Ultrabook™ and Tablet Windows* 8 Sensors Development Guide

Categories: 

• Microsoft Windows* 8 Desktop 
•  Sensors 
•  Laptop 
•  Tablet 
•  Microsoft Windows* 8

Introduction

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This guide gives developers an overview of the Microsoft Windows 8.1 sensors application programming interfaces (APIs) for Windows 8.1 Desktop and Windows Store applications with a specific focus on the various sensor capabilities available in Windows 8.1 Desktop mode. This Development Guide summarizes the APIs that enable creating interactive applications by including some of the common sensors such as accelerometers, magnetometers, and gyroscopes with Windows 8.1.

Programming Choices for Windows 8.1

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Developers have multiple API choices to program sensors on Windows 8.1. The touch-friendly app environment is called “Windows Store apps.” Windows Store apps can run software developed with the Windows Run-Time (WinRT) interface. The WinRT sensor API represents a portion of the overall WinRT library. For more details, please refer to the MSDN Sensor API library.

Traditional Win Forms, or MFC-style apps are called “Desktop Apps” because they run in the Desktop Windows Manager environment. Desktop apps can either use the native Win32*/COM API, a .NET-style API or a subset of select WinRT APIs. 

The following is a list of WinRT APIs that can be accessed by Desktop apps:

• Windows.Sensors (Accelerometer, Gyrometer, Ambient Light Sensor, Orientation Sensor...)
• Windows.Networking.Proximity.ProximityDevice (NFC)
• Windows.Device.Geolocation (GPS)
• Windows.UI.Notifications.ToastNotification
• Windows.Globalization
• Windows.Security.Authentication.OnlineId (including LiveID integration)
• Windows.Security.CryptographicBuffer (useful binary encoding/decoding functions)
• Windows.ApplicationModel.DataTransfer.Clipboard (access and monitor Windows 8 Clipboard)

In both cases, the APIs go through a Windows middleware component called the Windows Sensor Framework. The Windows Sensor Framework defines the Sensor Object Model. The different APIs “bind” to that object model in slightly different ways.

Differences in the Desktop and Windows Store app development will be discussed later in this document. For brevity, we will consider only Desktop app development. For Windows Store app development, please refer to the API Reference for Windows Store apps.

Sensors

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There are many kinds of sensors, but we are interested in the ones required for Windows 8.1, namely accelerometers, gyroscopes, ambient light sensors, compass, and GPS. Windows 8.1 represents the physical sensors with object-oriented abstractions. To manipulate the sensors, programmers use APIs to interact with the objects. The following table provides information on how the sensors can be accessed from both the Windows 8 Desktop apps as well as from Windows Store apps.

	Windows 8.1 Desktop Mode Apps			Windows Store Apps
Feature/Toolset	C++	C#/VB	JavaScript*/ HTML5	C++, C#, VB & XAML JavaScript/HTML5	Unity* 4.2
Orientation Sensors (accelerometer,  inclinometer, gyrometer)	Yes	Yes	Yes	Yes Yes Yes	Yes
Light Sensor	Yes	Yes	Yes	Yes	Yes
NFC	Yes	Yes	Yes	Yes	Yes
GPS	Yes	Yes	Yes	Yes	Yes

Table 1. Features Matrix for Windows* 8.1 Developer Environments 

Below, Figure 1 identifies that there are more objects than actual hardware. Windows defines some “logical sensor” objects by combining information from multiple physical sensors. This is called “Sensor Fusion.”

Figure 1.  Different sensors supported, starting on Windows* 8

Sensor Fusion

Physical sensor chips have some inherent natural limitations. For example:

• Accelerometers measure linear acceleration, which is a measurement of the combined relative motion and the force of Earth’s gravity. If you want to know the computer’s tilt, you’ll have to do some mathematical calculations.
• Magnetometers measure the strength of magnetic fields, which indicate the location of the Earth’s Magnetic North Pole.

These measurements are subject to an inherent drift problem, which can be corrected by using raw data from the Gyro. Both measurements are (scaled) dependent upon the tilt of the computer from level with respect to the Earth’s surface. For example, to obtain the computer’s heading with respect to the Earth’s True North Pole (Magnetic North Pole is in a different position and moves over time), corrections must be applied.

