Subscription class

Class representing a Subscription to be submitted to a Lightstreamer Server.

It contains subscription details and the listeners needed to process the real-time data.
After the creation, a Subscription object is in the "inactive" state. When a Subscription object is subscribed to on a LightstreamerClient object, through the LightstreamerClient.subscribe method, its state becomes "active". This means that the client activates a subscription to the required items through Lightstreamer Server and the Subscription object begins to receive real-time events.
A Subscription can be configured to use either an Item Group or an Item List to specify the items to be subscribed to and using either a Field Schema or Field List to specify the fields.
"Item Group" and "Item List" are defined as follows:

• "Item Group": an Item Group is a String identifier representing a list of items. Such Item Group has to be expanded into a list of items by the getItems method of the MetadataProvider of the associated Adapter Set. When using an Item Group, items in the subscription are identified by their 1-based index within the group.
  It is possible to configure the Subscription to use an "Item Group" using the [setItemGroup] method.
• "Item List": an Item List is an array of Strings each one representing an item. For the Item List to be correctly interpreted a LiteralBasedProvider or a MetadataProvider with a compatible implementation of getItems has to be configured in the associated Adapter Set.
  Note that no item in the list can be empty, can contain spaces or can be a number.
  When using an Item List, items in the subscription are identified by their name or by their 1-based index within the list.
  It is possible to configure the Subscription to use an "Item List" using the [setItems] method or by specifying it in the constructor.

• "Field Schema": a Field Schema is a String identifier representing a list of fields. Such Field Schema has to be expanded into a list of fields by the getFields method of the MetadataProvider of the associated Adapter Set. When using a Field Schema, fields in the subscription are identified by their 1-based index within the schema.
  It is possible to configure the Subscription to use a "Field Schema" using the [setFieldSchema] method.
• "Field List": a Field List is an array of Strings each one representing a field. For the Field List to be correctly interpreted a LiteralBasedProvider or a MetadataProvider with a compatible implementation of getFields has to be configured in the associated Adapter Set.
  Note that no field in the list can be empty or can contain spaces.
  When using a Field List, fields in the subscription are identified by their name or by their 1-based index within the list.
  It is possible to configure the Subscription to use a "Field List" using the [setFields] method or by specifying it in the constructor.