Event Manager

The Event Manager is a piece of software that supports development of consistent, modular, event-based application. In an event-based application, parts of the application functionality are separated into isolated modules that communicate with each other using events. Events are submitted by modules and other modules can subscribe and react to them. The Event Manager acts as coordinator of the event-based communication.

Architecture of an application based on Event Manager

For a usage example of the Event Manger module and event structure in the nRF Connect SDK, see the Common Application Framework (CAF).

See the Event Manager sample for a simple example of how to use the Event Manager.

Events

Events are structured data types that are defined by the application and can contain additional data.

The Event Manager handles the events by processing and propagating all of them to the modules (listeners) that subscribe to a specific event. Multiple modules can subscribe to the same event. As part of this communication, listeners can process events differently based on their type.

The Event Manager provides API for defining, creating, and subscribing events. See Implementing an event type for details about how to create custom event types.

Modules

Modules are separate source files that can subscribe to every defined event. You can use events for communication between modules.

There is no limitation as to how many modules each module can subscribe to. An application can have as many modules as required.

The Event Manager provides API for subscribing modules to a specific events defined in the application. When a module is subscribing to a specific event, it is called a listener module. Every listener is identified by a unique name.

Configuration

To use Event Manager, you must enable the following Kconfig options:

Initializing the Event Manager

You must initialize the Event Manager in your main.c file. Initializing the Event Manger allows it to handle submitted events and deliver them to modules that subscribe to the specified event type.

To initialize the Event Manager, complete the following steps:

  1. Include event_manager.h in your main.c file.

  2. Call event_manager_init().

Implementing events and modules

If an application module is supposed to react to an event, your application must implement an event type, submit the event, and register the module as listener. Read the following sections for details.

Implementing an event type

If you want to easily create and implement custom event types, the Event Manager provides macros to add a new event type in your application. Complete the following steps:

Create a header file

To create a header file for the event type you want to define:

  1. Make sure the header file includes the Event Manager header file:

    #include event_manager.h
    
  2. Define the new event type by creating a structure that contains event_header header as the first field.

  3. Optionally, add additional custom data fields to the structure.

  4. Declare the event type with the EVENT_TYPE_DECLARE macro, passing the name of the created structure as an argument.

The following code example shows a header file for the event type sample_event:

#include <event_manager.h>

struct sample_event {
        struct event_header header;

        /* Custom data fields. */
        int8_t value1;
        int16_t value2;
        int32_t value3;
};

EVENT_TYPE_DECLARE(sample_event);

In some use cases, the length of the data associated with an event may vary. You can use the EVENT_TYPE_DYNDATA_DECLARE macro instead of EVENT_TYPE_DECLARE to declare an event type with variable data size. In such case, add the data with the variable size as the last member of the event structure. For example, you can add the variable size data to a previously defined event by applying the following change to the code:

#include <event_manager.h>

struct sample_event {
        struct event_header header;

        /* Custom data fields. */
        int8_t value1;
        int16_t value2;
        int32_t value3;
        struct event_dyndata dyndata;
};

EVENT_TYPE_DYNDATA_DECLARE(sample_event);

In this example, the event_dyndata contains the following information:

Create a source file

To create a source file for the event type you defined in the header file:

  1. Include the header file for the new event type in your source file.

  2. Define the event type with the EVENT_TYPE_DEFINE macro. Passing the name of the event type as declared in the header and the additional parameters. For example, you can provide a function that fills a buffer with a string version of the event data (used for logging).

The following code example shows a source file for the event type sample_event:

#include "sample_event.h"

static int log_sample_event(const struct event_header *eh, char *buf,
                            size_t buf_len)
{
        struct sample_event *event = cast_sample_event(eh);

        return snprintf(buf, buf_len, "val1=%d val2=%d val3=%d", event->value1,
                        event->value2, event->value3);
}

EVENT_TYPE_DEFINE(sample_event,      /* Unique event name. */
                  true,              /* Event logged by default. */
                  log_sample_event,  /* Function logging event data. */
                  NULL);             /* No event info provided. */

Submitting an event

To submit an event of a given type, for example sample_event:

  1. Allocate the event by calling the function with the name new_event_type_name. For example, new_sample_event().

  2. Write values to the data fields.

  3. Use EVENT_SUBMIT to submit the event.

The following code example shows how to create and submit an event of type sample_event that has three data fields:

/* Allocate event. */
struct sample_event *event = new_sample_event();

/* Write data to datafields. */
event->value1 = value1;
event->value2 = value2;
event->value3 = value3;

/* Submit event. */
EVENT_SUBMIT(event);

If an event type also defines data with variable size, you must pass also the size of the data as an argument to the function that allocates the event. For example, if the sample_event also contains data with variable size, you must apply the following changes to the code:

/* Allocate event. */
struct sample_event *event = new_sample_event(my_data_size);

/* Write data to datafields. */
event->value1 = value1;
event->value2 = value2;
event->value3 = value3;

/* Write data with variable size. */
memcpy(event->dyndata.data, my_buf, my_data_size);

/* Submit event. */
EVENT_SUBMIT(event);

After the event is submitted, the Event Manager adds it to the processing queue. When the event is processed, the Event Manager notifies all modules that subscribe to this event type.