Sensor Fusion (Figure 2) is defined by obtaining raw data from multiple physical sensors, especially the Accelerometer, Gyro, and Magnetometer, performing mathematical calculations to correct for natural sensor limitations, computing more human-usable data, and representing those as logical sensor abstractions. The application developer must implement the necessary transformations required to translate physical sensor data to the abstract sensor data. If the system design has a SensorHub, the fusion operations will take place inside the microcontroller firmware. If the system design does not have a SensorHub, the fusion operations must be done inside one or more device drivers that the IHVs and/or OEMs provide.

Figure 2.  Sensor fusion via combining output from multiple sensors

Identifying Sensors

To manipulate a sensor, a system is needed to identify and refer to. The Windows Sensor Framework defines a number of categories that sensors are grouped into. It also defines a large number of specific sensor types. Table 2 lists some of the sensors available for Desktop applications.

“All”
Biometric	Electrical	Environmental	Light	Location	Mechanical	Motion	Orientation	Scanner
Human Presence	Capacitance	Atmospheric Pressure	Ambient Light	Broadcast	Boolean Switch	Accelerometer 1D	Compass 1D	Barcode
Human Proximity*	Current	Humidity		Gps	Boolean Switch Array	Accelerometer 2D	Compass 2D	Rfid
Touch	Electrical Power	Temperature		Static	Force	Accelerometer 3D	Compass 3D
	Inductance	Wind Direction			Multivalue Switch	Gyrometer 1D	Device Orientation
	Potentio-meter	Wind Speed			Pressure	Gyrometer 2D	Distance 1D
	Resistance				Strain	Gyrometer 3D	Distance 2D
	Voltage				Weight	Motion Detector	Distance 3D
						Speedometer	Inclinometer 1D
							Inclinometer 2D
							Inclinometer 3D

Table 2. Sensor types and categories 

The sensor types required by Windows 8 are shown in bold font:

• Accelerometer, Gyro, Compass, and Ambient Light are the required “real/physical” sensors
• Device Orientation and Inclinometer are the required “virtual/fusion” sensors (note that the Compass also includes fusion-enhanced/tilt-compensated data)
• GPS is a required sensor if a WWAN radio exists, otherwise GPS is optional
• Human Proximity is an oft-mentioned possible addition to the required list, but, for now, it’s not required.

All of these constants correspond to Globally Unique IDs GUIDs. Below, in Table 3, is a sample of some of the sensor categories and types, the names of the constants for Win32/COM and .NET, and their underlying GUID values.

Identifier	Constant (Win32*/COM)	Constant (.NET)	GUID
Category “All”	SENSOR_CATEGORY_ALL	SensorCategories.SensorCategoryAll	{C317C286-C468-4288-9975-D4C4587C442C}
Category Biometric	SENSOR_CATEGORY_BIOMETRIC	SensorCategories.SensorCategoryBiometric	{CA19690F-A2C7-477D-A99E-99EC6E2B5648}
Category Electrical	SENSOR_CATEGORY_ELECTRICAL	SensorCategories.SensorCategoryElectrical	{FB73FCD8-FC4A-483C-AC58-27B691C6BEFF}
Category Environmental	SENSOR_CATEGORY_ENVIRONMENTAL	SensorCategories.SensorCategoryEnvironmental	{323439AA-7F66-492B-BA0C-73E9AA0A65D5}
Category Light	SENSOR_CATEGORY_LIGHT	SensorCategories.SensorCategoryLight	{17A665C0-9063-4216-B202-5C7A255E18CE}
Category Location	SENSOR_CATEGORY_LOCATION	SensorCategories.SensorCategoryLocation	{BFA794E4-F964-4FDB-90F6-51056BFE4B44}
Category Mechanical	SENSOR_CATEGORY_MECHANICAL	SensorCategories.SensorCategoryMechanical	{8D131D68-8EF7-4656-80B5-CCCBD93791C5}
Category Motion	SENSOR_CATEGORY_MOTION	SensorCategories.SensorCategoryMotion	{CD09DAF1-3B2E-4C3D-B598-B5E5FF93FD46}
Category Orientation	SENSOR_CATEGORY_ORIENTATION	SensorCategories.SensorCategoryOrientation	{9E6C04B6-96FE-4954-B726-68682A473F69}
Category Scanner	SENSOR_CATEGORY_SCANNER	SensorCategories.SensorCategoryScanner	{B000E77E-F5B5-420F-815D-0270ª726F270}
Type HumanProximity	SENSOR_TYPE_HUMAN_PROXIMITY	SensorTypes.SensorTypeHumanProximity	{5220DAE9-3179-4430-9F90-06266D2A34DE}
Type AmbientLight	SENSOR_TYPE_AMBIENT_LIGHT	SensorTypes.SensorTypeAmbientLight	{97F115C8-599A-4153-8894-D2D12899918A}
Type Gps	SENSOR_TYPE_LOCATION_GPS	SensorTypes.SensorTypeLocationGps	{ED4CA589-327A-4FF9-A560-91DA4B48275E}
Type Accelerometer3D	SENSOR_TYPE_ACCELEROMETER_3D	SensorTypes.SensorTypeAccelerometer3D	{C2FB0F5F-E2D2-4C78-BCD0-352A9582819D}
Type Gyrometer3D	SENSOR_TYPE_GYROMETER_3D	SensorTypes.SensorTypeGyrometer3D	{09485F5A-759E-42C2-BD4B-A349B75C8643}
Type Compass3D	SENSOR_TYPE_COMPASS_3D	SensorTypes.SensorTypeCompass3D	{76B5CE0D-17DD-414D-93A1-E127F40BDF6E}
Type DeviceOrientation	SENSOR_TYPE_DEVICE_ORIENTATION	SensorTypes.SensorTypeDeviceOrientation	{CDB5D8F7-3CFD-41C8-8542-CCE622CF5D6E}
Type Inclinometer3D	SENSOR_TYPE_INCLINOMETER_3D	SensorTypes.SensorTypeInclinometer3D	{B84919FB-EA85-4976-8444-6F6F5C6D31DB}

