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libevents

This project contains the events interface, which includes the following:

  • a protocol to send events based on an ID (and possibly arguments) to inform other components (in a reliable way)
  • using MAVLink as a transport layer. Each MAVLink component can send and/or receive events to/from other components. The primary use-case is for an autopilot to inform a user (through a GCS) about certain events (such as a failsafe action).
  • metadata for each event, for example a description, stored as JSON. Metadata is typically distributed via the COMPONENT_INFORMATION MAVLink API.
  • a set of commonly used predefined events that can be used to build simple protocols
  • a parser (MAVLink-independent) to combine event ID's with the JSON metadata for display or analysis purposes. This also includes log processing.

Design

Initial design doc: https://docs.google.com/document/d/1VLdnvjG6r6Cw-_FSo9lcG-Nlk7eGA7DaEHhe-3bP4-Q

Event definitions

Event definitions are separated by namespaces. Each namespace has a name and an 8 bit component ID (typically matching the MAVLink component ID. For multi-instance components, it should be the first one, e.g. GIMBAL1). Within a namespace, each event has a unique name and sub ID (24 bits). The event ID is the combined component ID with sub ID. So events can be uniquely identified for the whole system by ID or <namespace>::<name>. This is generally not required, as metadata is queried independently per MAVLink component. It is only relevant when combining all events from a system without further component identification, e.g. for logging.

Each event can have a set of arguments, using either basic types or enums.

Message and description

Events have a single-line (short) message and possibly a more extensive description.

Supported format and parsing:

  • characters can be escaped with \

    These have to be escaped: '\\', '\<', '\{'.

  • tags:

    • Profiles: <profile name="[!]NAME">CONTENT</profile>

      CONTENT will only be shown if the name matches the configured profile. This can be used for example to hide developer information from end-users.

    • URLs: <a [href="URL"]>CONTENT</a>. If href is not set, use CONTENT as URL

    • Parameters: <param>PARAM_NAME</param>

    • no nested tags of the same type

  • arguments: following python syntax, with 1-based indexing (instead of 0)

    • general form: {ARG_IDX[:.NUM_DECIMAL_DIGITS][UNIT]}

      UNIT:

      • m: horizontal distance in meters
      • m_v: vertical distance in meters
      • m^2: area in m^2
      • m/s: speed in m/s
      • C: temperature in degrees celcius
      • These are used to be able to switch the display unit (e.g. metric vs imperial)

Log levels

Events have 2 log levels:

  • Internal: for logging purposes
  • External: log level used by a GCS

Typically the same log level is used for both cases, but they can differ. For example a low battery event could be shown as warning to the user, but is either an info or disabled in the log.

An event with log level 'Protocol' should not be shown.

Event groups

Every event belongs to a group. That is essentially just a tag, and can be used to treat a set of events different from others. A UI should not display unkown groups.

Special groups:

  • calibration: used for the sensor/RC calibration protocol

  • health and arming_check: used to report health and arming check results. Apart from the 2 summary messages (via type='summary'), each event needs to have at least 2 arguments:

    • <navigation_group>: affected flight modes (bitmask)
    • uint8_t health_component_index: affected health component index (to the <health_component> bitset), can be 0 if none applies.

    <navigation_group> and <health_component> are custom enums (bitsets) based on any unsigned integer. The mapping from the navigation groups to custom_modes is defined via navigation_mode_groups in the json. The first entry (1. bit) in <health_component> must always be 'none'.

    The order of sending is:

    • arming_check_summary
    • set of N health and arming check events
    • health_summary

    The set of checks can be split into multiple chunks that the receiver then combines.

    See libs/cpp/parse/health_and_arming_checks.h for an implementation.

    Events with type=append_health_and_arming_messages can be used to display a message with all the current arming and health warnings and errors appended to the message. These events need to specify the mode as the first argument.

common namespace

Since events in the common namespace are used by multiple components, they cannot be changed arbitrarily. Rules:

  • changing the description or message is ok, as long as the semantic is the same
  • arguments cannot be changed, it is only possible to add new arguments
  • for any other change (name, ID, ...), a new event needs to be defined, and the old one deprecated.
  • enums cannot be changed, except for adding new values

Serialization

  • Event arguments are serialized in the same order as defined. No extra padding, so accesses might be unaligned.
  • Everything assumes little endian

How to use the library

C++ (via cmake)

There are two options to include the library in a project:

  • using a submodule / subdirectory
  • using ExternalProject_Add

Using a Submodule

At the repository as a submodule:

cd <your_project>
git submodule add https://github.com/mavlink/libevents.git

In the CMakeLists.txt, add:

add_subdirectory(libevents/libs/cpp)
target_link_libraries(your_target PRIVATE libevents)

Then, in a header:

// Enable debug output, comment out to disable
#define LIBEVENTS_PARSER_DEBUG_PRINTF printf
#define LIBEVENTS_DEBUG_PRINTF printf

// Include the mavlink message definitions first
// (If only the parser is used, mavlink is not required)
#include <mavlink.h>

#include "libevents/libs/cpp/protocol/receive.h"
#include "libevents/libs/cpp/parse/parser.h"
#include "libevents/libs/cpp/parse/health_and_arming_checks.h"
#include "libevents/libs/cpp/generated/events_generated.h"

The headers can be used independently:

  • receive.h: Implements the receiving part of the events protocol over MAVLink.
  • parser.h: Allows to convert a serialized event (e.g. received from MAVLink) together with JSON metadata definitions back into the original message.
  • health_and_arming_checks.h: Implements the health and arming subprotocol (receiving part).
  • events_generated.h: Allows to serialize / deserialize commonly defined events.

Using ExternalProject_Add

In the CMakeLists.txt, add:

ExternalProject_Add(
        libevents
        GIT_REPOSITORY https://github.com/mavlink/libevents.git
        GIT_TAG main
        SOURCE_SUBDIR libs/cpp
        CMAKE_ARGS
          "-DCMAKE_PREFIX_PATH:PATH=${CMAKE_PREFIX_PATH}"
          "-DCMAKE_INSTALL_PREFIX:PATH=${CMAKE_INSTALL_PREFIX}"
          "-DCMAKE_TOOLCHAIN_FILE:PATH=${CMAKE_TOOLCHAIN_FILE}"
          "-DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"
          "-DBUILD_SHARED_LIBS=OFF"
          "-DENABLE_TESTING=OFF"
)

find_package(libevents REQUIRED)

target_link_libraries(your_target PRIVATE libevents)

Then, in a header:

// Include the mavlink message definitions first
// (If only the parser is used, mavlink is not required)
#include <mavlink.h>

#include <libevents/protocol/receive.h>
#include <libevents/parse/parser.h>
#include <libevents/parse/health_and_arming_checks.h>
#include <libevents/generated/events_generated.h>