Note

Events are dynamically allocated and must be submitted. If an event is not submitted, it will not be handled and the memory will not be freed.

Registering a module as listener

If you want a module to receive events managed by the Event Manager, you must register it as a listener and you must subscribe it to a given event type.

To turn a module into a listener for specific event types, complete the following steps:

  1. Include the header files for the respective event types, for example, #include "sample_event.h".

  2. Implement an Event handler function and define the module as a listener with the EVENT_LISTENER macro, passing both the name of the module and the event handler function as arguments.

  3. Subscribe the listener to specific event types.

For subscribing to an event type, the Event Manager provides three types of subscriptions, differing in priority. They can be registered with the following macros:

There is no defined order in which subscribers of the same priority are notified.

The module will receive events for the subscribed event types only. The listener name passed to the subscribe macro must be the same one used in the macro EVENT_LISTENER.

Implementing an event handler function

The event handler function is called when any of the subscribed event types are being processed. Only one event handler function can be registered per listener. Therefore, if a listener subscribes to multiple event types, the function must handle all of them.

The event handler gets a pointer to the event_header structure as the function argument. The function should return true to consume the event, which means that the event is not propagated to further listeners, or false, otherwise.

To check if an event has a given type, call the function with the name is_event_type_name (for example, is_sample_event()), passing the pointer to the event header as the argument. This function returns true if the event matches the given type, or false otherwise.

To access the event data, cast the event_header structure to a proper event type, using the function with the name cast_event_type_name (for example, cast_sample_event()), passing the pointer to the event header as the argument.

Code example

The following code example shows how to register an event listener with an event handler function and subscribe to the event type sample_event:

#include "sample_event.h"

static bool event_handler(const struct event_header *eh)
{
        if (is_sample_event(eh)) {

                /* Accessing event data. */
                struct sample_event *event = cast_sample_event(eh);

                int8_t v1 = event->value1;
                int16_t v2 = event->value2;
                int32_t v3 = event->value3;

                /* Actions when received given event type. */
                foo(v1, v2, v3);

                return false;
        }

        return false;
}

EVENT_LISTENER(sample_module, event_handler);
EVENT_SUBSCRIBE(sample_module, sample_event);

The variable size data is accessed in the same way as the other members of the structure defining an event.

Event Manager extensions

The Event Manager provides additional features that could be helpful when debugging event-based applications.

Tracing hooks

Event Manager provides tracing hooks that you can use at run time to get information about Event Manager initialization, event submission, and event execution. The hooks are provided as weak functions. You can override them for interacting with custom profiler or for other purposes.

The following weak functions are provided by Event Manager as hooks:

For details, refer to API documentation.

Shell integration

Shell integration is available to display additional information and to dynamically enable or disable logging for given event types.

The Event Manager is integrated with Zephyr’s Shell module. When the shell is turned on, an additional subcommand set (event_manager) is added.

This subcommand set contains the following commands:

show_listeners

Show all registered listeners.

show_subscribers

Show all registered subscribers.

show_events

Show all registered event types. The letters “E” or “D” indicate if logging is currently enabled or disabled for a given event type.

enable or disable

Enable or disable logging. If called without additional arguments, the command applies to all event types. To enable or disable logging for specific event types, pass the event type indexes, as displayed by show_events, as arguments.

API documentation

Header file: include/event_manager.h
Source files: subsys/event_manager/
group event_manager

Event Manager.

Defines

SUBS_PRIO_MIN

Index of the highest subscriber priority level.

SUBS_PRIO_MAX

Index of the lowest subscriber priority level.

SUBS_PRIO_COUNT

Number of subscriber priority levels.

EVENT_LISTENER(lname, cb_fn)

Create an event listener object.

Parameters
  • lname – Module name.

  • cb_fn – Pointer to the event handler function.

EVENT_SUBSCRIBE_EARLY(lname, ename)

Subscribe a listener to the early notification list for an event type.

Parameters
  • lname – Name of the listener.

  • ename – Name of the event.

EVENT_SUBSCRIBE(lname, ename)

Subscribe a listener to the normal notification list for an event type.

