Handling Communications Messages in Plugins

From 5.10 a new ObsListener class offers a simple way to listen to messages.

The simplified approach:

1. Declare a ObsListener listener object
2. Declare a method which handles the message.
3. Initiate the listener to invoke the method when message arrives.

The ObsListener object normally lives in the class context, like:

    class Dlg : public ShipDriverBase {
    public:
        ...
    private:
        ObsListener  listener;
        ...

The method is declared something like below.

   static void HandleGPGA(ObservedEvt ev) {
        NMEA0183Id id("GPGGA");
        std::string payload = GetN0183Payload(id, ev)
        ...

There are other ids for NMEA2000 and SignalK messages available. Likewise, there are similar methods to access the payload. See the ocpn_plugin.h header file.

To invoke this method when a GPGGA message arrives, initiate the ObsListener like:

    listener.Init(NMEA0183Id("GPGGA"),
                  [&](ObservedEvt ev) { HandleGPGA(ev); });

The last line is a lambda expression which could be used to much more than to just invoke a method, see Epilog: Using the lambda below.

Epilog: Using the lambda

The last step above was Bind(EVT_SHIPDRIVER [](wxCommandEvent ev) { HandleGPGA(ev); }). The part between the curly braces is actually a C++ lambda expression. There is no need to dive into this to get something running, but it offers far larger possibilities than to just call a function. Actually, if written like Bind(EVT_SHIPDRIVER, [&](wxCommandEvent ev) { ... }) any code could be written between the braces.

The interesting part here is that the [&] prefix makes this code "see" anything defined in the plugin. This is a convenient way to access plugin variables in the handler, something which otherwise is a problem.

To get the feeling one need to experiment. But then again, C++ lambdas is a complex step which is not necessary to get something running.