Added example to show Wi-Fi and a subscriber.

examples/micro-ros_subscriber_wifi/.gitignore

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+.pio
+.vscode/.browse.c_cpp.db*
+.vscode/c_cpp_properties.json
+.vscode/launch.json
+.vscode/ipch
+.vscode/extensions.json

examples/micro-ros_subscriber_wifi/platformio.ini

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+; This example shows micro-ROS can be used with Wi-Fi transport on the
+; ESP32-CAM board.
+
+[env:esp32cam]
+platform = espressif32
+board = esp32cam
+framework = arduino
+board_microros_distro = rolling
+board_microros_transport = wifi
+monitor_speed = 115200
+lib_deps =
+    https://github.com/micro-ROS/micro_ros_platformio

examples/micro-ros_subscriber_wifi/src/Main.cpp

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+/* This example shows micro-ROS can be used with Wi-Fi transport on the
+   ESP32-CAM board.
+
+  NOTE: There are two FIXMES in the code where you need to replace the
+  existing code with your Wi-Fi credentials and IP address of the agent.
+
+   To test the ROS 2 communication, you can use the following commands:
+    1. Subscribe to the topic /micro_ros_platformio_node_publisher:
+        ros2 topic echo /micro_ros_platformio_node_publisher std_msgs/msg/Int32
+    2. Publish a message to the topic /cmd_vel:
+        ros2 topic pub -1 /cmd_vel geometry_msgs/msg/Twist "{linear:{x: 1.0}, angular:{z: -1.0}}"
+*/
+
+#include <Arduino.h>
+#include <geometry_msgs/msg/twist.h>
+#include <micro_ros_platformio.h>
+#include <rcl/rcl.h>
+#include <rclc/executor.h>
+#include <rclc/rclc.h>
+#include <std_msgs/msg/int32.h>
+
+// FIXME: Replace with your Wi-Fi credentials.
+#define SSID "your ssid"
+#define SSID_PW "your password"
+
+// Publisher
+rcl_publisher_t publisher;
+std_msgs__msg__Int32 msg;
+
+// Subscriber
+static const char *k_twist = "cmd_vel";
+static rcl_subscription_t subscriber_twist;
+static geometry_msgs__msg__Twist twist_msg;
+
+// Node, executor and timer variables.
+rclc_executor_t executor;
+rclc_support_t support;
+rcl_allocator_t allocator;
+rcl_node_t node;
+rcl_timer_t timer;
+
+#define RCCHECK(fn)                                                    \
+  {                                                                    \
+    rcl_ret_t temp_rc = fn;                                            \
+    if ((temp_rc != RCL_RET_OK)) {                                     \
+      printf("Failed status on line %d: %d. Message: %s, Aborting.\n", \
+             __LINE__, (int)temp_rc, rcl_get_error_string().str);      \
+      error_loop(temp_rc);                                             \
+    }                                                                  \
+  }
+
+#define RCSOFTCHECK(fn)                                               \
+  {                                                                   \
+    rcl_ret_t temp_rc = fn;                                           \
+    if ((temp_rc != RCL_RET_OK)) {                                    \
+      printf("Failed status on line %d: %d. Continuing.\n", __LINE__, \
+             (int)temp_rc);                                           \
+    }                                                                 \
+  }
+
+// Error handle loop
+void error_loop(rcl_ret_t rc) {
+  Serial.println("Error loop");
+  while (1) {
+    delay(100);
+  }
+}
+
+void timer_callback(rcl_timer_t *timer, int64_t last_call_time) {
+  RCLC_UNUSED(last_call_time);
+  if (timer != NULL) {
+    RCSOFTCHECK(rcl_publish(&publisher, &msg, NULL));
+    msg.data++;
+    Serial.println("Published message");
+  }
+}
+
+void twist_callback(const void *msg_in) {
+  const geometry_msgs__msg__Twist *msg =
+      (const geometry_msgs__msg__Twist *)msg_in;
+  Serial.println("Received twist message");
+  Serial.print("Linear: ");
+  Serial.print(msg->linear.x);
+  Serial.print(", ");
+  Serial.print("Angular: ");
+  Serial.print(msg->angular.z);
+  Serial.println();
+}
+
+void setup() {
+  // Configure serial transport
+  Serial.begin(115200);
+  Serial.println("Started");
+
+  char ssid[] = SSID;
+  char ssid_pw[] = SSID_PW;
+  // FIXME: Replace with your Wi-Fi credentials.
+  IPAddress agent_ip(192, 168, 0, 2);
+  const uint16_t k_agent_port = 8888;
+  set_microros_wifi_transports(ssid, ssid_pw, agent_ip, k_agent_port);
+  delay(2000);
+  Serial.println("Connected");
+
+  allocator = rcl_get_default_allocator();
+
+  // create init_options
+  RCCHECK(rclc_support_init(&support, 0, NULL, &allocator));
+
+  // create node
+  RCCHECK(
+      rclc_node_init_default(&node, "micro_ros_platformio_node", "", &support));
+
+  // create publisher
+  RCCHECK(rclc_publisher_init_default(
+      &publisher, &node, ROSIDL_GET_MSG_TYPE_SUPPORT(std_msgs, msg, Int32),
+      "micro_ros_platformio_node_publisher"));
+
+  // create timer.
+  const unsigned int timer_timeout = 1000;
+  RCCHECK(rclc_timer_init_default(&timer, &support, RCL_MS_TO_NS(timer_timeout),
+                                  timer_callback));
+
+  // Create subscriber.
+  RCCHECK(rclc_subscription_init_best_effort(
+      &subscriber_twist, &node,
+      ROSIDL_GET_MSG_TYPE_SUPPORT(geometry_msgs, msg, Twist), k_twist));
+
+  // create executor
+  RCCHECK(rclc_executor_init(&executor, &support.context, 2, &allocator));
+  RCCHECK(rclc_executor_add_timer(&executor, &timer));
+  RCCHECK(rclc_executor_add_subscription(
+      &executor, &subscriber_twist, &twist_msg, &twist_callback, ON_NEW_DATA));
+}
+
+void loop() {
+  delay(100);
+  RCSOFTCHECK(rclc_executor_spin_some(&executor, RCL_MS_TO_NS(100)));
+}