Chromium Sensor Backend Modernization on Windows

1. Summary

Windows has two sets of sensor APIs:

• ISensor API: The Win32 sensor API on Windows, which is now deprecated.
• Windows.Devices.Sensors WinRT API: The WinRT sensor API on Windows.

The Chromium sensor backend on Windows is forked, systems running Windows 19H1 or later will use the WinRT based stack while older systems will use the ISensor stack. This document outlines both of those implementations:

1. Summary
2. General Chromium Sensor Backend Model
3. ISensor Chromium Sensor Backend Model on Windows
4. Windows.Devices.Sensors Chromium Sensor Backend Model on Windows

2. General Chromium Sensor Backend Model

For context, the Chromium sensor backend is abstracted into these interfaces:

• SensorProvider – Sensor factory class responsible for discovering sensors on system, querying capabilities, and creating new sensor instances.
• Sensor – Sensor device wrapper class used to control the sensor.
• SensorClient – Event class that observes sensor reading changes and error conditions.

These interfaces are ultimately what the W3C sensor interface communicates with - each platform has its own implementation.

3. ISensor Chromium Sensor Backend Model on Windows

The following diagram shows the ISensor based Chromium backend implementation on Windows:

The implementation contains three main classes:

• PlatformSensorProviderWin
  • Windows implementation of the SensorProvider interface. It is responsible for the creation of PlatformSensorWin and PlatformSensorReaderWin. It also manages ISensorManager (overall manager for the ISensor API) and the sensor thread where all COM objects are created.
• PlatformSensorWin
  • Windows implementation of the Sensor and SensorClient interface, manages the state of PlatformSensorReaderWin.
• PlatformSensorReaderWin
  • Wrapper class around an ISensor instance.

The following diagram shows the typical usage flow:

Support for the SensorTypes defined by the Mojo interface on Windows is summarized below:

*The LINEAR_ACCELEROMETER sensor type is provided by implementing a low-pass-filter over the values returned by the ACCELEROMETER in order to remove the contribution of the gravitational force.

*The GRAVITY sensor type is provided by implementing a low-pass-filter over the values returned by the ACCELEROMETER in order to get the contribution of the gravitational force.

The “Sensor GUID” column specifies the names of the sensor type GUIDs used to provide data for a SensorType. Any SensorType not mentioned by this table are not supported on Windows.

4. Windows.Devices.Sensors Chromium Sensor Backend Model on Windows

The overall sensor backend design on Windows remains the same (as detailed in section 3). The classes which take a direct dependency on the ISensor API have their interfaces abstracted out and reimplemented with Windows.Devices.Sensors:

• The PlatformSensorProviderWin class:
  • platform_sensor_provider_win.h
  • platform_sensor_provider_win.cc
• The PlatformSensorReaderWin class:
  • platform_sensor_reader_win.h
  • platform_sensor_reader_win.cc

This allows the other classes, which are agnostic to the underlying API, to interchangeability use the ISensor vs. Windows.Devices.Sensors implementation. This is advantageous as the two implementations live side by side.

4.1 Support For Adapting Between ISensor and Windows.Devices.Sensors Sensor Implementations

Please refer to platform_sensor_provider.cc.

4.2 Windows.Devices.Sensors Sensor Implementation

The overall Windows.Devices.Sensors design is the same as the current design detailed in section 3 except the classes that take a dependency on the ISensor API have been reimplemented with these new classes:

• PlatformSensorProviderWinrt
  • Windows.Devices.Sensors based implementation of PlatformSensorProvider, functional equivalent of PlatformSensorProviderWin.
• PlatformSensorReaderWinrt
  • Wrapper class around the actual Windows.Devices.Sensors APIs, functional equivalent of PlatformSensorReaderWin.

4.2.1 PlatformSensorReaderWinrt

The existing PlatformSensorReaderWin class has been pulled out into its own interface in platform_sensor_reader_win_base.h.

The existing (ISensor) PlatformSensorReaderWin32 class and new (Windows.Devices.Sensors) PlatformSensorReaderWinrt class inherit from this interface so both classes can be interchangeably used with the ISensor/Windows.Devices.Sensors agnostic PlatformSensorWin, which simply consume the PlatformSensorReaderWinBase interface.

Since there are several Windows.Devices.Sensors sensor functions which are identical between the different sensor types, an abstract class encapsulates the common functionality. Please refer to platform_sensor_reader_winrt.h.