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Arduino EventManager

Using an event-driven design is a common way to code Arduino projects that interact with the environment around them. EventManager is a single C++ class that provides an event handling system for Arduino. With EventManager you can register functions that "listen" for particular events and when things happen you can "post" events to EventManager. You then use the loop() function to regularly tell EventManager to process events and the appropriate listeners will be called.

EventManger is designed to be interrupt safe, so that you can post events from interrupt handlers. The corresponding listeners will be called from outside the interrupt handler in your loop() function when you tell EventManager to process events.

In keeping with the limited resources of an Arduino system, EventManager is light-weight. There is no dynamic memory allocation. Event queuing is very fast (so you can be comfortable queuing events from interrupt handlers). To keep the footprint minimal, the event queue and the listener list are both small (although you can make them bigger if needed).

NOTE: There are two versions of EventManager. The master branch has a version of EventManager that uses functions as listeners (also known as event handlers). Most users will find that this version meets their needs. The GenericListeners branch contains a version of EventManager that accepts more general types of listeners such as callable member functions and callable objects. If you don't know what these are, stick to the master branch.

Installation

Copy the folder EventManager into your Arduino Libraries folder, as described in the Arduino documentation.

Usage

At the top of your sketch you must include the EventManager header file

    #include <EventManager.h>

And then at global scope you should instantiate an EventManager object

    EventManager gMyEventManager;

You can safely instantiate more than one EventManager object, if so desired, but the two objects will be completely independent. This might be useful if perhaps you need to have separate event processes in different components of your code.

Events

EventManager Events consist of an event code and an event parameter. Both of these are integer values. The event code identifies the type of event. For your convenience, EventManager.h provides a set of constants you can use to identify events

    EventManager::kEventKeyPress
    EventManager::kEventKeyRelease
    EventManager::kEventChar
    EventManager::kEventTime
    EventManager::kEventTimer0
    EventManager::kEventTimer1
    EventManager::kEventTimer2
    EventManager::kEventTimer3
    EventManager::kEventAnalog0
    EventManager::kEventAnalog1
    EventManager::kEventAnalog2
    EventManager::kEventAnalog3
    EventManager::kEventAnalog4
    EventManager::kEventAnalog5
    EventManager::kEventMenu0
    EventManager::kEventMenu1
    EventManager::kEventMenu2
    EventManager::kEventMenu3
    EventManager::kEventMenu4
    EventManager::kEventMenu5
    EventManager::kEventMenu6
    EventManager::kEventMenu7
    EventManager::kEventMenu8
    EventManager::kEventMenu9
    EventManager::kEventSerial
    EventManager::kEventPaint
    EventManager::kEventUser0
    EventManager::kEventUser1
    EventManager::kEventUser2
    EventManager::kEventUser3
    EventManager::kEventUser4
    EventManager::kEventUser5
    EventManager::kEventUser6
    EventManager::kEventUser7
    EventManager::kEventUser8
    EventManager::kEventUser9

These are purely for your convenience; EventManager only uses the value to match events to listeners, so you are free to use any event codes you wish. The event parameter is also whatever you want it to be: for a key press event it could be the corresponding key code. For an analog event it could be the value read from that analog pin or a pin number. The event parameter will be passed to every listener that is associated with that event code.

You post events using the queueEvent() function

    gMyEventManager.queueEvent( EventManager::kEventUser0, 1234 );

The queueEvent() function is lightweight and interrupt safe, so you can call it from inside an interrupt handler.

By default the event queue holds 8 events, but you can make the queue any size you want by defining the macro EVENTMANAGER_EVENT_QUEUE_SIZE to whatever value you desire (see Increase Event Queue Size below).

Listeners

Listeners are functions of type

    typedef void ( *EventListener )( int eventCode, int eventParam );

You add listeners using the addListener() function

    void myListener( int eventCode, int eventParam )
    {
        // Do something with the event
    }

    void setup()
    {
        gMyEventManager.addListener( EventManager::kEventUser0, myListener );

        // Do more set up
    }

Do not add listeners from within an interrupt routine.

By default the list of listeners holds 8 listeners, but you can make the list any size you want by defining the macro EVENTMANAGER_LISTENER_LIST_SIZE to whatever value you desire (see Increase Listener List Size below).

Processing Events

To actually process events in the event queue and dispatch them to listeners you call the processEvent() function

    void loop()
    {
        gMyEventManager.processEvent();
    }

This call processes one event from the event queue every time it is called. The standard usage is to call processEvent() once in your loop() function so that one event is handled every time through the loop. This is usually more than adequate to keep up with incoming events. Events are normally processed in a first-in, first-out fashion (but see the section on Event Priority below).

