[display] Optimize display writers with function pointers for stateless lambdas (#11629)

esphome/components/display/display.h

@@ -176,7 +176,117 @@ class Display;
 class DisplayPage;
 class DisplayOnPageChangeTrigger;
 
-using display_writer_t = std::function<void(Display &)>;
+/** Optimized display writer that uses function pointers for stateless lambdas.
+ *
+ * Similar to TemplatableValue but specialized for display writer callbacks.
+ * Saves ~8 bytes per stateless lambda on 32-bit platforms (16 bytes std::function → ~8 bytes discriminator+pointer).
+ *
+ * Supports both:
+ * - Stateless lambdas (from YAML) → function pointer (4 bytes)
+ * - Stateful lambdas/std::function (from C++ code) → std::function* (heap allocated)
+ *
+ * @tparam T The display type (e.g., Display, Nextion, GPIOLCDDisplay)
+ */
+template<typename T> class DisplayWriter {
+ public:
+  DisplayWriter() : type_(NONE) {}
+
+  // For stateless lambdas (convertible to function pointer): use function pointer (4 bytes)
+  template<typename F>
+  DisplayWriter(F f) requires std::invocable<F, T &> && std::convertible_to<F, void (*)(T &)>
+      : type_(STATELESS_LAMBDA) {
+    this->stateless_f_ = f;  // Implicit conversion to function pointer
+  }
+
+  // For stateful lambdas and std::function (not convertible to function pointer): use std::function* (heap allocated)
+  // This handles backwards compatibility with external components
+  template<typename F>
+  DisplayWriter(F f) requires std::invocable<F, T &> &&(!std::convertible_to<F, void (*)(T &)>) : type_(LAMBDA) {
+    this->f_ = new std::function<void(T &)>(std::move(f));
+  }
+
+  // Copy constructor
+  DisplayWriter(const DisplayWriter &other) : type_(other.type_) {
+    if (type_ == LAMBDA) {
+      this->f_ = new std::function<void(T &)>(*other.f_);
+    } else if (type_ == STATELESS_LAMBDA) {
+      this->stateless_f_ = other.stateless_f_;
+    }
+  }
+
+  // Move constructor
+  DisplayWriter(DisplayWriter &&other) noexcept : type_(other.type_) {
+    if (type_ == LAMBDA) {
+      this->f_ = other.f_;
+      other.f_ = nullptr;
+    } else if (type_ == STATELESS_LAMBDA) {
+      this->stateless_f_ = other.stateless_f_;
+    }
+    other.type_ = NONE;
+  }
+
+  // Assignment operators
+  DisplayWriter &operator=(const DisplayWriter &other) {
+    if (this != &other) {
+      this->~DisplayWriter();
+      new (this) DisplayWriter(other);
+    }
+    return *this;
+  }
+
+  DisplayWriter &operator=(DisplayWriter &&other) noexcept {
+    if (this != &other) {
+      this->~DisplayWriter();
+      new (this) DisplayWriter(std::move(other));
+    }
+    return *this;
+  }
+
+  ~DisplayWriter() {
+    if (type_ == LAMBDA) {
+      delete this->f_;
+    }
+    // STATELESS_LAMBDA/NONE: no cleanup needed (function pointer or empty)
+  }
+
+  bool has_value() const { return this->type_ != NONE; }
+
+  void call(T &display) const {
+    switch (this->type_) {
+      case STATELESS_LAMBDA:
+        this->stateless_f_(display);  // Direct function pointer call
+        break;
+      case LAMBDA:
+        (*this->f_)(display);  // std::function call
+        break;
+      case NONE:
+      default:
+        break;
+    }
+  }
+
+  // Operator() for convenience
+  void operator()(T &display) const { this->call(display); }
+
+  // Operator* for backwards compatibility with (*writer_)(*this) pattern
+  DisplayWriter &operator*() { return *this; }
+  const DisplayWriter &operator*() const { return *this; }
+
+ protected:
+  enum : uint8_t {
+    NONE,
+    LAMBDA,
+    STATELESS_LAMBDA,
+  } type_;
+
+  union {
+    std::function<void(T &)> *f_;
+    void (*stateless_f_)(T &);
+  };
+};
+
+// Type alias for Display writer - uses optimized DisplayWriter instead of std::function
+using display_writer_t = DisplayWriter<Display>;
 
