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221 lines
12 KiB
ReStructuredText
221 lines
12 KiB
ReStructuredText
Event Loop Library
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==================
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Overview
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--------
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The event loop library allows components to declare events to which other components can register handlers -- code which will
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execute when those events occur. This allows loosely coupled components to attach desired behavior to changes in state of other components
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without application involvement. For instance, a high level connection handling library may subscribe to events produced
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by the wifi subsystem directly and act on those events. This also simplifies event processing by serializing and deferring
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code execution to another context.
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Using ``esp_event`` APIs
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------------------------
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There are two objects of concern for users of this library: events and event loops.
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Events are occurences of note. For example, for WiFi, a successful connection to the access point may be an event.
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Events are referenced using a two part identifier which are discussed more :ref:`here <esp-event-declaring-defining-events>`.
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Event loops are the vehicle by which events get posted by event sources and handled by event handler functions.
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These two appear prominently in the event loop library APIs.
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Using this library roughly entails the following flow:
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1. A user defines a function that should run when an event is posted to a loop. This function is referred to as the event handler. It should have the same signature as :cpp:type:`esp_event_handler_t`.
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2. An event loop is created using :cpp:func:`esp_event_loop_create`, which outputs a handle to the loop of type :cpp:type:`esp_event_loop_handle_t`. Event loops created using this API are referred to as user event loops. There is, however, a special type of event loop called the default event loop which are discussed :ref:`here <esp-event-default-loops>`.
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3. Components register event handlers to the loop using :cpp:func:`esp_event_handler_register_with`. Handlers can be registered with multiple loops, more on that :ref:`here <esp-event-handler-registration>`.
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4. Event sources post an event to the loop using :cpp:func:`esp_event_post_to`.
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5. Components wanting to remove their handlers from being called can do so by unregistering from the loop using :cpp:func:`esp_event_handler_unregister_with`.
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6. Event loops which are no longer needed can be deleted using :cpp:func:`esp_event_loop_delete`.
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In code, the flow above may look like as follows:
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.. code-block:: c
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// 1. Define the event handler
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void run_on_event(void* handler_arg, esp_event_base_t base, int32_t id, void* event_data)
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{
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// Event handler logic
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}
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void app_main()
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{
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// 2. A configuration structure of type esp_event_loop_args_t is needed to specify the properties of the loop to be
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// created. A handle of type esp_event_loop_handle_t is obtained, which is needed by the other APIs to reference the loop
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// to perform their operations on.
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esp_event_loop_args_t loop_args = {
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.queue_size = ...,
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.task_name = ...
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.task_priority = ...,
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.task_stack_size = ...,
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.task_core_id = ...
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};
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esp_event_loop_handle_t loop_handle;
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esp_event_loop_create(&loop_args, &loop_handle)
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// 3. Register event handler defined in (1). MY_EVENT_BASE and MY_EVENT_ID specifies a hypothetical
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// event that handler run_on_event should execute on when it gets posted to the loop.
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esp_event_handler_register_with(loop_handle, MY_EVENT_BASE, MY_EVENT_ID, run_on_event, ...);
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...
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// 4. Post events to the loop. This queues the event on the event loop. At some point in time
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// the event loop executes the event handler registered to the posted event, in this case run_on_event.
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// For simplicity sake this example calls esp_event_post_to from app_main, but posting can be done from
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// any other tasks (which is the more interesting use case).
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esp_event_post_to(loop_handle, MY_EVENT_BASE, MY_EVENT_ID, ...);
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...
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// 5. Unregistering an unneeded handler
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esp_event_handler_unregister_with(loop_handle, MY_EVENT_BASE, MY_EVENT_ID, run_on_event);
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...
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// 6. Deleting an unneeded event loop
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esp_event_loop_delete(loop_handle);
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}
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.. _esp-event-declaring-defining-events:
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Declaring and defining events
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-----------------------------
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As mentioned previously, events consists of two-part identifers: the event base and the event ID. The event base identifies an independent group
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of events; the event ID identifies the event within that group. Think of the event base and event ID as a
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person's last name and first name, respectively. A last name identifies a family, and the first name identifies a person within that family.
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The event loop library provides macros to declare and define the event base easily.
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Event base declaration:
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.. code-block:: c
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ESP_EVENT_DECLARE_BASE(EVENT_BASE)
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Event base definition:
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.. code-block:: c
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ESP_EVENT_DEFINE_BASE(EVENT_BASE)
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.. note::
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In IDF, the base identifiers for system events are uppercase and are postfixed with ``_EVENT``. For example, the base for wifi events is declared and defined
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as ``WIFI_EVENT``, the ethernet event base ``ETHERNET_EVENT``, and so on. The purpose is to have event bases look like constants (although
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they are global variables considering the defintions of macros ``ESP_EVENT_DECLARE_BASE`` and ``ESP_EVENT_DEFINE_BASE``).
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For event ID's, declaring them as enumerations is recommended. Once again, for visibility, these are typically placed in public header files.
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Event ID:
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.. code-block:: c
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enum {
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EVENT_ID_1,
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EVENT_ID_2,
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EVENT_ID_3,
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...
