An object which has at least two properties called expression and listener. The watch.expression
is added to the formly-form directive's scope (to allow it to run even when hide is true). You
can specify a type ($watchCollection or $watchGroup) via the watcher.type property (defaults to
$watch) and whether you want it to be a deep watch via the watcher.deep property (defaults to false).
An object which has at least two properties called expression and listener. The watch.expression is added to the formly-form directive's scope (to allow it to run even when hide is true). You can specify a type ($watchCollection or $watchGroup) via the watcher.type property (defaults to $watch) and whether you want it to be a deep watch via the watcher.deep property (defaults to false).
see http://docs.angular-formly.com/docs/field-configuration-object#watcher-objectarray-of-watches