 #define LOG_DISPLAY(prefix, type, obj) \
   if ((obj) != nullptr) { \
@@ -678,7 +788,7 @@ class Display : public PollingComponent {
   void sort_triangle_points_by_y_(int *x1, int *y1, int *x2, int *y2, int *x3, int *y3);
 
   DisplayRotation rotation_{DISPLAY_ROTATION_0_DEGREES};
-  optional<display_writer_t> writer_{};
+  display_writer_t writer_{};
   DisplayPage *page_{nullptr};
   DisplayPage *previous_page_{nullptr};
   std::vector<DisplayOnPageChangeTrigger *> on_page_change_triggers_;

esphome/components/lcd_gpio/gpio_lcd_display.h

@@ -2,13 +2,18 @@
 
 #include "esphome/core/hal.h"
 #include "esphome/components/lcd_base/lcd_display.h"
+#include "esphome/components/display/display.h"
 
 namespace esphome {
 namespace lcd_gpio {
 
+class GPIOLCDDisplay;
+
+using gpio_lcd_writer_t = display::DisplayWriter<GPIOLCDDisplay>;
+
 class GPIOLCDDisplay : public lcd_base::LCDDisplay {
  public:
-  void set_writer(std::function<void(GPIOLCDDisplay &)> &&writer) { this->writer_ = std::move(writer); }
+  void set_writer(gpio_lcd_writer_t &&writer) { this->writer_ = std::move(writer); }
   void setup() override;
   void set_data_pins(GPIOPin *d0, GPIOPin *d1, GPIOPin *d2, GPIOPin *d3) {
     this->data_pins_[0] = d0;
@@ -43,7 +48,7 @@ class GPIOLCDDisplay : public lcd_base::LCDDisplay {
   GPIOPin *rw_pin_{nullptr};
   GPIOPin *enable_pin_{nullptr};
   GPIOPin *data_pins_[8]{nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr, nullptr};
-  std::function<void(GPIOLCDDisplay &)> writer_;
+  gpio_lcd_writer_t writer_;
 };
 
 }  // namespace lcd_gpio

esphome/components/lcd_pcf8574/pcf8574_display.h

@@ -3,13 +3,18 @@
 #include "esphome/core/component.h"
 #include "esphome/components/lcd_base/lcd_display.h"
 #include "esphome/components/i2c/i2c.h"
+#include "esphome/components/display/display.h"
 
 namespace esphome {
 namespace lcd_pcf8574 {
 
+class PCF8574LCDDisplay;
+
+using pcf8574_lcd_writer_t = display::DisplayWriter<PCF8574LCDDisplay>;
+
 class PCF8574LCDDisplay : public lcd_base::LCDDisplay, public i2c::I2CDevice {
  public:
-  void set_writer(std::function<void(PCF8574LCDDisplay &)> &&writer) { this->writer_ = std::move(writer); }
+  void set_writer(pcf8574_lcd_writer_t &&writer) { this->writer_ = std::move(writer); }
   void setup() override;
   void dump_config() override;
   void backlight();
@@ -24,7 +29,7 @@ class PCF8574LCDDisplay : public lcd_base::LCDDisplay, public i2c::I2CDevice {
 
   // Stores the current state of the backlight.
   uint8_t backlight_value_;
-  std::function<void(PCF8574LCDDisplay &)> writer_;
+  pcf8574_lcd_writer_t writer_;
 };
 
 }  // namespace lcd_pcf8574

esphome/components/max7219/max7219.h

@@ -4,13 +4,14 @@
 #include "esphome/core/time.h"
 
 #include "esphome/components/spi/spi.h"
+#include "esphome/components/display/display.h"
 
 namespace esphome {
 namespace max7219 {
 
 class MAX7219Component;
 