Table 3. Example of Constants and Globally Unique IDs (GUIDs) 

Above are the most commonly used GUIDs; there are many available. At first you might think that the GUIDs are silly and tedious, but there is one good reason for using them: extensibility. Since the APIs don’t care about the actual sensor names (they just pass GUIDs around), it is possible for vendors to invent new GUIDs for “value add” sensors.

Generating New GUIDs

Microsoft provides a tool in Visual Studio* for generating new GUIDs. Figure 3 shows a screenshot from Visual Studio for doing this. All the vendor has to do is publish them, and new functionality can be exposed without the need to change the Microsoft APIs or any operating system code at all.

Figure 3. Defining new GUIDs for value add sensors

Using Sensor Manager Object

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In order for an app to use a sensor, Microsoft Sensor Framework needs a way to “bind” the object to actual hardware. It does this via Plug and Play, using a special object called the Sensor Manager Object.

Ask by Type

An app can ask for a specific type of sensor, such as Gyrometer3D. The Sensor Manager consults the list of sensor hardware present on the computer and returns a collection of matching objects bound to that hardware. Although the Sensor Collection may have 0, 1, or more objects, it usually has only one. Below is a C++ code sample illustrating the use of the Sensor Manager object’s GetSensorsByType method to search for 3-axis Gyros and return them in a Sensor Collection. Note that a ::CoCreateInstance() must be made for the Sensor Manager Object first.

      
      
      
      
01 // Additional includes for sensors
02 #include
03 #include
04 #include
05 // Create a COM interface to the SensorManager object.
06 ISensorManager* pSensorManager = NULL;
07 HRESULT hr = ::CoCreateInstance(CLSID_SensorManager, NULL, CLSCTX_INPROC_SERVER,
08     IID_PPV_ARGS(&pSensorManager));
09 if (FAILED(hr))
10 {
11     ::MessageBox(NULL, _T("Unable to CoCreateInstance() the SensorManager."),
12         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
13     return -1;
14 }
15 // Get a collection of all 3-axis Gyros on the computer.
16 ISensorCollection* pSensorCollection = NULL;
17 hr = pSensorManager->GetSensorsByType(SENSOR_TYPE_GYROMETER_3D, &pSensorCollection);
18 if (FAILED(hr))
19 {
20     ::MessageBox(NULL, _T("Unable to find any Gyros on the computer."),
21         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
22     return -1;
23 }

Ask by Category

An app can request sensors by category, such as all motion sensors. The Sensor Manager consults the list of sensor hardware on the computer and returns a collection of motion objects bound to that hardware. The SensorCollection may have 0, 1, or more objects in it. On most computers, the collection will have two motion objects: Accelerometer3D and Gyrometer3D.

The C++ code sample below illustrates the use of the Sensor Manager object’s GetSensorsByCategory method to search for motion sensors and return them in a sensor collection.

