9.3. Event callback mechanism

9.3.1. Overview

The PikaPython kernel provides an event callback mechanism that supports triggering Python defined callback functions in C events/interrupts.

Note: requires kernel version no less than: v1.8.7

9.3.2. Headers

#include "PikaObj.h"

9.3.3. Data types

typedef PikaObj PikaEventListener;

The event callback mechanism relies on the PikaEventListener event listener, which records the ID of each registered event. When a signal is sent to the event listener, the event listener will call the corresponding Python callback function based on the event ID, and pass the semaphore.

9.3.4. The Event Model

The core of the event model is the PikaEventListener event listener.


The PikaEventListener model is shown above. After registering an event to the event listener, an event item Event Item will be recorded inside the PikaEventListener, including.

  • Event ID the unique ID of the event

  • Event Handler Object event object, which records all the information about the event item

  • Event CallBack event callback function ( Python function )

When the Event Signal event signal arrives, the event listener will match the Event ID to find the corresponding event item, then pass the signal code Event Code to Event CallBak to trigger the callback function.


9.3.5. Event callback mechanism flow

  1. Initialize the event listener

  2. register callback functions in Python

  3. Signal the event listener in C (usually in an interrupt or a callback in C)

  4. the callback function registered in Python is executed

9.3.6. Support event callbacks via PikaStdDevice

Inheriting PikaStdDevice is the easiest way to support event callbacks, the PikaStdDevice.BaseDev device base class already supports the event registration method addEventCallBack.

class BaseDev:
    def addEventCallBack(self, eventCallback: any): ...

    # need override
    def platformGetEventId(self): ...
  • The device classes in PikaStdDevice (e.g. GPIO) all inherit from BaseDev, so they all get the addEventCallBack method and can register callbacks.


class GPIO(BaseDev):

After the platform driver inherits from PikaStdDevice.GPIO, it also gets the addEventCallBack method.


# TemplateDevice.pyi
class GPIO(PikaStdDevice.GPIO):
    # overrid
    def platformGetEventId(self): ...

Just override the platformGetEventId platform method to be able to support registration callbacks.

For example.


const uint32_t GPIO_PA8_EVENT_ID = 0x08;
void TemplateDevice_GPIO_platformGetEventId(PikaObj* self) {
    char* pin = obj_getStr(self, "pin");
    if (strEqu(pin, "PA8")) {
        obj_setInt(self, "eventId", GPIO_PA8_EVENT_ID);

9.3.7. Registering callback functions in Python

  • Define a callback function callBack1 that takes an input parameter signal, signal can receive the incoming signal number.


import TemplateDevice

io1 = TemplateDevice.GPIO()


def callBack1(signal):
    if signal == EVENT_SIGAL_IO_RISING_EDGE:
        print('get rising edge!')
    elif signal == EVENT_SIGAL_IO_FALLING_EDGE:
        print('get falling edge!')


9.3.8. Signal triggering

Send a signal to PikaEventListener when an event callback needs to be triggered.

Example: /port/linux/test/event-test.cpp

  • Get the event listener provided by PikaStdDevice via extern PikaEventListener* g_pika_device_event_listener.

  • Send eventID and signal code via pks_eventLisener_sendSignal.

extern PikaEventListener* g_pika_device_event_listener;
#define GPIO_PA8_EVENT_ID 0x08
TEST(event, gpio) {
    /* init */
    PikaObj* pikaMain = newRootObj("pikaMain", New_PikaMain);
    /* run */
    pikaVM_runFile(pikaMain, "... /... /examples/TemplateDevice/gpio_cb.py");
    /* simulate run in the call back */
    pks_eventLisener_sendSignal(g_pika_device_event_listener, GPIO_PA8_EVENT_ID,
    pks_eventLisener_sendSignal(g_pika_device_event_listener, GPIO_PA8_EVENT_ID,
  • Running results.

get rising edge!
get falling edg! Waiting for the return value

Event callback functions can have return values, such as returning signal directly.

def callBack1(signal):
    return signal

This function requires OS support, and the __platform_thread_delay() method needs to be overridden to be able to dispatch events to the main process while waiting for a return value. If a return value is required, the trigger event can use pks_eventLisener_sendSignalAwaitResult to get the return value of the callback function, which is an ``Arg*` type.

Arg* res_123 = pks_eventLisener_sendSignalAwaitResult(
        g_pika_device_event_listener, GPIO_PA8_EVENT_ID, 123);
int res = arg_getInt(res_123);

Note: requires kernel version >= v1.11.7

9.3.9. Advanced: Custom event registration functions

  • In addition to event callbacks supported by PikaStdDevice, you can also customize event registration functions, which is an advanced part.

  • Custom event registration requires a better understanding of PikaPython’s C-module mechanism and object mechanism.

  • Define a Python interface to a C module that receives incoming event callback functions.

For example.


class BaseDev:
    def addEventCallBack(self, eventCallback: any): ...

The type annotation for the event callback function is any.

  • Registering events in the C module implementation

Example: /package/PikaStdDevice/PikaStdDevice_BaseDev.c

PikaEventListener* g_pika_device_event_listener;

void PikaStdDevice_BaseDev_addEventCallBack(PikaObj* self, Arg* eventCallBack) {
    obj_setArg(self, "eventCallBack", eventCallBack);
    /* init event_listener for the first time */
    if (NULL == g_pika_device_event_listener) {
    if (PIKA_RES_OK != obj_runNativeMethod(self, "platformGetEventId", NULL)) {
        obj_setErrorCode(self, 1);
        __platform_printf("Error: Method %s no found.\r\n",
    uint32_t eventId = obj_getInt(self, "eventId");
    pks_eventLicener_registEvent(g_pika_device_event_listener, eventId, self);
  • Create a global PikaEventListener: g_pika_device_event_listener.

  • Pass self as event handler object and evnetCallBack into self.

  • Get evnetID.

    • This example gets the eventID by calling the platformGetEventId() platform function, which requires BaseDev inheritance, then rewrites platformGetEventId() and sets self. eventId in the overridden platformGetEventId().

    • For example: /package/TemplateDevice/TemplateDevice_GPIO.c

  • Call pks_eventLicener_registEvent to register eventId and self into the event listener.