +64,10 @@ void MQTTClientComponent::setup() {
   });
 #ifdef USE_LOGGER
   if (this->is_log_message_enabled() && logger::global_logger != nullptr) {
-    logger::global_logger->add_log_listener(this);
+    logger::global_logger->add_log_callback(
+        this, [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+          static_cast<MQTTClientComponent *>(self)->on_log(level, tag, message, message_len);
+        });
   }
 #endif
 

esphome/components/mqtt/mqtt_client.h

@@ -99,12 +99,7 @@ enum MQTTClientState {
 
 class MQTTComponent;
 
-class MQTTClientComponent : public Component
-#ifdef USE_LOGGER
-    ,
-                            public logger::LogListener
-#endif
-{
+class MQTTClientComponent : public Component {
  public:
   MQTTClientComponent();
 
@@ -252,7 +247,7 @@ class MQTTClientComponent : public Component
   float get_setup_priority() const override;
 
 #ifdef USE_LOGGER
-  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override;
+  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len);
 #endif
 
   void on_message(const std::string &topic, const std::string &payload);

esphome/components/syslog/esphome_syslog.cpp

@@ -18,7 +18,12 @@ constexpr int LOG_LEVEL_TO_SYSLOG_SEVERITY[] = {
     7   // VERY_VERBOSE
 };
 
-void Syslog::setup() { logger::global_logger->add_log_listener(this); }
+void Syslog::setup() {
+  logger::global_logger->add_log_callback(
+      this, [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+        static_cast<Syslog *>(self)->on_log(level, tag, message, message_len);
+      });
+}
 
 void Syslog::on_log(uint8_t level, const char *tag, const char *message, size_t message_len) {
   this->log_(level, tag, message, message_len);

esphome/components/syslog/esphome_syslog.h

@@ -2,17 +2,16 @@
 #include "esphome/core/component.h"
 #include "esphome/core/helpers.h"
 #include "esphome/core/log.h"
-#include "esphome/components/logger/logger.h"
 #include "esphome/components/udp/udp_component.h"
 #include "esphome/components/time/real_time_clock.h"
 
 #ifdef USE_NETWORK
 namespace esphome::syslog {
-class Syslog : public Component, public Parented<udp::UDPComponent>, public logger::LogListener {
+class Syslog : public Component, public Parented<udp::UDPComponent> {
  public:
   Syslog(int level, time::RealTimeClock *time) : log_level_(level), time_(time) {}
   void setup() override;
-  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override;
+  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len);
   void set_strip(bool strip) { this->strip_ = strip; }
   void set_facility(int facility) { this->facility_ = facility; }
 

esphome/components/web_server/web_server.cpp

@@ -395,7 +395,10 @@ void WebServer::setup() {
 
 #ifdef USE_LOGGER
   if (logger::global_logger != nullptr && this->expose_log_) {
-    logger::global_logger->add_log_listener(this);
+    logger::global_logger->add_log_callback(
+        this, [](void *self, uint8_t level, const char *tag, const char *message, size_t message_len) {
+          static_cast<WebServer *>(self)->on_log(level, tag, message, message_len);
+        });
   }
 #endif
 

esphome/components/web_server/web_server.h

@@ -186,14 +186,7 @@ class DeferredUpdateEventSourceList : public std::list<DeferredUpdateEventSource
  * under the '/light/...', '/sensor/...', ... URLs. A full documentation for this API
  * can be found under https://esphome.io/web-api/.
  */
-class WebServer : public Controller,
-                  public Component,
-                  public AsyncWebHandler
-#ifdef USE_LOGGER
-    ,
-                  public logger::LogListener
-#endif
-{
+class WebServer : public Controller, public Component, public AsyncWebHandler {
 #if !defined(USE_ESP32) && defined(USE_ARDUINO)
   friend class DeferredUpdateEventSourceList;
 #endif
@@ -254,7 +247,7 @@ class WebServer : public Controller,
   void dump_config() override;
 
 #ifdef USE_LOGGER
-  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len) override;
+  void on_log(uint8_t level, const char *tag, const char *message, size_t message_len);
 #endif
 
   /// MQTT setup priority.