Example

Here is a simple example illustrating how to blink the LED on pin 13 using EventManager

    #include <Arduino.h>
    #include <EventManager.h>

    boolean pin13State;
    unsigned long lastToggled;

    EventManager gEM;

    // Our listener will simply toggle the state of pin 13
    void listener( int event, int param )
    {
        // event and param are not used in this example function
        pin13State = pin13State ? false : true;
        digitalWrite( 13, pin13State ? HIGH : LOW  );
        lastToggled = millis();
    }

    void setup()
    {
        // Setup
        pinMode( 13, OUTPUT );
        digitalWrite( 13, HIGH );
        pin13State = true;
        lastToggled = millis();

        // Add our listener
        gEM.addListener( EventManager::kEventUser0, listener );
    }

    void loop()
    {
        // Handle any events that are in the queue
        gEM.processEvent();

        // Add events into the queue
        addPinEvents();
     }

    // Add events to toggle pin 13 every second
    // NOTE:  doesn't handle millis() turnover
    void addPinEvents()
    {
        if ( ( millis() - lastToggled ) > 1000 )
        {
            gEM.queueEvent( EventManager::kEventUser0, 0 );
        }
    }

The examples that come with the EventManager library (accessible via the Arduino File/Examples menu) provide more sophisticated illustrations of how you can use EventManager.

Advanced Details

Event Priority

EventManager recognizes high and low priority events. You can specify the priority when you queue the event. By default, events are considered low priority. You indicate an event is high priority by passing an additional constant to queueEvent(), like so

    gMyEventManager.queueEvent( EventManager::kEventUser0, 1234, EventManager::kHighPriority );

The difference between high and low priority events is that processEvent() will process a high priority event ahead of any low priority events. In effect, high priority events jump to the front of the queue (multiple high priority events are processed first-in, first-out, but all of them are processed before any low priority events).

Note that if high priority events are queued faster than low priority events, EventManager may never get to processing any of the low priority events. So use high priority events judiciously.

Interrupt Safety

EventManager was designed to be interrupt safe, so that you can queue events both from within interrupt handlers and also from normal functions without having to worry about queue corruption. However, this safety comes at the price of slightly slower queueEvent() and processEvent() functions and the need to globally disable interrupts while certain small snippets of code are executing.

Processing All Events

Normally calling processEvent() once every time through the loop() function is more than adequate to service incoming events. However, there may be times when you want to process all the events in the queue. For this purpose you can call processAllEvents(). Note that if you call this function at the same time that a series of events are being rapidly added to the queue asynchronously (via interrupt handlers), the processAllEvents() function might not return until the series of additions to the event queue stops.

Increase Event Queue Size

Define EVENTMANAGER_EVENT_QUEUE_SIZE to whatever size you need at the very beginning of EventManager.h like so

    #ifndef EventManager_h
    #define EventManager_h

    #define EVENTMANAGER_EVENT_QUEUE_SIZE   16

    #include <Arduino.h>

If you are using the Arduino IDE, it is not enough to define this constant the usual C/C++ way by defining the constant before including EventManager.h in your own files. This is because the Arduino IDE has no way to pass the definition to the library code unless you actually edit EventManager.h. The Arduino IDE lacks a way to pass precompile constants to all the files in the project. Given that the underlying compiler is GCC, it would be nice if the Arduino IDE had a dialog to set things like -D EVENTMANAGER_EVENT_QUEUE_SIZE=16 and have this constant definition passed directly to the compiler.

The event queue requires 4*sizeof(int) = 8 bytes for each unit of size. There is a factor of 4 (instead of 2) because internally EventManager maintains two separate queues: a high-priority queue and a low-priority queue.

Increase Listener List Size

Define EVENTMANAGER_LISTENER_LIST_SIZE to whatever size you need at the very beginning of EventManager.h like so

    #ifndef EventManager_h
    #define EventManager_h

    #define EVENTMANAGER_LISTENER_LIST_SIZE   16

    #include <Arduino.h>

If you are using the Arduino IDE, it is not enough to define this constant the usual C/C++ way by defining the constant before including EventManager.h in your own files. This is because the Arduino IDE has no way to pass the definition to the library code unless you actually edit EventManager.h. The Arduino IDE lacks a way to pass precompile constants to all the files in the project. Given that the underlying compiler is GCC, it would be nice if the Arduino IDE had a dialog to set things like -D EVENTMANAGER_LISTENER_LIST_SIZE=16 and have this constant definition passed directly to the compiler.

The listener list requires sizeof(*f()) + sizeof(int) + sizeof(boolean) = 5 bytes for each unit of size.

Additional Features

There are various class functions for managing the listeners:

You can remove listeners (removeListener()),
Disable and enable specific listeners (enableListener()),
Set a default listener that will handle any events not handled by other listeners and manipulate the default listener just like any other listener (setDefaultListener(), removeDefaultListener(), and enableDefaultListener())
Check the status of the listener list (isListenerListEmpty(), isListenerListFull())

There are various class functions that provide information about the event queue:

Check the status of the event queue (isEventQueueEmpty(), isEventQueueFull())
See how many events are in the queue (getNumEventsInQueue())

For details on these functions you should review EventManager.h.

Feedback

If you find a bug or if you would like a specific feature, please report it at:

https://github.com/igormiktor/arduino-EventManager/issues

If you would like to hack on this project, don't hesitate to fork it on GitHub. If you would like me to incorporate changes you made, don't hesitate to send me a Pull Request.

Credits

EventManager was inspired by and adapted from the Arduino Event System library created by mromani@ottotecnica.com of OTTOTECNICA Italy, which was kindly released under a LGPL 2.1 license.

License

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

A copy of the license is included in the EventManager package.