wifi: add SoftAPModeProbeRequestReceived event handler (#2917)

- add probe request event handler (#2910)
- update WiFi events handling example to use new handler

Pro tip: replace blinking LED with ‘analogWrite’ and connect the pin to
a loudspeaker (or use a servo to hit a bell). Get notified when someone
with a smartphone wanders around your country house.

libraries/ESP8266WiFi/examples/WiFiEvents/WiFiEvents.ino

@@ -0,0 +1,99 @@
+/*
+    This sketch shows how to use WiFi event handlers.
+
+    In this example, ESP8266 works in AP mode.
+    Three event handlers are demonstrated:
+    - station connects to the ESP8266 AP
+    - station disconnects from the ESP8266 AP
+    - ESP8266 AP receives a probe request from a station
+
+    Written by Markus Sattler, 2015-12-29.
+    Updated for new event handlers by Ivan Grokhotkov, 2017-02-02.
+    This example is released into public domain,
+    or, at your option, CC0 licensed.
+*/
+
+#include <ESP8266WiFi.h>
+#include <stdio.h>
+
+const char* ssid     = "ap-ssid";
+const char* password = "ap-password";
+
+WiFiEventHandler stationConnectedHandler;
+WiFiEventHandler stationDisconnectedHandler;
+WiFiEventHandler probeRequestPrintHandler;
+WiFiEventHandler probeRequestBlinkHandler;
+
+bool blinkFlag;
+
+void setup() {
+  Serial.begin(115200);
+  pinMode(LED_BUILTIN, OUTPUT);
+  digitalWrite(LED_BUILTIN, HIGH);
+
+  // Don't save WiFi configuration in flash - optional
+  WiFi.persistent(false);
+
+  // Set up an access point
+  WiFi.mode(WIFI_AP);
+  WiFi.softAP(ssid, password);
+
+  // Register event handlers.
+  // Callback functions will be called as long as these handler objects exist.
+  // Call "onStationConnected" each time a station connects
+  stationConnectedHandler = WiFi.onSoftAPModeStationConnected(&onStationConnected);
+  // Call "onStationDisconnected" each time a station disconnects
+  stationDisconnectedHandler = WiFi.onSoftAPModeStationDisconnected(&onStationDisconnected);
+  // Call "onProbeRequestPrint" and "onProbeRequestBlink" each time
+  // a probe request is received.
+  // Former will print MAC address of the station and RSSI to Serial,
+  // latter will blink an LED.
+  probeRequestPrintHandler = WiFi.onSoftAPModeProbeRequestReceived(&onProbeRequestPrint);
+  probeRequestBlinkHandler = WiFi.onSoftAPModeProbeRequestReceived(&onProbeRequestBlink);
+}
+
+void onStationConnected(const WiFiEventSoftAPModeStationConnected& evt) {
+  Serial.print("Station connected: ");
+  Serial.println(macToString(evt.mac));
+}
+
+void onStationDisconnected(const WiFiEventSoftAPModeStationDisconnected& evt) {
+  Serial.print("Station disconnected: ");
+  Serial.println(macToString(evt.mac));
+}
+
+void onProbeRequestPrint(const WiFiEventSoftAPModeProbeRequestReceived& evt) {
+  Serial.print("Probe request from: ");
+  Serial.print(macToString(evt.mac));
+  Serial.print(" RSSI: ");
+  Serial.println(evt.rssi);
+}
+
+void onProbeRequestBlink(const WiFiEventSoftAPModeProbeRequestReceived&) {
+  // We can't use "delay" or other blocking functions in the event handler.
+  // Therefore we set a flag here and then check it inside "loop" function.
+  blinkFlag = true;
+}
+
+void loop() {
+  if (millis() > 10000 && probeRequestPrintHandler) {
+    // After 10 seconds, disable the probe request event handler which prints.
+    // Other three event handlers remain active.
+    Serial.println("Not printing probe requests any more (LED should still blink)");
+    probeRequestPrintHandler = WiFiEventHandler();
+  }
+  if (blinkFlag) {
+    blinkFlag = false;
+    digitalWrite(LED_BUILTIN, LOW);
+    delay(100);
+    digitalWrite(LED_BUILTIN, HIGH);
+  }
+  delay(10);
+}
+
+String macToString(const unsigned char* mac) {
+  char buf[20];
+  snprintf(buf, sizeof(buf), "%02x:%02x:%02x:%02x:%02x:%02x",
+           mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
+  return String(buf);
+}