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The Events service

As you may know from the page about the Logging, system events can pass parameters with the first non-string argument.

console.log('#money, #input', {amount: 20})

There is a system service Events, which listens to the log stream and notifies the subscribers about events.

A comprehensive example

Suppose that a BillValidator instance sends log messages about the entered amount every 500 milliseconds, and messages about the opening of the cassette - every 5 seconds.

The example below shows how to use this information log by implementing a simple cash monitoring service.

// tell the Domain to start the Events service. This service is always a singleton.
// if it is already up and running, this function returns a reference to it
Domain.SvcDomain.startService('Events', function (svc) {
    // this variable counts the entered amount of cash
    var count = 0;
    // the Events service defines the method 'subscribe', which accepts an object...
    svc.subscribe({
        // ...with the 'listen' key, in which you can pass a listener (as an object) or an array of listeners
        // in this example, there is an array of 2 listeners
        listen: [{
            // the event is passed as the first argument of the callback function
            // it may be skipped if distinguishing events is not required
            event: 'billInsert',
            // the logging namespace
            namespace: 'Devices.BillValidator',
            // tags to filter the messages
            // if multiple tags are specified, they ALL must be present in the message
            // if no tags are specified, all messages from the namespace will be listened
            filter: ['bill']
        }, {
            event: 'cashCollection',
            namespace: 'Devices.BillValidator',
            filter: ['cassetteopen']
        }],
        // the callback function invoked on each message that satisfies the filtering conditions
        fn: function (
            // the event attribute of the fitting subscriber
            event, 
            // data object: the first non-string argument of the console.log() call
            // (be careful - this puts restrictions on the data logging in system events)
            info,
            // The following arguments are auxiliary and are not often used:
            // the object that contains all tags specified in the 'filter' argument of the listener (these tags are keys of this object)
            matched,
            // the listener object after the filtration; can be used to save the state
            listener,
            // the entire LogObject containing all information connected with the message; avoid using this argument
            logObject
        ) {
            // the function is invoked in the context of the listener object

            if(event === 'billInsert') {
                /* if the event of the listener corresonds to 'billInsert' -> 
                add the amount extracted from the first non-string argument */
                count += info.amount;
            }else if(event === 'cashCollection'){
                /* if a cash collection has taken place -> empty the amount count */
                console.log('#cashMonitoringService #billStackEmpty', count);
                count = 0;
            }
            console.log('#cashMonitoringService #amount', count);
        }
    });
});