Reading Sensor Data Asynchronously and Handling Sensor Events in Visual Basic .NET

Creation QR Code in Visual Basic .NET Reading Sensor Data Asynchronously and Handling Sensor Events

As we mentioned in the previous section, you can also read sensor data asynchronously. To do so, you need to use the Sensor API events, which provide event notifications for sensor data updates and state changes. To receive sensor event notifications, you must implement a set of callback methods defined by the ISensorEvents COM Interface: OnEvent Provides a generic event notification that includes an ID indicating which event was received: data update, sensors leave, or sensor state change

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OnDataUpdated The event you want to use to receive sensor data updates and read new sensor data values OnLeave The event you ll need to handle when a sensor is no longer available OnStateChanged Provides a notification that a sensor state has changed

ISensorEvents is a COM interface that inherits from IUnknown. If you want to read sensor data asynchronously, you need to implement the ISensorEvents.OnDataUpdate callback method. This method receives pointers to a ISensor object and to a ISensorDataReport as shown in the following code snippet:

HRESULT CAmbientLightAwareSensorEvents::OnDataUpdated(ISensor *pSensor, ISensorDataReport *pNewData) { HRESULT hr = S_OK; if ((NULL != pSensor) && (NULL != pNewData)) { // Helper function, gets LUX data from a sensor and updates the application's UI hr = GetSensorData(pSensor, pNewData); } else { hr = E_UNEXPECTED; } return hr; }

Here you can see that after verifying that the sensor and its data report are valid (not null pointers), you call a helper function, GetSensorData, that reads the LUX value from the sensor report and updates the application s UI. This is the same helper function we described in the Reading Sensor Data Synchronously section. The OnStateChange event is also an important event to handle. This is mainly because when you ask a user to enable a sensor, if the user does enable the sensor, you can expect the sensor to generate a state change event. In this case, the state change event indicates that the sensor state has changed from SENSOR_STATE_ACCESS_DENIED to SENSOR_STATE_READY. When implementing the OnStateChanged event handler, you should handle all the possible states a sensor can have, which were described in the What Is a Sensor section earlier in the chapter. The following code snippet is the simple (and not complete) implementation of the OnStateChanged method in our Ambient Light application.

HRESULT CAmbientLightAwareSensorEvents::OnStateChanged(ISensor *pSensor, SensorState state) { HRESULT hr = S_OK; if (NULL != pSensor) {

SENSOR_ID idSensor = GUID_NULL; hr = pSensor->GetID(&idSensor); if (SUCCEEDED(hr)) { if (SENSOR_STATE_READY == state) { hr = GetSensorData(pSensor); } else { // If the sensor is not ready, its lux value is ignored m_mapLux[idSensor] = -1.0; hr = UpdateLux(); } } } else { hr = E_POINTER; } return hr; }

Here you can see that first you verify the sensor state, and if it s ready, you synchronously read its data using the GetSensorData method that we described in the Reading Sensor Data Synchronously section.

There is just one more thing you need to do to receive sensor events. You need to register your ISensorEvents implementation to specify the interface that receives sensor event notifications. We touched upon this topic in the Discovering Sensors section earlier in the chapter. During the Ambient Light application initialization phase, for each sensor you discover, you call the AddSensor function. This is a helper function that sets up event sinking for a specific sensor, and its code looks like this:

HRESULT CAmbientLightAwareSensorManagerEvents::AddSensor(ISensor *pSensor) { HRESULT hr = S_OK; if (NULL != pSensor) { hr = pSensor->SetEventSink(m_pSensorEvents); if (SUCCEEDED(hr)) { // Get the sensor's ID to be used as a key to store the sensor SENSOR_ID idSensor = GUID_NULL; hr = pSensor->GetID(&idSensor); if (SUCCEEDED(hr)) { // Enter the sensor into the map and take the ownership of its lifetime pSensor->AddRef(); // the sensor is released in the destructor m_Sensors[idSensor] = pSensor;