Custom formatters should be a javascript class. The constructor will be an options object which is the options defined with --format-options and the following:
colorFns: a series of helper functions for outputting colors. See here. RespectscolorsEnabledoptioncwd: the current working directoryeventBroadcaster: an event emitter that emits the event protocol (which is still being defined). See current docs and the proposed updates which were implemented.eventDataCollector: an instance of EventDataCollector which handles the complexity of grouping the data for related eventslog: function which will write the passed string to the the designated stream (stdout or the to file the formatter output is being redirected to).snippetBuilder: an object with abuildmethod that should be called with{keywordType, pickleStep}. ThepickleStepcan be retrieved with theeventDataCollectorwhile thekeywordTypeis complex to compute (see the SnippetsFormatter for an example).stream: the underlying stream the formatter is writing to.logis a shortcut for writing to it.
The constructor of custom formatters should add listeners to the eventBroadcaster.
See a couple examples here and the built in formatters here
The base Formatter does very little aside from saving some of the options on the instance. You can extend the SummaryFormatter (as the ProgressFormatter and PrettyFormatter) do in order to get the same error reporting at the end.
formatterHelpers are also exposed to give some of the functionality in more modular pieces.
If there is any other formatter functionality you would like access to, please create an issue.