# IDataSource.RequestNotification()

Description:

The `RequestNotification` method registers the widget to receive notifications of changes to the event log.

This is an asynchronous method.

Syntax:

    result = so.IDataSource.RequestNotification(criteria, callback);

Arguments:

- `criteria`:

  This is an object that specifies the request for notification of changes to the event log. For more information about the object properties and how to define them, see section Criteria for requesting notification.

- `callback`:

  The `callback` argument is the name of the method that is executed when an asynchronous `RequestNotification` call has results or status information to return. You must define this method separately. Follow the instructions in section Defining the callback handler for an asynchronous method to define the `callback` method.

Return value:

The `RequestNotification` method returns an object that contains a transaction ID for the `callback` instance, an error code, and an error message (see the following table). When the asynchronous call has completed, `callback` returns an object that contains an error code and an error message (see Table: Callback return value).

Table: Return value properties for an asynchronous RequestNotification
Property | Description | Value
--- | --- | ---
`result.TransactionID` | This is a number used as an identification to match transactions started with the asynchronous `GetList` call to one or more calls it generates to `callback`. This property is only valid for asynchronous calls. |
`result.ErrorCode` | This is a number that specifies a predefined error code. | See Service API error codes.
`result.ErrorMessage` | This is a text string that describes the error. | See Logging Service API error messages.

Example code:

For the complete source of a sample widget that demonstrates how to use this Service API, see the full example.