Quickstart:Responding to changes in lighting (C++)
You can use the ambient light sensor to detect changes in lighting with an app written in C++.
An ambient light sensor is one of several types of environmental sensors that allow apps to respond to changes in the user's environment.
The Windows.Devices.Sensors namespace includes support for a LightSensor object that apps can use to retrieve the current illuminance as a LUX value. For more information about LUX values, refer to the Sensor API documentation on MSDN.
Objective: After completing this quickstart you will understand how to use the light sensor to detect changes in lighting.
Prerequisites
You should be familiar with XAML, Visual C++, and events.
The device or emulator that you're using must support an ambient light sensor.
Time to complete: 25 minutes.
Instructions
1. Open Microsoft Visual Studio
Open an instance of Microsoft Visual Studio.
2. Create a new project
Create a new project, choosing a Blank App from the Visual C++/Store Apps project types.
3. Replace the header file contents
Open your project's MainPage.xaml.h file and replace the existing comments, code, and definitions with the following.
//
// MainPage.xaml.h
// Declaration of the MainPage.xaml class.
//
#pragma once
#include "MainPage.g.h"
namespace App1
{
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public ref class MainPage sealed
{
public:
MainPage();
private:
void ReadingChanged(Windows::Devices::Sensors::LightSensor^ sender, Windows::Devices::Sensors::LightSensorReadingChangedEventArgs^ e);
Windows::Devices::Sensors::LightSensor^ lightsensor;
Windows::Foundation::EventRegistrationToken listenerToken;
Windows::UI::Xaml::DispatcherTimer^ dispatcherTimer;
protected:
virtual void OnNavigatedTo(Windows::UI::Xaml::Navigation::NavigationEventArgs^ e) override;
};
}
You'll need to replace the class name in the previous snippet with the name of your app's class. For example, if you created a project named LightingCPP, you'd replace namespace App1 with namespace LightingCPP.
4. Replace the C++ code
Open your project's MainPage.xaml.cpp file and replace the existing code with the following.
//
// MainPage.xaml.cpp
// Implementation of the MainPage.xaml class.
//
#include "pch.h"
#include "MainPage.xaml.h"
using namespace App1;
using namespace Platform;
using namespace Windows::Foundation;
using namespace Windows::Foundation::Collections;
using namespace Windows::UI::Xaml;
using namespace Windows::UI::Xaml::Controls;
using namespace Windows::UI::Xaml::Controls::Primitives;
using namespace Windows::UI::Xaml::Data;
using namespace Windows::UI::Xaml::Input;
using namespace Windows::UI::Xaml::Media;
using namespace Windows::UI::Xaml::Navigation;
// Below namespaces are required to support sensors and events
using namespace Windows::System;
using namespace Windows::Foundation;
using namespace Platform;
using namespace Windows::Devices::Sensors;
using namespace Windows::UI::Core;
// The Blank Page item template is documented at https://go.microsoft.com/fwlink/p/?linkid=234238
MainPage::MainPage()
{
InitializeComponent();
lightsensor = LightSensor::GetDefault();
if (lightsensor != nullptr)
{
// Establish the report interval for all scenarios
uint32 minReportInterval = lightsensor->MinimumReportInterval;
uint32 reportInterval = minReportInterval > 16 ? minReportInterval : 16;
lightsensor->ReportInterval = reportInterval;
// Establish the event handler
listenerToken = lightsensor->ReadingChanged::add(ref new TypedEventHandler<LightSensor^, LightSensorReadingChangedEventArgs^>(this, &MainPage::ReadingChanged));
}
}
void MainPage::ReadingChanged(LightSensor^ /* sender */, LightSensorReadingChangedEventArgs^ e)
{
auto ignored = Dispatcher->RunAsync(
CoreDispatcherPriority::Normal,
ref new DispatchedHandler(
[this, e]()
{
LightSensorReading^ reading = e->Reading;
txtLuxValue->Text = reading->IlluminanceInLux.ToString();
},
CallbackContext::Any
)
);
}
/// <summary>
/// Invoked when this page is about to be displayed in a Frame.
/// </summary>
/// <param name="e">Event data that describes how this page was reached. The Parameter
/// property is typically used to configure the page.</param>
void MainPage::OnNavigatedTo(NavigationEventArgs^ e)
{
}
You'll need to rename the namespace in the previous snippet with the name you gave your project. For example, if you created a project named LightingCS, you'd replace using namespace App1; with using namespace LightingCPP;.
5. Replace the XAML code
Open the file MainPage.xaml and copy the following XML into this file (replacing its original contents).
<Page
x:Class="LightSensorCPP.MainPage"
xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="https://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:LightSensorCPP"
xmlns:d="https://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="https://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d">
<Grid Background="{StaticResource ApplicationPageBackgroundBrush}">
<TextBlock HorizontalAlignment="Left" Height="44" Margin="52,38,0,0" TextWrapping="Wrap" Text="LUX Reading" VerticalAlignment="Top" Width="150"/>
<TextBlock x:Name="txtLuxValue" HorizontalAlignment="Left" Height="44" Margin="224,38,0,0" TextWrapping="Wrap" Text="TextBlock" VerticalAlignment="Top" Width="217"/>
</Grid>
</Page>
You'll need to replace the first part of the class name in the previous snippet with the namespace of your app. For example, if you created a project named LightingCPP, you'd replace x:Class="App1.MainPage" with x:Class="LightingCPP.MainPage". You should also replace xmlns:local="using:App1" with xmlns:local="using:LightingCPP".
6. Build, deploy and run the app
Press F5 or select Debug > Start Debugging to build, deploy, and run the app.
Once the app is running, you can change the accelerometer values by moving the device or using the emulator tools.
7. Stop the app
- Press ALT+Tab to return to Visual Studio.
- Press Shift+F5 or select Debug > Stop Debugging to stop the app.
Summary
The previous example demonstrates how little code you'll need to write in order to integrate light-sensor input in your app.
The app establishes a connection with the default sensor in the MainPage constructor.
lightsensor = LightSensor::GetDefault();
The app also establishes the report interval within the MainPage constructor. This code retrieves the minimum interval supported by the device and compares it to a requested interval of 16 milliseconds (which approximates a 60-Hz refresh rate). If the minimum supported interval is greater than the requested interval, the code sets the value to the minimum. Otherwise, it sets the value to the requested interval.
uint32 minReportInterval = lightsensor->MinimumReportInterval;
uint32 reportInterval = minReportInterval > 16 ? minReportInterval : 16;
lightsensor->ReportInterval = reportInterval;
The new light-sensor data is captured in the ReadingChanged method. Each time the sensor driver receives new data from the sensor, it passes the value to your app using this event handler. The app registers this event handler on the following line.
// Establish the event handler
listenerToken = lightsensor->ReadingChanged::add(ref new TypedEventHandler<LightSensor^,
LightSensorReadingChangedEventArgs^>(this, &MainPage::ReadingChanged));
These new values are written to a TextBlock found in the project's XAML.
<TextBlock HorizontalAlignment="Left" Height="44" Margin="52,38,0,0" TextWrapping="Wrap" Text="LUX Reading" VerticalAlignment="Top" Width="150"/>
<TextBlock x:Name="txtLuxValue" HorizontalAlignment="Left" Height="44" Margin="224,38,0,0" TextWrapping="Wrap" Text="TextBlock" VerticalAlignment="Top" Width="217"/>
Related topics
Roadmap for creating Windows Store apps using C#, C++, or Visual Basic