Message subscribers

The message subscriber is the most common observer in the Asset Tracker Template. A message subscriber receives messages asynchronously. It is used by any module that has its own thread.

A message subscriber queues up messages that are received while the module is busy processing another message. The module will then process the messages in the order they were received. An incoming message can never interrupt the processing of another message.

A message subscriber is defined using ZBUS_MSG_SUBSCRIBER_DEFINE, and the subscriber is added to a channel using ZBUS_CHAN_ADD_OBS. For example, in the Network module:

ZBUS_MSG_SUBSCRIBER_DEFINE(network);
ZBUS_CHAN_ADD_OBS(network_chan, network, 0);

The messages are received in the module's thread loop by calling zbus_sub_wait_msg():

err = zbus_sub_wait_msg(&network, &network_state.chan,
                        network_state.msg_buf, zbus_wait_ms);

As with all the modules in the Asset Tracker Template with a state machine, the channel and the message contents are stored in the module's state object in preparation to run the state handler, where the message will be processed.

In modules that subscribe to multiple channels, a channel list macro is used to simplify subscribing to all the channels. The same macro is used to determine the size of a message buffer to handle the largest possible message. For example, in app/src/main.c:

#define CHANNEL_LIST(X)                                \
        X(CLOUD_CHAN,           struct cloud_msg)      \
        X(BUTTON_CHAN,          struct button_msg)     \
        /* ... more channels ... */

#define ADD_OBSERVERS(_chan, _type) ZBUS_CHAN_ADD_OBS(_chan, main_subscriber, 0);
CHANNEL_LIST(ADD_OBSERVERS)