feat(wire): std::functional Wire slave callback functions (#11582)

This PR enhances the Wire library to support std::function–based callbacks for I2C slave mode, enabling the use of lambdas and captured contexts.

- Replaces raw function pointers in TwoWire and HardwareI2C with std::function for onRequest and onReceive
- Updates constructors, method signatures, and default initializations to use std::function
- Adds new example sketch, CI config, and documentation updates demonstrating the functional callback API

cores/esp32/HardwareI2C.h

@@ -20,6 +20,7 @@
 
 #include <inttypes.h>
 #include "Stream.h"
+#include <functional>
 
 class HardwareI2C : public Stream {
 public:
@@ -36,6 +37,7 @@ public:
   virtual size_t requestFrom(uint8_t address, size_t len, bool stopBit) = 0;
   virtual size_t requestFrom(uint8_t address, size_t len) = 0;
 
-  virtual void onReceive(void (*)(int)) = 0;
+  // Update base class to use std::function
-  virtual void onRequest(void (*)(void)) = 0;
+  virtual void onReceive(const std::function<void(int)> &) = 0;
+  virtual void onRequest(const std::function<void()> &) = 0;
 };

docs/en/api/i2c.rst

@@ -347,20 +347,147 @@ This function will return ``true`` if the peripheral was initialized correctly.
 onReceive
 ^^^^^^^^^
 
-The ``onReceive`` function is used to define the callback for the data received from the master.
+The ``onReceive`` function is used to define the callback for data received from the master device.
 
 .. code-block:: arduino
 
-    void onReceive( void (*)(int) );
+    void onReceive(const std::function<void(int)>& callback);
+
+**Function Signature:**
+
+The callback function must have the signature ``void(int numBytes)`` where ``numBytes`` indicates how many bytes were received from the master.
+
+**Usage Examples:**
+
+.. code-block:: arduino
+
+    // Method 1: Regular function
+    void handleReceive(int numBytes) {
+        Serial.printf("Received %d bytes: ", numBytes);
+        while (Wire.available()) {
+            char c = Wire.read();
+            Serial.print(c);
+        }
+        Serial.println();
+    }
+    Wire.onReceive(handleReceive);
+
+    // Method 2: Lambda function
+    Wire.onReceive([](int numBytes) {
+        Serial.printf("Master sent %d bytes\n", numBytes);
+        while (Wire.available()) {
+            uint8_t data = Wire.read();
+            // Process received data
+            Serial.printf("Data: 0x%02X\n", data);
+        }
+    });
+
+    // Method 3: Lambda with capture (for accessing variables)
+    int deviceId = 42;
+    Wire.onReceive([deviceId](int numBytes) {
+        Serial.printf("Device %d received %d bytes\n", deviceId, numBytes);
+        // Process data...
+    });
+
+    // Method 4: Using std::function variable
+    std::function<void(int)> receiveHandler = [](int bytes) {
+        Serial.printf("Handling %d received bytes\n", bytes);
+    };
+    Wire.onReceive(receiveHandler);
+
+    // Method 5: Class member function (using lambda wrapper)
+    class I2CDevice {
+    private:
+        int deviceAddress;
+    public:
+        I2CDevice(int addr) : deviceAddress(addr) {}
+
+        void handleReceive(int numBytes) {
+            Serial.printf("Device 0x%02X received %d bytes\n", deviceAddress, numBytes);
+        }
+
+        void setup() {
+            Wire.onReceive([this](int bytes) {
+                this->handleReceive(bytes);
+            });
+        }
+    };
+
+.. note::
+    The ``onReceive`` callback is triggered when the I2C master sends data to this slave device.
+    Use ``Wire.available()`` and ``Wire.read()`` inside the callback to retrieve the received data.
 
 onRequest
 ^^^^^^^^^
 
-The ``onRequest`` function is used to define the callback for the data to be send to the master.
+The ``onRequest`` function is used to define the callback for responding to master read requests.
 
 .. code-block:: arduino
 
-    void onRequest( void (*)(void) );
+    void onRequest(const std::function<void()>& callback);
+
+**Function Signature:**
+
+The callback function must have the signature ``void()`` with no parameters. This callback is triggered when the master requests data from this slave device.
+
+**Usage Examples:**
+
+.. code-block:: arduino
+
+    // Method 1: Regular function
+    void handleRequest() {
+        static int counter = 0;
+        Wire.printf("Response #%d", counter++);
+    }
+    Wire.onRequest(handleRequest);
+
+    // Method 2: Lambda function
+    Wire.onRequest([]() {
+        // Send sensor data to master
+        int sensorValue = analogRead(A0);
+        Wire.write(sensorValue >> 8);    // High byte
+        Wire.write(sensorValue & 0xFF);  // Low byte
+    });
+
+    // Method 3: Lambda with capture (for accessing variables)
+    int deviceStatus = 1;
+    String deviceName = "Sensor1";
+    Wire.onRequest([&deviceStatus, &deviceName]() {
+        Wire.write(deviceStatus);
+        Wire.write(deviceName.c_str(), deviceName.length());
+    });
+
+    // Method 4: Using std::function variable
+    std::function<void()> requestHandler = []() {
+        Wire.write("Hello Master!");
+    };
+    Wire.onRequest(requestHandler);
+
+    // Method 5: Class member function (using lambda wrapper)
+    class TemperatureSensor {
+    private:
+        float temperature;
+    public:
+        void updateTemperature() {
+            temperature = 25.5; // Read from actual sensor
+        }
+
+        void sendTemperature() {
+            // Convert float to bytes and send
+            uint8_t* tempBytes = (uint8_t*)&temperature;
+            Wire.write(tempBytes, sizeof(float));
+        }
+
+        void setup() {
+            Wire.onRequest([this]() {
+                this->sendTemperature();
+            });
+        }
+    };
+
+.. note::
+    The ``onRequest`` callback is triggered when the I2C master requests data from this slave device.
+    Use ``Wire.write()`` inside the callback to send response data back to the master.
 
