# ISensor.RegisterForNotification()

Description:

The `RegisterForNotification` method registers the client to receive data from one sensor channel.

This is an asynchronous method.

Syntax:

    result = so.ISensor.RegisterForNotification(criteria, callback);

Arguments:

- `criteria`:

  This is an object that specifies the sensor channel to listen for data. For more information about the object properties and how to define them, see section Criteria for receiving sensor data.

- `callback`:

  The `callback` argument is the name of the method that is executed when a `RegisterForNotification` 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 `RegisterForNotification` method returns an object that contains the initial return value for the asynchronous call it started (see the following table). The actual sensor data is returned by the `callback` method in the `ReturnValue` property of its result object. The returned data is described in section Returned sensor data.

Table: Return value properties for RegisterForNotification
Property | Description | Value
--- | --- | ---
`result.TransactionID` | This is a number used as an identification to match transactions started with the `RegisterForNotification` call to one or more calls it generates to `callback`. |
`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 Sensor Service API error messages.

Remarks:

- `RegisterForNotification` retrieves sensor data until cancelled with `Cancel`.

- Invoking the same call on the same sensor channel from the same user without cancelling the previous `RegisterForNotification` request results in an error, since the channel is already listening on the pre-registered notifications.

Example code:

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