Writing a location sensor driver for Windows 8.1

The Sensor and Location Platform provides the Windows Location API to enable software developers to add location features to their applications. If you are writing a driver for a location sensor, you must understand how to make the driver compatible with the Location API and follow the guidelines in Location Driver Guidelines for Power and Performance.

Windows Hardware Certification Program requirements

The Windows Hardware Certification Program enables hardware manufacturers to receive certification that their devices meet the required standards for working with Windows. The certification program describes the requirements for location sensors and other types of sensors. You should make your location sensor driver comply with all the certification program requirements. These requirements include the following:

• Location sensors must support the required set of data and sensor properties.

• Location sensors must support the required data fields for at least one built-in data report type.

Generally, the recommendations in this WDK documentation match the Certification Program requirements. However, you must review the official Certification Program documentation when you create sensor drivers that you intend to submit for approval. For more information about the Windows Hardware Certification Program, see the Windows Hardware Developer Central website.

Location API requirements

You create drivers for location sensors by using the same driver model and class extension as for any other category of sensor. At a minimum, to work as a location sensor, the driver must:

• Identify the location sensor as belonging to the Location category.

• Set the sensor type to one of the location sensor types.

• Identify the location report data fields the sensor provides.

• Support the required properties.

• Provide data, when it is requested.

• Manage state transitions.

• Raise data-updated and state-changed events.

The rest of this section describes these minimum requirements

Identifying the category

When it is called through ISensorDriver::OnGetProperties, set the WPD_FUNCTIONAL_OBJECT_CATEGORY property value to SENSOR_CATEGORY_LOCATION. The following code example shows how to set this constant through a pointer to IPortableDeviceValues named pValues.

hr = pValues->SetGuidValue(WPD_FUNCTIONAL_OBJECT_CATEGORY, SENSOR_CATEGORY_LOCATION);

Setting the location sensor type

When it is called through ISensorDriver::OnGetProperties, set the SENSOR_PROPERTY_TYPE property value to the correct value. The following code example shows how to set the sensor type by using the SENSOR_TYPE_LOCATION_GPS constant through a pointer to IPortableDeviceValues named pValues.

hr = pValues->SetGuidValue(SENSOR_PROPERTY_TYPE, SENSOR_TYPE_LOCATION_GPS);

Identifying the supported data fields

The Location API defines two kinds of location reports. These are objects that organize location data. LatLong reports contain latitude, longitude, and altitude data fields, together with data fields that contain error range information. Civic address reports contain street address data fields, such as city and postal code. Your location driver must support the required data fields for at least one of these two data report types.

To support a LatLong report, the following data fields are required:

• SENSOR_DATA_TYPE_LATITUDE_DEGREES

• SENSOR_DATA_TYPE_LONGITUDE_DEGREES

• SENSOR_DATA_TYPE_ERROR_RADIUS_METERS

To support a civic address report, at least one of the following data fields is required:

• SENSOR_DATA_TYPE_COUNTRY_REGION

To view the complete set of platform-defined location data fields, see SENSOR_CATEGORY_LOCATION in the Windows Sensor Reference section.

When they are called through ISensorDriver::OnGetSupportedDataFields, add the supported data field property key constants to the IPortableDeviceKeyCollection that you return through the ppSupportedDataFields parameter. The following code example shows how to add the postal code data field to IPortableDeviceKeyCollection through a variable named pKeyCollection.

pKeyCollection->Add(SENSOR_DATA_TYPE_POSTALCODE);

Support the required properties

Like other sensor drivers, location drivers provide information about the sensor itself through a set of properties. The Windows Hardware Certification Program specifies the minimum required set of properties that a location sensor must support. For more information about sensor properties, their meanings, and which properties are required for sensor drivers, see Sensor Properties. The following list contains the required properties:

• WPD_FUNCTIONAL_OBJECT_CATEGORY

• SENSOR_PROPERTY_TYPE

• SENSOR_PROPERTY_STATE

• SENSOR_PROPERTY_PERSISTENT_UNIQUE_ID

• SENSOR_PROPERTY_MANUFACTURER

• SENSOR_PROPERTY_MODEL

• SENSOR_PROPERTY_SERIAL_NUMBER

• SENSOR_PROPERTY_FRIENDLY_NAME

• SENSOR_PROPERTY_MIN_REPORT_INTERVAL

• SENSOR_PROPERTY_CURRENT_REPORT_INTERVAL

• SENSOR_PROPERTY_LOCATION_DESIRED_ACCURACY

Providing data

Location drivers provide data through the same mechanisms as other sensor drivers. That is, the sensor class extension calls the driver through ISensorDriver::OnGetDataFields and the driver returns the values through the ppDataValues parameter.

The following requirements apply to providing data from a location sensor:

• Provide data both through synchronous requests and by raising events.

• Maintain a copy of your most recent data report. If new data is not available when you request it, return the cached report. Do not update the time stamp.

• Do not provide values for SENSOR_DATA_TYPE_LATITUDE_DEGREES and SENSOR_DATA_TYPE_LONGITUDE_DEGREES that fall outside the range of real-world latitudes and longitudes.

• Do not report a value for SENSOR_DATA_TYPE_ERROR_RADIUS_METERS that is zero or less.

• Set the value for SENSOR_DATA_TYPE_COUNTRY_REGION to a valid ISO 3166 1-alpha-2 country code.

• If your driver supports both latitude/longitude and civic address reports, the location data in these reports should correspond to the same physical location.

The following table describes the sensor data fields that correspond to Location API data report fields. Use these data field constants when you provide data reports for a location.

Managing state transitions

At any time, a sensor driver can be in one of a number of states. Sensor states are defined by the SensorState enumeration. To work correctly with the Location API, location sensors must follow these rules for handling state transitions.

• Always start in the SENSOR_STATE_INITIALIZING state, but do not raise a state-changed event at startup.

• The driver should transition from SENSOR_STATE_INITIALIZING to SENSOR_STATE_READY when data is available.

• The driver should transition back to SENSOR_STATE_INITIALIZING when the driver does not have current data to report. The driver should decide when that transition occurs. If you have lost a signal, but still have a means to provide valid data, stay in the SENSOR_STATE_READY state.

• Always raise events in the correct order. First, establish that data is available. Then, raise a state-changed event. Finally, raise the data-updated event.

• Always raise a state-changed event when the driver's state changes.

-The Location API does not use data from sensors that are in the following states: SENSOR_STATE_NO_DATA, SENSOR_STATE_NOT_AVAILABLE, SENSOR_STATE_ERROR.

The various sensor states for location sensor drivers are described in the following table.

The following diagram shows how state transitions may occur in a location sensor.

Raising data-updated and state-changed events

The Location API, requires location sensors, such as GPS sensors, to raise events that provide data and state-change information. For more information about raising sensor events, see About Sensor Driver Events.

When raising these events, location drivers must follow these rules:

• Raise state change events by calling the sensor class extension's ISensorClassExtension::PostStateChange method. Do not call PostEvent to raise state change events.

• Raise data-updated events by calling PostEvent.

• Raise a data-updated event only if the data is up to date and accurate.

• Do not raise a data-updated event twice. This means that you should not raise a data-updated event by using cached data. You can provide cached data in response to a synchronous request for data.

• Always include all the required data fields when you send a latitude/longitude report through an event.

• Always raise a data-updated event when the sensor accuracy changes.

• Report a valid value for SENSOR_DATA_TYPE_ERROR_RADIUS_METERS before raising events or changing the value for SENSOR_PROPERTY_STATE to SENSOR_STATE_READY.

• Do not provide incomplete data reports.

• You might not have current data for the required data fields, such as when a GPS sensor has lost its fix. In this case, you might still want to provide notifications about updates to extended data fields, such as SENSOR_DATA_TYPE_NMEA_SENTENCE. To provide such notifications, you must use a custom event type and raise only the custom event until data for the required data fields becomes available. For information about how to define custom types, see Defining Custom Values for Constants.

Related topics

Location Driver Guidelines for Power and Performance