-using max7219_writer_t = std::function<void(MAX7219Component &)>;
+using max7219_writer_t = display::DisplayWriter<MAX7219Component>;
 
 class MAX7219Component : public PollingComponent,
                          public spi::SPIDevice<spi::BIT_ORDER_MSB_FIRST, spi::CLOCK_POLARITY_LOW,
@@ -57,7 +58,7 @@ class MAX7219Component : public PollingComponent,
   uint8_t num_chips_{1};
   uint8_t *buffer_;
   bool reverse_{false};
-  optional<max7219_writer_t> writer_{};
+  max7219_writer_t writer_{};
 };
 
 }  // namespace max7219

esphome/components/max7219digit/max7219digit.h

@@ -23,7 +23,7 @@ enum ScrollMode {
 
 class MAX7219Component;
 
-using max7219_writer_t = std::function<void(MAX7219Component &)>;
+using max7219_writer_t = display::DisplayWriter<MAX7219Component>;
 
 class MAX7219Component : public display::DisplayBuffer,
                          public spi::SPIDevice<spi::BIT_ORDER_MSB_FIRST, spi::CLOCK_POLARITY_LOW,
@@ -117,7 +117,7 @@ class MAX7219Component : public display::DisplayBuffer,
   uint32_t last_scroll_ = 0;
   uint16_t stepsleft_;
   size_t get_buffer_length_();
-  optional<max7219_writer_t> writer_local_{};
+  max7219_writer_t writer_local_{};
 };
 
 }  // namespace max7219digit

esphome/components/nextion/nextion.h

@@ -9,6 +9,7 @@
 #include "esphome/components/uart/uart.h"
 #include "nextion_base.h"
 #include "nextion_component.h"
+#include "esphome/components/display/display.h"
 #include "esphome/components/display/display_color_utils.h"
 
 #ifdef USE_NEXTION_TFT_UPLOAD
@@ -31,7 +32,7 @@ namespace nextion {
 class Nextion;
 class NextionComponentBase;
 
-using nextion_writer_t = std::function<void(Nextion &)>;
+using nextion_writer_t = display::DisplayWriter<Nextion>;
 
 static const std::string COMMAND_DELIMITER{static_cast<char>(255), static_cast<char>(255), static_cast<char>(255)};
 
@@ -1471,7 +1472,7 @@ class Nextion : public NextionBase, public PollingComponent, public uart::UARTDe
   CallbackManager<void(uint8_t, uint8_t, bool)> touch_callback_{};
   CallbackManager<void()> buffer_overflow_callback_{};
 
-  optional<nextion_writer_t> writer_;
+  nextion_writer_t writer_;
   optional<float> brightness_;
 
 #ifdef USE_NEXTION_CONFIG_DUMP_DEVICE_INFO

esphome/components/pvvx_mithermometer/display/pvvx_display.h

@@ -3,6 +3,7 @@
 #include "esphome/core/component.h"
 #include "esphome/core/defines.h"
 #include "esphome/components/ble_client/ble_client.h"
+#include "esphome/components/display/display.h"
 
 #include <cinttypes>
 
@@ -29,7 +30,7 @@ enum UNIT {
   UNIT_DEG_E,     ///< show "°E"
 };
 
-using pvvx_writer_t = std::function<void(PVVXDisplay &)>;
+using pvvx_writer_t = display::DisplayWriter<PVVXDisplay>;
 
 class PVVXDisplay : public ble_client::BLEClientNode, public PollingComponent {
  public:
@@ -126,7 +127,7 @@ class PVVXDisplay : public ble_client::BLEClientNode, public PollingComponent {
   esp32_ble_tracker::ESPBTUUID char_uuid_ =
       esp32_ble_tracker::ESPBTUUID::from_raw("00001f1f-0000-1000-8000-00805f9b34fb");
 
-  optional<pvvx_writer_t> writer_{};
+  pvvx_writer_t writer_{};
 };
 
 }  // namespace pvvx_mithermometer

esphome/components/st7920/st7920.h

@@ -9,7 +9,7 @@ namespace st7920 {
 
 class ST7920;
 