01 // Additional includes for sensors
02 #include
03 #include
04 #include
05 // Create a COM interface to the SensorManager object.
06 ISensorManager* pSensorManager = NULL;
07 HRESULT hr = ::CoCreateInstance(CLSID_SensorManager, NULL, CLSCTX_INPROC_SERVER,
08     IID_PPV_ARGS(&pSensorManager));
09 if (FAILED(hr))
10 {
11     ::MessageBox(NULL, _T("Unable to CoCreateInstance() the SensorManager."),
12         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
13     return -1;
14 }
15 // Get a collection of all 3-axis Gyros on the computer.
16 ISensorCollection* pSensorCollection = NULL;
17 hr = pSensorManager->GetSensorsByCategory(SENSOR_CATEGORY_MOTION, &pSensorCollection);
18 if (FAILED(hr))
19 {
20     ::MessageBox(NULL, _T("Unable to find any Motion sensors on the computer."),
21         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
22     return -1;
23 }

Ask by Category “All”

In practice, it is most efficient for an app to request all of the sensors on the computer at once. The Sensor Manager consults the list of sensor hardware on the computer and returns a collection of all the objects bound to that hardware. The Sensor Collection may have 0, 1, or more objects in it. On most computers, the collection will have seven or more objects.

C++ does not have a GetAllSensors call, so you must use GetSensorsByCategory(SENSOR_CATEGORY_ALL, …) instead as shown in the sample code below.

      
      
      
      
01 C++ does not have a GetAllSensors call, so you must use GetSensorsByCategory(SENSOR_CATEGORY_ALL, …) instead as shown in the sample code below.
02 // Additional includes for sensors
03 #include
04 #include
05 #include
06 // Create a COM interface to the SensorManager object.
07 ISensorManager* pSensorManager = NULL;
08 HRESULT hr = ::CoCreateInstance(CLSID_SensorManager, NULL, CLSCTX_INPROC_SERVER,
09     IID_PPV_ARGS(&pSensorManager));
10 if (FAILED(hr))
11 {
12     ::MessageBox(NULL, _T("Unable to CoCreateInstance() the SensorManager."),
13         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
14     return -1;
15 }
16 // Get a collection of all 3sensors on the computer.
17 ISensorCollection* pSensorCollection = NULL;
18 hr = pSensorManager->GetSensorsByCategory(SENSOR_CATEGORY_ALL, &pSensorCollection);
19 if (FAILED(hr))
20 {
21     ::MessageBox(NULL, _T("Unable to find any sensors on the computer."),
22         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
23     return -1;
24 }

Sensor Life Cycle – Enter and Leave Events

On Windows, as with most hardware devices, sensors are treated as Plug and Play devices. There are a few different scenarios where sensors can be connected/disconnected:

1. It is possible to have USB-based sensors external to the system and plugged in to a USB port. 
2. It is conceivable to have sensors that are attached by an unreliable wireless interface (such as Bluetooth*) or wired interface (such as Ethernet), where connects and disconnects are common.
3. If a Windows Update upgrades the device driver for the sensors, they will appear to disconnect and then reconnect.
4. When Windows shuts down (to S4 or S5), the sensors appear to disconnect.

In the context of sensors, a Plug and Play connect is called an Enter event, and disconnect is called a Leave event. Resilient apps need to be able to handle both.

Enter Event Callback

If the app is already running at the time a sensor is plugged in, the Sensor Manager reports the sensor Enter event; however, if the sensors are already plugged in when the app starts running, this action will not result in Enter events for those sensors. In C++/COM, you must use the SetEventSinkmethod to hook the callback. The callback must be an entire class that inherits from ISensorManagerEvents and must implement IUnknown. Additionally, the ISensorManagerEvents interface must have callback function implementations for:

      
      
      
      
01     STDMETHODIMP OnSensorEnter(ISensor *pSensor, SensorState state);
02 // Hook the SensorManager for any SensorEnter events.
03 pSensorManagerEventClass = new SensorManagerEventSink();  // create C++ class instance
04 // get the ISensorManagerEvents COM interface pointer
05 HRESULT hr = pSensorManagerEventClass->QueryInterface(IID_PPV_ARGS(&pSensorManagerEvents));
06 if (FAILED(hr))
07 {
08     ::MessageBox(NULL, _T("Cannot query ISensorManagerEvents interface for our callback class."),
09         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
10     return -1;
11 }
12 // hook COM interface of our class to SensorManager eventer
13 hr = pSensorManager->SetEventSink(pSensorManagerEvents);
14 if (FAILED(hr))
15 {
16     ::MessageBox(NULL, _T("Cannot SetEventSink on SensorManager to our callback class."),
17         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
18     return -1;
19 }

Code:Hook Callback for Enter event 

Below is the C++/COM equivalent of the Enter callback. All the initialization steps from the main loop would be performed in this function. In fact, it is more efficient to refactor the code so that the main loop merely calls OnSensorEnter to simulate an Enter event.