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}
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.. _esp-event-default-loops:
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Default Event Loop
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------------------
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The default event loop is a special type of loop used for system events (WiFi events, for example). The handle for this
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loop is hidden from the user. The creation, deletion, handler registration/unregistration and posting of events is done
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through a variant of the APIs for user event loops. The table below enumerates those variants, and the user event
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loops equivalent.
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+---------------------------------------------------+---------------------------------------------------+
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| User Event Loops | Default Event Loops |
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+===================================================+===================================================+
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| :cpp:func:`esp_event_loop_create` | :cpp:func:`esp_event_loop_create_default` |
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+---------------------------------------------------+---------------------------------------------------+
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| :cpp:func:`esp_event_loop_delete` | :cpp:func:`esp_event_loop_delete_default` |
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+---------------------------------------------------+---------------------------------------------------+
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| :cpp:func:`esp_event_handler_register_with` | :cpp:func:`esp_event_handler_register` |
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+---------------------------------------------------+---------------------------------------------------+
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| :cpp:func:`esp_event_handler_unregister_with` | :cpp:func:`esp_event_handler_unregister` |
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+---------------------------------------------------+---------------------------------------------------+
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| :cpp:func:`esp_event_post_to` | :cpp:func:`esp_event_post` |
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+---------------------------------------------------+---------------------------------------------------+
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If you compare the signatures for both, they are mostly similar except the for the lack of loop handle
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specification for the default event loop APIs.
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Other than the API difference and the special designation to which system events are posted to, there is no difference
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to how default event loops and user event loops behave. It is even possible for users to post their own events
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to the default event loop, should the user opt to not create their own loops to save memory.
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.. _esp-event-handler-registration:
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Notes on Handler Registration
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-----------------------------
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It is possible to register a single handler to multiple events individually, i.e. using multiple calls to :cpp:func:`esp_event_handler_register_with`.
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For those multiple calls, the specific event base and event ID can be specified with which the handler should execute.
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However, in some cases it is desirable for a handler to execute on (1) all events that get posted to a loop or (2) all events
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of a particular base identifier. This is possible using the special event base identifier ``ESP_EVENT_ANY_BASE`` and
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special event ID ``ESP_EVENT_ANY_ID``. These special identifiers may be passed as the event base and event ID arguments
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for :cpp:func:`esp_event_handler_register_with`.
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Therefore, the valid arguments to :cpp:func:`esp_event_handler_register_with` are:
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1. <event base>, <event ID> - handler executes when the event with base <event base> and event ID <event ID> gets posted to the loop
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2. <event base>, ESP_EVENT_ANY_ID - handler executes when any event with base <event base> gets posted to the loop
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3. ESP_EVENT_ANY_BASE, ESP_EVENT_ANY_ID - handler executes when any event gets posted to the loop
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As an example, suppose the following handler registrations were performed:
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.. code-block:: c
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esp_event_handler_register_with(loop_handle, MY_EVENT_BASE, MY_EVENT_ID, run_on_event_1, ...);
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esp_event_handler_register_with(loop_handle, MY_EVENT_BASE, ESP_EVENT_ANY_ID, run_on_event_2, ...);
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esp_event_handler_register_with(loop_handle, ESP_EVENT_ANY_BASE, ESP_EVENT_ANY_ID, run_on_event_3, ...);
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If the hypothetical event ``MY_EVENT_BASE``, ``MY_EVENT_ID`` is posted, all three handlers ``run_on_event_1``, ``run_on_event_2``,
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and ``run_on_event_3`` would execute.
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If the hypothetical event ``MY_EVENT_BASE``, ``MY_OTHER_EVENT_ID`` is posted, only ``run_on_event_2`` and ``run_on_event_3`` would execute.
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If the hypothetical event ``MY_OTHER_EVENT_BASE``, ``MY_OTHER_EVENT_ID`` is posted, only ``run_on_event_3`` would execute.
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Handler Registration and Handler Dispatch Order
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The general rule is that for handlers that match a certain posted event during dispatch, those which are registered first also gets executed first. The user can then control which handlers get executed first by
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registering them before other handlers, provided that all registrations are performed using a single task.
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If the user plans to take advantage of this behavior, caution must be exercised if there are multiple tasks registering handlers. While the 'first registered, first executed'
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behavior still holds true, the task which gets executed first will also get their handlers registered first. Handlers registered one after the other by a single task
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will still be dispatched in the order relative to each other, but if that task gets pre-empted in between registration by another task which also registers handlers; then during dispatch those
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handlers will also get executed in between.
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Event loop profiling
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--------------------
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A configuration option :envvar:`CONFIG_EVENT_LOOP_PROFILING` can be enabled in order to activate statistics collection for all event loops created.
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The function :cpp:func:`esp_event_dump` can be used to output the collected statistics to a file stream. More details on the information included in the dump
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can be found in the :cpp:func:`esp_event_dump` API Reference.
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Application Example
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-------------------
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Examples on using the ``esp_event`` library can be found in :example:`system/esp_event`. The examples cover event declaration, loop creation, handler registration and unregistration and event posting.
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Other examples which also adopt esp_event library:
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* :example:`NMEA Parser <peripherals/uart/nmea0183_parser>`, which will decode the statements received from GPS.
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API Reference
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-------------
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.. include:: /_build/inc/esp_event.inc
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.. include:: /_build/inc/esp_event_base.inc
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