-using st7920_writer_t = std::function<void(ST7920 &)>;
+using st7920_writer_t = display::DisplayWriter<ST7920>;
 
 class ST7920 : public display::DisplayBuffer,
                public spi::SPIDevice<spi::BIT_ORDER_MSB_FIRST, spi::CLOCK_POLARITY_HIGH, spi::CLOCK_PHASE_TRAILING,
@@ -44,7 +44,7 @@ class ST7920 : public display::DisplayBuffer,
   void end_transaction_();
 
   int16_t width_ = 128, height_ = 64;
-  optional<st7920_writer_t> writer_local_{};
+  st7920_writer_t writer_local_{};
 };
 
 }  // namespace st7920

esphome/components/tm1621/tm1621.h

@@ -3,13 +3,14 @@
 #include "esphome/core/component.h"
 #include "esphome/core/defines.h"
 #include "esphome/core/hal.h"
+#include "esphome/components/display/display.h"
 
 namespace esphome {
 namespace tm1621 {
 
 class TM1621Display;
 
-using tm1621_writer_t = std::function<void(TM1621Display &)>;
+using tm1621_writer_t = display::DisplayWriter<TM1621Display>;
 
 class TM1621Display : public PollingComponent {
  public:
@@ -59,7 +60,7 @@ class TM1621Display : public PollingComponent {
   GPIOPin *cs_pin_;
   GPIOPin *read_pin_;
   GPIOPin *write_pin_;
-  optional<tm1621_writer_t> writer_{};
+  tm1621_writer_t writer_{};
   char row_[2][12];
   uint8_t state_;
   uint8_t device_;

esphome/components/tm1637/tm1637.h

@@ -4,6 +4,7 @@
 #include "esphome/core/defines.h"
 #include "esphome/core/hal.h"
 #include "esphome/core/time.h"
+#include "esphome/components/display/display.h"
 
 #include <vector>
 
@@ -19,7 +20,7 @@ class TM1637Display;
 class TM1637Key;
 #endif
 
-using tm1637_writer_t = std::function<void(TM1637Display &)>;
+using tm1637_writer_t = display::DisplayWriter<TM1637Display>;
 
 class TM1637Display : public PollingComponent {
  public:
@@ -78,7 +79,7 @@ class TM1637Display : public PollingComponent {
   uint8_t length_;
   bool inverted_;
   bool on_{true};
-  optional<tm1637_writer_t> writer_{};
+  tm1637_writer_t writer_{};
   uint8_t buffer_[6] = {0};
 #ifdef USE_BINARY_SENSOR
   std::vector<TM1637Key *> tm1637_keys_{};

esphome/components/tm1638/tm1638.h

@@ -5,6 +5,7 @@
 #include "esphome/core/defines.h"
 #include "esphome/core/hal.h"
 #include "esphome/core/time.h"
+#include "esphome/components/display/display.h"
 
 #include <vector>
 
@@ -18,7 +19,7 @@ class KeyListener {
 
 class TM1638Component;
 
-using tm1638_writer_t = std::function<void(TM1638Component &)>;
+using tm1638_writer_t = display::DisplayWriter<TM1638Component>;
 
 class TM1638Component : public PollingComponent {
  public:
@@ -70,7 +71,7 @@ class TM1638Component : public PollingComponent {
   GPIOPin *stb_pin_;
   GPIOPin *dio_pin_;
   uint8_t *buffer_ = new uint8_t[8];
-  optional<tm1638_writer_t> writer_{};
+  tm1638_writer_t writer_{};
   std::vector<KeyListener *> listeners_{};
 };
 

tests/components/image/test.host.yaml

@@ -1,5 +1,6 @@
 display:
   - platform: sdl
+    id: image_display
     auto_clear_enabled: false
     dimensions:
       width: 480

tests/components/sdl/common.yaml

@@ -13,11 +13,14 @@ display:
 
 binary_sensor:
   - platform: sdl
+    sdl_id: sdl_display
     id: key_up
     key: SDLK_UP
   - platform: sdl
+    sdl_id: sdl_display
     id: key_down
     key: SDLK_DOWN
   - platform: sdl
+    sdl_id: sdl_display
     id: key_enter
     key: SDLK_RETURN