Event Handling

Several ESP-IDF components use events to inform application about state changes, such as connection or disconnection. This document gives an overview of these event mechanisms.

Wi-Fi, Ethernet, and IP Events

Before the introduction of esp_event library, events from Wi-Fi driver, Ethernet driver, and TCP/IP stack were dispatched using the so-called legacy event loop. The following sections explain each of the methods.

esp_event Library Event Loop

esp_event library is designed to supersede the legacy event loop for the purposes of event handling in ESP-IDF. In the legacy event loop, all possible event types and event data structures had to be defined in system_event_id_t enumeration and system_event_info_t union, which made it impossible to send custom events to the event loop, and use the event loop for other kinds of events (e.g. Mesh). Legacy event loop also supported only one event handler function, therefore application components could not handle some of Wi-Fi or IP events themselves, and required application to forward these events from its event handler function.

See esp_event library API reference for general information on using this library. Wi-Fi, Ethernet, and IP events are sent to the default event loop provided by this library.

Legacy Event Loop

This event loop implementation is started using esp_event_loop_init() function. Application typically supplies an event handler, a function with the following signature:

esp_err_t event_handler(void *ctx, system_event_t *event)
{
}

Both the pointer to event handler function, and an arbitrary context pointer are passed to esp_event_loop_init().

When Wi-Fi, Ethernet, or IP stack generate an event, this event is sent to a high-priority event task via a queue. Application-provided event handler function is called in the context of this task. Event task stack size and event queue size can be adjusted using CONFIG_SYSTEM_EVENT_TASK_STACK_SIZE and CONFIG_SYSTEM_EVENT_QUEUE_SIZE options, respectively.

Event handler receives a pointer to the event structure (system_event_t) which describes current event. This structure follows a tagged union pattern: event_id member indicates the type of event, and event_info member is a union of description structures. Application event handler will typically use switch(event->event_id) to handle different kinds of events.

If application event handler needs to relay the event to some other task, it is important to note that event pointer passed to the event handler is a pointer to temporary structure. To pass the event to another task, application has to make a copy of the entire structure.

Event IDs and Corresponding Data Structures

Event ID (legacy event ID) Event data structure
Wi-Fi
WIFI_EVENT_WIFI_READY (SYSTEM_EVENT_WIFI_READY) n/a
WIFI_EVENT_SCAN_DONE (SYSTEM_EVENT_SCAN_DONE) wifi_event_sta_scan_done_t
WIFI_EVENT_STA_START (SYSTEM_EVENT_STA_START) n/a
WIFI_EVENT_STA_STOP (SYSTEM_EVENT_STA_STOP) n/a
WIFI_EVENT_STA_CONNECTED (SYSTEM_EVENT_STA_CONNECTED) wifi_event_sta_connected_t
WIFI_EVENT_STA_DISCONNECTED (SYSTEM_EVENT_STA_DISCONNECTED) wifi_event_sta_disconnected_t
WIFI_EVENT_STA_AUTHMODE_CHANGE (SYSTEM_EVENT_STA_AUTHMODE_CHANGE) wifi_event_sta_authmode_change_t
WIFI_EVENT_STA_WPS_ER_SUCCESS (SYSTEM_EVENT_STA_WPS_ER_SUCCESS) n/a
WIFI_EVENT_STA_WPS_ER_FAILED (SYSTEM_EVENT_STA_WPS_ER_FAILED) wifi_event_sta_wps_fail_reason_t
WIFI_EVENT_STA_WPS_ER_TIMEOUT (SYSTEM_EVENT_STA_WPS_ER_TIMEOUT) n/a
WIFI_EVENT_STA_WPS_ER_PIN (SYSTEM_EVENT_STA_WPS_ER_PIN) wifi_event_sta_wps_er_pin_t
WIFI_EVENT_AP_START (SYSTEM_EVENT_AP_START) n/a
WIFI_EVENT_AP_STOP (SYSTEM_EVENT_AP_STOP) n/a
WIFI_EVENT_AP_STACONNECTED (SYSTEM_EVENT_AP_STACONNECTED) wifi_event_ap_staconnected_t
WIFI_EVENT_AP_STADISCONNECTED (SYSTEM_EVENT_AP_STADISCONNECTED) wifi_event_ap_stadisconnected_t
WIFI_EVENT_AP_PROBEREQRECVED (SYSTEM_EVENT_AP_PROBEREQRECVED) wifi_event_ap_probe_req_rx_t
Ethernet
ETHERNET_EVENT_START (SYSTEM_EVENT_ETH_START) n/a
ETHERNET_EVENT_STOP (SYSTEM_EVENT_ETH_STOP) n/a
ETHERNET_EVENT_CONNECTED (SYSTEM_EVENT_ETH_CONNECTED) n/a
ETHERNET_EVENT_DISCONNECTED (SYSTEM_EVENT_ETH_DISCONNECTED) n/a
IP
IP_EVENT_STA_GOT_IP (SYSTEM_EVENT_STA_GOT_IP) ip_event_got_ip_t
IP_EVENT_STA_LOST_IP (SYSTEM_EVENT_STA_LOST_IP) n/a
IP_EVENT_AP_STAIPASSIGNED (SYSTEM_EVENT_AP_STAIPASSIGNED) n/a
IP_EVENT_GOT_IP6 (SYSTEM_EVENT_GOT_IP6) ip_event_got_ip6_t
IP_EVENT_ETH_GOT_IP (SYSTEM_EVENT_ETH_GOT_IP) ip_event_got_ip_t

Mesh Events

ESP-MESH uses a system similar to the Legacy Event Loop to deliver events to the application. See System Events for details.

Bluetooth Events

Various modules of the Bluetooth stack deliver events to applications via dedicated callback functions. Callback functions receive the event type (enumerated value) and event data (union of structures for each event type). The following list gives the registration API name, event enumeration type, and event parameters type.