Parameters
  • lname – Name of the listener.

  • ename – Name of the event.

EVENT_SUBSCRIBE_FINAL(lname, ename)

Subscribe a listener to an event type as final module that is being notified.

Parameters
  • lname – Name of the listener.

  • ename – Name of the event.

EVENT_TYPE_DECLARE(ename)

Declare an event type.

This macro provides declarations required for an event to be used by other modules.

Parameters
  • ename – Name of the event.

EVENT_TYPE_DYNDATA_DECLARE(ename)

Declare an event type with dynamic data size.

This macro provides declarations required for an event to be used by other modules. Declared event will use dynamic data.

Parameters
  • ename – Name of the event.

EVENT_TYPE_DEFINE(ename, init_log_en, log_fn, ev_info_struct)

Define an event type.

This macro defines an event type. In addition, it defines functions specific to the event type and the event type structure.

For every defined event, the following functions are created, where event_type is replaced with the given event type name ename (for example, button_event):

  • new_event_type - Allocates an event of a given type.

  • is_event_type - Checks if the event header that is provided as argument represents the given event type.

  • cast_event_type - Casts the event header that is provided as argument to an event of the given type.

Parameters
  • ename – Name of the event.

  • init_log_en – Bool indicating if the event is logged by default.

  • log_fn – Function to stringify an event of this type.

  • ev_info_struct – Data structure describing the event type.

ASSERT_EVENT_ID(id)

Verify if an event ID is valid.

The pointer to an event type structure is used as its ID. This macro validates that the provided pointer is within the range where event type structures are defined.

Parameters
  • id – ID.

EVENT_SUBMIT(event)

Submit an event.

This helper macro simplifies the event submission.

Parameters
  • event – Pointer to the event object.

Functions

int event_manager_init(void)

Initialize the Event Manager.

Returns

0 – If the operation was successful.

void event_manager_trace_event_execution(const struct event_header *eh, bool is_start)

Trace event execution. The behavior of this function depends on the actual implementation. The default implementation of this function is no-operation. It is annotated as weak and is meant to be overridden by layer adding support for profiling mechanism.

void event_manager_trace_event_submission(const struct event_header *eh, const void *trace_info)

Trace event submission. The behavior of this function depends on the actual implementation. The default implementation of this function is no-operation. It is annotated as weak and is meant to be overridden by layer adding support for profiling mechanism.

int event_manager_trace_event_init(void)

Initialize tracing in the Event Manager. The behavior of this function depends on the actual implementation. The default implementation of this function is no-operation. It is annotated as weak and is meant to be overridden by layer adding support for profiling mechanism.

void *event_manager_alloc(size_t size)

Allocate event. The behavior of this function depends on the actual implementation. The default implementation of this function is same as k_malloc. It is annotated as weak and can be overridden by user.

Parameters
  • size – Amount of memory requested (in bytes).

Returns

Address – of the allocated memory if successful, otherwise NULL.

void event_manager_free(void *addr)

Free memory occupied by the event. The behavior of this function depends on the actual implementation. The default implementation of this function is same as k_free. It is annotated as weak and can be overridden by user.

Parameters
  • addr – Pointer to previously allocated memory.

struct event_header
#include <event_manager.h>

Event header.

When defining an event structure, the event header must be placed as the first field.

Public Members

sys_snode_t node

Linked list node used to chain events.

const struct event_type *type_id

Pointer to the event type object.

struct event_dyndata
#include <event_manager.h>

Dynamic event data.

When defining an event structure, the dynamic event data must be placed as the last field.

Public Members

size_t size

Size of the dynamic data.

uint8_t data[0]

Dynamic data.

struct event_listener
#include <event_manager.h>

Event listener.

All event listeners must be defined using EVENT_LISTENER.

Public Members

const char *name

Name of this listener.

bool (*notification)(const struct event_header *eh)

Pointer to the function that is called when an event is handled.

struct event_subscriber
#include <event_manager.h>

Event subscriber.

Public Members

const struct event_listener *listener

Pointer to the listener.

struct event_type
#include <event_manager.h>

Event type.

Public Members

const char *name

Event name.

const struct event_subscriber *subs_start[(_SUBS_PRIO_FINAL - _SUBS_PRIO_FIRST + 1)]

Array of pointers to the array of subscribers.

const struct event_subscriber *subs_stop[(_SUBS_PRIO_FINAL - _SUBS_PRIO_FIRST + 1)]

Array of pointers to the element directly after the array of subscribers.

bool init_log_enable

Bool indicating if the event is logged by default.

int (*log_event)(const struct event_header *eh, char *buf, size_t buf_len)

Function to log data from this event.

const void *trace_data

Custom data related to tracking.