 slaveWrite
 ^^^^^^^^^^

libraries/Wire/examples/WireSlaveFunctionalCallback/WireSlaveFunctionalCallback.ino

@@ -0,0 +1,37 @@
+// This example demonstrates the use of functional callbacks with the Wire library
+// for I2C slave communication. It shows how to handle requests and data reception
+
+#include "Wire.h"
+
+#define I2C_DEV_ADDR 0x55
+
+uint32_t i = 0;
+
+void setup() {
+  Serial.begin(115200);
+  Serial.setDebugOutput(true);
+
+  Wire.onRequest([]() {
+    Wire.print(i++);
+    Wire.print(" Packets.");
+    Serial.println("onRequest");
+  });
+
+  Wire.onReceive([](int len) {
+    Serial.printf("onReceive[%d]: ", len);
+    while (Wire.available()) {
+      Serial.write(Wire.read());
+    }
+    Serial.println();
+  });
+
+  Wire.begin((uint8_t)I2C_DEV_ADDR);
+
+#if CONFIG_IDF_TARGET_ESP32
+  char message[64];
+  snprintf(message, 64, "%lu Packets.", i++);
+  Wire.slaveWrite((uint8_t *)message, strlen(message));
+#endif
+}
+
+void loop() {}

libraries/Wire/examples/WireSlaveFunctionalCallback/ci.json

@@ -0,0 +1,5 @@
+{
+  "requires": [
+    "CONFIG_SOC_I2C_SUPPORT_SLAVE=y"
+  ]
+}

libraries/Wire/src/Wire.cpp

@@ -48,7 +48,7 @@ TwoWire::TwoWire(uint8_t bus_num)
 #endif
 #if SOC_I2C_SUPPORT_SLAVE
     ,
-    is_slave(false), user_onRequest(NULL), user_onReceive(NULL)
+    is_slave(false), user_onRequest(nullptr), user_onReceive(nullptr)
 #endif /* SOC_I2C_SUPPORT_SLAVE */
 {
 }
@@ -596,14 +596,14 @@ void TwoWire::flush() {
   //i2cFlush(num); // cleanup
 }
 
-void TwoWire::onReceive(void (*function)(int)) {
+void TwoWire::onReceive(const std::function<void(int)> &function) {
 #if SOC_I2C_SUPPORT_SLAVE
   user_onReceive = function;
 #endif
 }
 
 // sets function called on slave read
-void TwoWire::onRequest(void (*function)(void)) {
+void TwoWire::onRequest(const std::function<void()> &function) {
 #if SOC_I2C_SUPPORT_SLAVE
   user_onRequest = function;
 #endif

libraries/Wire/src/Wire.h

@@ -48,10 +48,6 @@
 #ifndef I2C_BUFFER_LENGTH
 #define I2C_BUFFER_LENGTH 128  // Default size, if none is set using Wire::setBuffersize(size_t)
 #endif
-#if SOC_I2C_SUPPORT_SLAVE
-typedef void (*user_onRequest)(void);
-typedef void (*user_onReceive)(uint8_t *, int);
-#endif /* SOC_I2C_SUPPORT_SLAVE */
 
 class TwoWire : public HardwareI2C {
 protected:
@@ -77,8 +73,8 @@ protected:
 private:
 #if SOC_I2C_SUPPORT_SLAVE
   bool is_slave;
-  void (*user_onRequest)(void);
+  std::function<void()> user_onRequest;
-  void (*user_onReceive)(int);
+  std::function<void(int)> user_onReceive;
   static void onRequestService(uint8_t, void *);
   static void onReceiveService(uint8_t, uint8_t *, size_t, bool, void *);
 #endif /* SOC_I2C_SUPPORT_SLAVE */
@@ -116,8 +112,8 @@ public:
   size_t requestFrom(uint8_t address, size_t len, bool stopBit) override;
   size_t requestFrom(uint8_t address, size_t len) override;
 
-  void onReceive(void (*)(int)) override;
+  void onReceive(const std::function<void(int)> &) override;
-  void onRequest(void (*)(void)) override;
+  void onRequest(const std::function<void()> &) override;
 
   //call setPins() first, so that begin() can be called without arguments from libraries
   bool setPins(int sda, int scl);