      
      
      
      
01 STDMETHODIMP SensorManagerEventSink::OnSensorEnter(ISensor *pSensor, SensorState state)
02 {
03     // Examine the SupportsDataField for SENSOR_DATA_TYPE_LIGHT_LEVEL_LUX.
04     VARIANT_BOOL bSupported = VARIANT_FALSE;
05     HRESULT hr = pSensor->SupportsDataField(SENSOR_DATA_TYPE_LIGHT_LEVEL_LUX, &bSupported);
06     if (FAILED(hr))
07     {
08         ::MessageBox(NULL, _T("Cannot check SupportsDataField for SENSOR_DATA_TYPE_LIGHT_LEVEL_LUX."),
09             _T("Sensor C++ Sample"), MB_OK | MB_ICONINFORMATION);
10         return hr;
11     }
12     if (bSupported == VARIANT_FALSE)
13     {
14         // This is not the sensor we want.
15         return -1;
16     }
17     ISensor *pAls = pSensor;  // It looks like an ALS, memorize it.
18     ::MessageBox(NULL, _T("Ambient Light Sensor has entered."),
19         _T("Sensor C++ Sample"), MB_OK | MB_ICONINFORMATION);
20     .
21     .
22     .
23     return hr;
24 }

Code: Callback for Enter event

Leave Event

The individual sensor (not the Sensor Manager) reports when the Leave event happens. This code is the same as the previous hook callback for anEnter event.

      
      
      
      
01 // Hook the Sensor for any DataUpdated, Leave, or StateChanged events.
02 SensorEventSink* pSensorEventClass = new SensorEventSink();  // create C++ class instance
03 ISensorEvents* pSensorEvents = NULL;
04 // get the ISensorEvents COM interface pointer
05 HRESULT hr = pSensorEventClass->QueryInterface(IID_PPV_ARGS(&pSensorEvents));
06 if (FAILED(hr))
07 {
08     ::MessageBox(NULL, _T("Cannot query ISensorEvents interface for our callback class."),
09         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
10     return -1;
11 }
12 hr = pSensor->SetEventSink(pSensorEvents); // hook COM interface of our class to Sensor eventer
13 if (FAILED(hr))
14 {
15     ::MessageBox(NULL, _T("Cannot SetEventSink on the Sensor to our callback class."),
16         _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
17     return -1;
18 }

Code: Hook Callback for Leave event

The OnLeave event handler receives the ID of the leaving sensor as an argument.

      
      
      
      
01 STDMETHODIMP SensorEventSink::OnLeave(REFSENSOR_ID sensorID)
02 {
03     HRESULT hr = S_OK;
04     ::MessageBox(NULL, _T("Ambient Light Sensor has left."),
05         _T("Sensor C++ Sample"), MB_OK | MB_ICONINFORMATION);
06     // Perform any housekeeping tasks for the sensor that is leaving.
07     // For example, if you have maintained a reference to the sensor,
08     // release it now and set the pointer to NULL.
09     return hr;
10 }

Code: Callback for Leave event

Picking Sensors for an App

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Different types of sensors report different information. Microsoft calls these pieces of information Data Fields, and they are grouped together in a SensorDataReport. A computer may (potentially) have more than one type of sensor that an app can use. The app won’t care which sensor the information came from, so long as it is available.

Table 4 shows the constant names for the most commonly used Data Fields for Win32/COM and.NET. Just like sensor identifiers, these constants are just human-readable names for their associated GUIDs.  This method of association provides for extensibility of Data Fields beyond those “well known” fields that Microsoft has pre-defined.

