BeagleQueue is a wrapper of POSIX MessageQueues used by the BeagleBoom System. The POSIX standard needs each message to have a type in form of a long value. The receive command allows to specify a type, to only get messages of the desired type. Our system uses this value to distinguish the recipient for the events. The payload of an event is the event id, followed by an arbitrary number of additional paramaters, encoded as an JSON-Array.

Usage

Events

For each event-type, an entry has to be added to the enum found at [include/EventType.h]. An event has to be created with the desired event-type. The event can later be retrieved with the method getType. After instantiation, multiple values can be added to the event via the methods addInt, addBool, addFloat and addString. Likewise the methods getInt, getBool, getFloat and getString can read a value at a specified zero based array position. In order to check the datatype of a paramater the methods isInt, isBool, isFloat and isString can be used.

Event e=Event(EventType::BUTTON_UP); //create an event of type "BUTTON_UP"
e.addInt(5); //add an integer parameter with the value "5"
e.addString("Hi!");
e.isString(1); //=>true
e.isBool(1); //=>false
e.getInt(0); //=>5
e.getType(); //=> BUTTON_UP

Queue

To send and receive the beforementioned events, the MessageQueue class can be used. The Queue has to be initilaized with the desired id on which the queue should listen. Additionally the default recipient of events can be changed (defaults to 1, which is the Menu in the BeagleBoom system).

MessageQueue queue1 = MessageQueue(5); //Listen as recipient 5 and send events to recipient 1
MessageQueue queue2 = MessageQueue(8, 2); //Listen as recipient 8 and send events to recipient 2

To send an event, it can be passed to the send method. If the recipient shuld differ to the default one, it can be specified in the second parameter.

MessageQueue queue = MessageQueue(3);
Event event = Event(EventType::BUTTON_UP); 
queue.send(event); // send to recipient 1
queue.send(event, 5); // send to recipient 5

An event can be recieved in a blocking (receive) or non-blocking (receiveNoWait) way. Both methods return an event. As receiveNoWait directly returns without waiting, it is not guaranteed that there was an actual event to return. In this cases, the returned event will have the type EventType::NONE to indicate that it is not an actual event.

MessageQueue queue = MessageQueue(3);
Event e1 = queue.receive(); // Wait until an event is present in the queue

Event e2 = queue.receiveNoWait(); //Return event if one is present
if(e2.getType()==EventType::NONE){
  //There was no event :-(
}else{
  //Event found!
}

Installation

• You need an installed version of PocoLib which includes the JSON namespace.
• Cmake

These commands shall install the BeagleQueue:

cmake .
make && make install

About this repository

This repository is part of the BeagleBoom-Sampler project developed by the academic working group "Creative Technologies AG" (ctag) at the University of Applied Sciences Kiel.

License

The content and source code of this project itself is licensed under the Creative Commons Attribution 4.0 International license.