How to use QSensor? reading() returns a nullptr

[Donation]

I want to get the some sensor's information of Android device, so I use these class to make it: QGyroscope, QAccelerometer and QRotationSensor. The code is below:

//SensorWidget.h
#include <QGyroscope>
#include <QAccelerometer>
#include <QRotationSensor>
#include <QWidget>
#include <thread>

namespace Ui {
class SensorWidget;
}

class SensorWidget : public QWidget
{
    Q_OBJECT

public:
    explicit SensorWidget(QWidget *parent = nullptr);
    ~SensorWidget();

private:
    void showData();

private:
    Ui::SensorWidget *ui;

    bool m_thread_flag;
    std::thread* m_thread;

    QGyroscope* m_gyroscope;
    QGyroscopeReading* m_gyro_reader;

    QAccelerometer* m_accelerometer;
    QAccelerometerReading* m_acc_reader;

    QRotationSensor* m_rotation_sensor;
    QRotationReading* m_rotation_reader;
};

//SensorWidget.cpp
#include "SensorWidget.h"
#include "ui_SensorWidget.h"
#include <QDebug>

SensorWidget::SensorWidget(QWidget *parent) :
                                                QWidget(parent),
                                                ui(new Ui::SensorWidget),
                                                m_thread_flag(true),
                                                m_thread(new std::thread(&SensorWidget::showData, this)),
                                                m_gyroscope(new QGyroscope(this)),
                                                m_gyro_reader(nullptr),
                                                m_accelerometer(new QAccelerometer(this)),
                                                m_acc_reader(nullptr),
                                                m_rotation_sensor(new QRotationSensor(this)),
                                                m_rotation_reader(nullptr)
{
    ui->setupUi(this);
    m_gyroscope->start();
    m_accelerometer->start();
    m_rotation_sensor->start();
}

SensorWidget::~SensorWidget()
{
    m_thread_flag = false;
    m_thread->join();
    delete ui;
}

void SensorWidget::showData()
{
    while(m_thread_flag)
    {
        m_gyro_reader = m_gyroscope->reading();
        qDebug() << "Phone's X gyroscope = " << (m_gyro_reader->x());
        qDebug() << "Phone's Y gyroscope = " << (m_gyro_reader->y());
        qDebug() << "Phone's Z gyroscope = " << (m_gyro_reader->z());
        ui->lineEdit->setText(QString::number(m_gyro_reader->x()));
        ui->lineEdit_2->setText(QString::number(m_gyro_reader->y()));
        ui->lineEdit_3->setText(QString::number(m_gyro_reader->z()));


        m_acc_reader = m_accelerometer->reading();
        qDebug() << "Phone's X accelerometer = " << (m_acc_reader->x());
        qDebug() << "Phone's Y accelerometer = " << (m_acc_reader->y());
        qDebug() << "Phone's Z accelerometer = " << (m_acc_reader->z());
        ui->lineEdit_4->setText(QString::number(m_acc_reader->x()));
        ui->lineEdit_5->setText(QString::number(m_acc_reader->y()));
        ui->lineEdit_6->setText(QString::number(m_acc_reader->z()));

        m_rotation_reader = m_rotation_sensor->reading();
        qDebug() << "Phone's X rotation = " << (m_rotation_reader->x());
        qDebug() << "Phone's Y rotation = " << (m_rotation_reader->y());
        qDebug() << "Phone's Z rotation = " << (m_rotation_reader->z());
        ui->lineEdit_7->setText(QString::number(m_rotation_reader->x()));
        ui->lineEdit_8->setText(QString::number(m_rotation_reader->y()));
        ui->lineEdit_9->setText(QString::number(m_rotation_reader->z()));

        std::this_thread::sleep_for(std::chrono::milliseconds(50));
    }
}

I got an error when run the app on the Android and Windows, then I debug it and found that, m_gyroscope->reading() returns nothing but a nullptr. At last, I found m_gyroscope->start() is failed, it return false.
Is there any mistake I made in my code. What should I do to get those information?

[aha_1980]

@Donation

Have you already had a look at the examples: https://doc.qt.io/qt-5/qtsensors-examples.html ?

Also, it seems you are trying to operate with threads. Don't do that. Qt is asynchronous by nature, so you should use signals&slots extensively.

Threading is seldom necessary therefore and requires deep understanding of what's going on.

Regards

[Donation]

@aha_1980 thanks, I tried to move off the thread before, and got the same problem. I will take a good look at examples you give me, thanks very much.

[Donation]

The problem has been solved. Firstly, I removed the thread, QSensor can't works with thread.
And then, in .pro file, add these lines:

CONFIG += mobility

In head file, add these lines:

#include <QGyroscope>
#include <qgyroscope.h>
#include <QAccelerometer>
#include <qaccelerometer.h>
#include <QRotationSensor>
#include <qrotationsensor.h>

In cpp file, use signal&slot instead of thread

connect(m_gyroscope, SIGNAL(readingChanged()), this, SLOT(onGyroReadingChanged()));
connect(m_accelerometer, SIGNAL(readingChanged()), this, SLOT(onAccReadingChanged()));
connect(m_rotation_sensor, SIGNAL(readingChanged()), this, SLOT(onRotationReadingChanged()));

I haven't yet identified the cause of the problem. I'll update this when I check it out.
Personal opinion：We don't have to deal the problem by these sensor, only if the Gestures provided by Qt can not meeting your needs. It's too difficult to make a clear sense of these sensors, you may have to research on the knowledge about physics and how it relates to mobile device.

[Edit aha_1980: fixed typo]

[J.Hilk]

@Donation said in How to use QSensor? reading() returns a nullptr:

CONFIG += mobolity

that's not going to make much of a difference. I Assume you mean CONFIG += mobility ?

[Donation]

@J.Hilk sorry, it's a misspelling. yeah I'm not sure what makes the error now, maybe CONFIG+=mobility makes no difference, it seems would be added by Qt by default when I use Android Kits to build the project. I will check it out later. Thanks your reminding.
By the way, can you give some suggestions to me, what cause the error do you think? thanks very much.

[aha_1980]

@Donation said in How to use QSensor? reading() returns a nullptr:

#include <QGyroscope>
#include <qgyroscope.h>
#include <QAccelerometer>
#include <qaccelerometer.h>
#include <QRotationSensor>
#include <qrotationsensor.h>

I think the following is enough:

#include <QGyroscope>
#include <QAccelerometer>
#include <QRotationSensor>

[J.Hilk]

@Donation said in How to use QSensor? reading() returns a nullptr:

By the way, can you give some suggestions to me, what cause the error do you think? thanks very much.

you did add QT += sensors in your pro file ?
the sensors module is only supported for Android, iOS, SailFish and WinRT So building your app on Windows, Linux MacOS is going to be complicated(Windows UWP) or impossible(the rest)

[Donation]

@J.Hilk sure I did add QT += sensors in my pro file. And I build my project on Andorid.


I just don't know why it cause that error at the very beginning, now it runs well.

[J.Hilk]

@Donation
may be a manifest issue ? Sensor access requires special permission on android, IIRC

---

android.hardware.sensor.gyroscope
android.hardware.sensor.accelerometer

[Donation]

@aha_1980 I have tried, and it does enough. Now I can ruled it out, thanks a million.

[Donation]

@J.Hilk I don't know, I am a newcomer to Qt for Android. But all the differences between before and after are showing there, I didn't change any other codes or my device. Haha, I will check it out later when I finish my work today. Very very thanks for your help.