Constant (Win32*/COM)	Constant (.NET)	PROPERTYKEY (GUID,PID)
SENSOR_DATA_TYPE_TIMESTAMP	SensorDataTypeTimestamp	{DB5E0CF2-CF1F-4C18-B46C-D86011D62150},2
SENSOR_DATA_TYPE_LIGHT_LEVEL_LUX	SensorDataTypeLightLevelLux	{E4C77CE2-DCB7-46E9-8439-4FEC548833A6},2
SENSOR_DATA_TYPE_ACCELERATION_X_G	SensorDataTypeAccelerationXG	{3F8A69A2-07C5-4E48-A965-CD797AAB56D5},2
SENSOR_DATA_TYPE_ACCELERATION_Y_G	SensorDataTypeAccelerationYG	{3F8A69A2-07C5-4E48-A965-CD797AAB56D5},3
SENSOR_DATA_TYPE_ACCELERATION_Z_G	SensorDataTypeAccelerationZG	{3F8A69A2-07C5-4E48-A965-CD797AAB56D5},4
SENSOR_DATA_TYPE_ANGULAR_VELOCITY_X_DEGRE ES_PER_SECOND	SensorDataTypeAngularVelocityXDegreesPerSecond	{3F8A69A2-07C5-4E48-A965-CD797AAB56D5},10
SENSOR_DATA_TYPE_ANGULAR_VELOCITY_Y_DEGRE ES_PER_SECOND	SensorDataTypeAngularVelocityYDegreesPerSecond	{3F8A69A2-07C5-4E48-A965-CD797AAB56D5},11
SENSOR_DATA_TYPE_ANGULAR_VELOCITY_Z_DEGRE ES_PER_SECOND	SensorDataTypeAngularVelocityZDegreesPerSecond	{3F8A69A2-07C5-4E48-A965-CD797AAB56D5},12
SENSOR_DATA_TYPE_TILT_X_DEGREES	SensorDataTypeTiltXDegrees	{1637D8A2-4248-4275-865D-558DE84AEDFD},2
SENSOR_DATA_TYPE_TILT_Y_DEGREES	SensorDataTypeTiltYDegrees	{1637D8A2-4248-4275-865D-558DE84AEDFD},3
SENSOR_DATA_TYPE_TILT_Z_DEGREES	SensorDataTypeTiltZDegrees	{1637D8A2-4248-4275-865D-558DE84AEDFD},4
SENSOR_DATA_TYPE_MAGNETIC_HEADING_COMPEN SATED_MAGNETIC_NORTH_DEGREES	SensorDataTypeMagneticHeadingCompen satedTrueNorthDegrees	{1637D8A2-4248-4275-865D-558DE84AEDFD},11
SENSOR_DATA_TYPE_MAGNETIC_FIELD_STRENGTH_ X_MILLIGAUSS	SensorDataTypeMagneticFieldStrengthXMilligauss	{1637D8A2-4248-4275-865D-558DE84AEDFD},19
SENSOR_DATA_TYPE_MAGNETIC_FIELD_STRENGTH_ Y_MILLIGAUSS	SensorDataTypeMagneticFieldStrengthYMilligauss	{1637D8A2-4248-4275-865D-558DE84AEDFD},20
SENSOR_DATA_TYPE_MAGNETIC_FIELD_STRENGTH_ Z_MILLIGAUSS	SensorDataTypeMagneticFieldStrengthZMilligauss	{1637D8A2-4248-4275-865D-558DE84AEDFD},21
SENSOR_DATA_TYPE_QUATERNION	SensorDataTypeQuaternion	{1637D8A2-4248-4275-865D-558DE84AEDFD},17
SENSOR_DATA_TYPE_ROTATION_MATRIX	SensorDataTypeRotationMatrix	{1637D8A2-4248-4275-865D-558DE84AEDFD},16
SENSOR_DATA_TYPE_LATITUDE_DEGREES	SensorDataTypeLatitudeDegrees	{055C74D8-CA6F-47D6-95C6-1ED3637A0FF4},2
SENSOR_DATA_TYPE_LONGITUDE_DEGREES	SensorDataTypeLongitudeDegrees	{055C74D8-CA6F-47D6-95C6-1ED3637A0FF4},3
SENSOR_DATA_TYPE_ALTITUDE_ELLIPSOID_METERS	SensorDataTypeAltitudeEllipsoidMeters	{055C74D8-CA6F-47D6-95C6-1ED3637A0FF4},5

Table 4. Data Field identifier constants  

One thing that makes Data Field identifiers different from sensor IDs is the use of a data type called PROPERTYKEY. A PROPERTYKEY consists of a GUID (similar to what sensors have), plus an extra number called a “PID” (property ID). You might notice that the GUID part of a PROPERTYKEY is common for sensors that are in the same category. Data Fields have a native data type for all of their values, such as Boolean, unsigned char, int, float, double, etc.

