Node.js Client 9.1.0

ItemUpdate

ItemUpdate

Contains all the information related to an update of the field values for an item. It reports all the new values of the fields.

COMMAND Subscription
If the involved Subscription is a COMMAND Subscription, then the values for the current update are meant as relative to the same key.
Moreover, if the involved Subscription has a two-level behavior enabled, then each update may be associated with either a first-level or a second-level item. In this case, the reported fields are always the union of the first-level and second-level fields and each single update can only change either the first-level or the second-level fields (but for the "command" field, which is first-level and is always set to "UPDATE" upon a second-level update); note that the second-level field values are always null until the first second-level update occurs). When the two-level behavior is enabled, in all methods where a field name has to be supplied, the following convention should be followed:

  • The field name can always be used, both for the first-level and the second-level fields. In case of name conflict, the first-level field is meant.
  • The field position can always be used; however, the field positions for the second-level fields start at the highest position of the first-level field list + 1. If a field schema had been specified for either first-level or second-level Subscriptions, then client-side knowledge of the first-level schema length would be required.

Constructor

new ItemUpdate()

Used by the client library to provide a value object to each call of the SubscriptionListener#onItemUpdate event.

Methods

forEachChangedField(iterator)

Receives an iterator function and invokes it once per each field such that ItemUpdate#isValueChanged is true.
Note that if the Subscription mode of the involved Subscription is COMMAND, then changed fields are meant as relative to the previous update for the same key. On such tables if a DELETE command is received, all the fields, excluding the key field, will be iterated as changed, with null value. All of this is also true on tables that have the two-level behavior enabled, but in case of DELETE commands second-level fields will not be iterated.
Note that the iterator is executed before this method returns.

Parameters:
Name Type Description
iterator ItemUpdateChangedFieldCallback

Function instance that will be called once per each field changed on the last update received from the server.

forEachField(iterator)

Receives an iterator function and invokes it once per each field in the Subscription.
Note that the iterator is executed before this method returns.

Parameters:
Name Type Description
iterator ItemUpdateChangedFieldCallback

Function instance that will be called once per each field in the Subscription.

getItemName() → {String}

Inquiry method that retrieves the name of the item to which this update pertains.
The name will be null if the related Subscription was initialized using an "Item Group".

See:
Returns:

the name of the item to which this update pertains.

Type
String

getItemPos() → {Number}

Inquiry method that retrieves the position in the "Item List" or "Item Group" of the item to which this update pertains.

See:
Returns:

the 1-based position of the item to which this update pertains.

Type
Number

getValue(fieldNameOrPos) → {String}

Inquiry method that gets the value for a specified field, as received from the Server with the current or previous update.

Parameters:
Name Type Description
fieldNameOrPos String

The field name or the 1-based position of the field within the "Field List" or "Field Schema".
See:
Throws:

if the specified field is not part of the Subscription.

Type
IllegalArgumentException
Returns:

The value of the specified field; it can be null in the following cases:

  • a null value has been received from the Server, as null is a possible value for a field;
  • no value has been received for the field yet;
  • the item is subscribed to with the COMMAND mode and a DELETE command is received (only the fields used to carry key and command information are valued).
Type
String

getValueAsJSONPatchIfAvailable(fieldNameOrPos) → {Object}

Inquiry method that gets the difference between the new value and the previous one as a JSON Patch structure, provided that the Server has used the JSON Patch format to send this difference, as part of the "delta delivery" mechanism. This, in turn, requires that:

  • the Data Adapter has explicitly indicated JSON Patch as the privileged type of compression for this field;
  • both the previous and new value are suitable for the JSON Patch computation (i.e. they are valid JSON representations);
  • the item was subscribed to in MERGE or DISTINCT mode (note that, in case of two-level behavior, this holds for all fields related with second-level items, as these items are in MERGE mode);
  • sending the JSON Patch difference has been evaluated by the Server as more efficient than sending the full new value.
Note that the last condition can be enforced by leveraging the Server's <jsonpatch_min_length> configuration flag, so that the availability of the JSON Patch form would only depend on the Client and the Data Adapter.
When the above conditions are not met, the method just returns null; in this case, the new value can only be determined through ItemUpdate#getValue. For instance, this will always be needed to get the first value received.
Parameters:
Name Type Description
fieldNameOrPos String

The field name or the 1-based position of the field within the "Field List" or "Field Schema".
See:
Throws:

if the specified field is not part of the Subscription.

Type
IllegalArgumentException
Returns:

A JSON Patch structure representing the difference between the new value and the previous one, or null if the difference in JSON Patch format is not available for any reason.

Type
Object

isSnapshot() → {boolean}

Inquiry method that asks whether the current update belongs to the item snapshot (which carries the current item state at the time of Subscription). Snapshot events are sent only if snapshot information was requested for the items through Subscription#setRequestedSnapshot and precede the real time events. Snapshot information take different forms in different subscription modes and can be spanned across zero, one or several update events. In particular:

  • if the item is subscribed to with the RAW subscription mode, then no snapshot is sent by the Server;
  • if the item is subscribed to with the MERGE subscription mode, then the snapshot consists of exactly one event, carrying the current value for all fields;
  • if the item is subscribed to with the DISTINCT subscription mode, then the snapshot consists of some of the most recent updates; these updates are as many as specified through Subscription#setRequestedSnapshot, unless fewer are available;
  • if the item is subscribed to with the COMMAND subscription mode, then the snapshot consists of an "ADD" event for each key that is currently present.
Note that, in case of two-level behavior, snapshot-related updates for both the first-level item (which is in COMMAND mode) and any second-level items (which are in MERGE mode) are qualified with this flag.
Returns:

true if the current update event belongs to the item snapshot; false otherwise.

Type
boolean

isValueChanged(fieldNameOrPos) → {boolean}

Inquiry method that asks whether the value for a field has changed after the reception of the last update from the Server for an item. If the Subscription mode is COMMAND then the change is meant as relative to the same key.

Parameters:
Name Type Description
fieldNameOrPos String

The field name or the 1-based position of the field within the field list or field schema.
Throws:

if the specified field is not part of the Subscription.

Type
IllegalArgumentException
Returns:

Unless the Subscription mode is COMMAND, the return value is true in the following cases:

  • It is the first update for the item;
  • the new field value is different than the previous field value received for the item.
If the Subscription mode is COMMAND, the return value is true in the following cases:
  • it is the first update for the involved key value (i.e. the event carries an "ADD" command);
  • the new field value is different than the previous field value received for the item, relative to the same key value (the event must carry an "UPDATE" command);
  • the event carries a "DELETE" command (this applies to all fields other than the field used to carry key information).
In all other cases, the return value is false.
Type
boolean