In Win32/COM, the value of a Data Field is stored in a polymorphic data type called PROPVARIANT. In .NET, there is a CLR (Common Language Runtime) data type called “object” that does the same thing. The polymorphic data type will need to be queried and/or typecast to the “expected”/”documented” data type.

The SupportsDataField() method of the sensor should be used to check the sensors for the Data Fields of interest. This is the most common programming idiom that is used to select sensors. Depending on the usage model of the app, only a subset of the Data Field may be required. Sensors that support the desired Data Fields should be selected. Type casting will be required to assign the sub-classed member variables from the base class sensor.

      
      
      
      
01 ISensor* m_pAls;
02 ISensor* m_pAccel;
03 ISensor* m_pTilt;
04 // Cycle through the collection looking for sensors we care about.
05 ULONG ulCount = 0;
06 HRESULT hr = pSensorCollection->GetCount(&ulCount);
07 if (FAILED(hr))
08 {
09     ::MessageBox(NULL, _T("Unable to get count of sensors on the computer."), _T("Sensor C++ Sample"), MB_OK | MB_ICONERROR);
10     return -1;
11 }
12 for (int i = 0; i < (int)ulCount; i++)
13 {
14     hr = pSensorCollection->GetAt(i, &pSensor);
15     if (SUCCEEDED(hr))
16     {
17         VARIANT_BOOL bSupported = VARIANT_FALSE;
18         hr = pSensor->SupportsDataField(SENSOR_DATA_TYPE_LIGHT_LEVEL_LUX, &bSupported);
19         if (SUCCEEDED(hr) && (bSupported == VARIANT_TRUE)) m_pAls = pSensor;
20         hr = pSensor->SupportsDataField(SENSOR_DATA_TYPE_ACCELERATION_Z_G, &bSupported);
21         if (SUCCEEDED(hr) && (bSupported == VARIANT_TRUE)) m_pAccel = pSensor;
22         hr = pSensor->SupportsDataField(SENSOR_DATA_TYPE_TILT_Z_DEGREES, &bSupported);
23         if (SUCCEEDED(hr) && (bSupported == VARIANT_TRUE)) m_pTilt = pSensor;
24         .
25         .
26         .
27     }
28 }

Code: Use of the SupportsDataField() method of the sensor to check for supported data field

Sensor Properties

In addition to Data Fields, sensors have Properties that can be used for identification and configuration. Table 5 shows the most commonly-used Properties. Just like Data Fields, Properties have constant names used by Win32/COM and .NET, and those constants are really PROPERTYKEY numbers underneath. Properties are extensible by vendors and also have PROPVARIANT polymorphic data types. Unlike Data Fields that are read-only, Properties have the ability to Read/Write. It is up to the individual sensor’s discretion as to whether or not it rejects Write attempts. Because no exception is thrown when a write attempt fails, a write-read-verification will need to be performed. 

Identification (Win32*/COM)	Identification (.NET)		PROPERTYKEY (GUID,PID)
SENSOR_PROPERTY_PERSISTENT_UNIQUE_ID	SensorID		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},5
WPD_FUNCTIONAL_OBJECT_CATEGORY	CategoryID		{8F052D93-ABCA-4FC5-A5AC-B01DF4DBE598},2
SENSOR_PROPERTY_TYPE	TypeID		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},2
SENSOR_PROPERTY_STATE	State		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},3
SENSOR_PROPERTY_MANUFACTURER	SensorManufacturer		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},6
SENSOR_PROPERTY_MODEL	SensorModel		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},7
SENSOR_PROPERTY_SERIAL_NUMBER	SensorSerialNumber		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},8
SENSOR_PROPERTY_FRIENDLY_NAME	FriendlyName		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},9
SENSOR_PROPERTY_DESCRIPTION	SensorDescription		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},10
SENSOR_PROPERTY_MIN_REPORT_INTERVAL	MinReportInterval		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},12
SENSOR_PROPERTY_CONNECTION_TYPE	SensorConnectionType		{7F8383EC-D3EC-495C-A8CF-B8BBE85C2920},11