Best practice accessing Q_PROPERTY in qml from another thread - c++

Question

I have a larger project that crashes randomly after a few hours. I believe it is due to qml accessing Q_PROPERTY in my C++ QObject, where the Q_PROPERTY is set in another thread, causing a data race.
I have a simple toy project here. In it main.qml has a label that accesses a QProperty test.testChild.time. test.testChild.time is set in another thread when a QTimer expires and calls Test::timerExpired(). Test::timerExpired() has a QThread::usleep(100000) to simulate a long operation. When I comment the moveToThread(&m_workerThread) line below, the ui is much less responsive than before but everything is in the same thread so I believe data race cannot occur. If I don't comment moveToThread(&m_workerThread) and have timer.setInterval(0), the ui is very responsive but a crash occurs usually within a few minutes on my Ubuntu VM.
The code below is a section from test.cpp in my simple toy project here
Test::Test(QObject *parent) : QObject(parent)
// , testChild(this) // uncommenting this line will cause testChild to move to m_workerThread
{
uiThread = QThread::currentThreadId();
// move this object to a separate thread so does not interrupt UI thread
moveToThread(&m_workerThread); // commenting this line will make test::timerExpired() run on UI thread and slow down UI
timer.setInterval(0); // with interval 0, more likely to seg fault due to data race
connect(&timer, SIGNAL(timeout()), this, SLOT(timerExpired()));
connect(&m_workerThread, SIGNAL(started()), &timer, SLOT(start()));
m_workerThread.start();
}
void Test::timerExpired()
{
if (uiThread == QThread::currentThreadId())
qCritical() << "timerExpired() Controller thread is same as UI thread";
QMutexLocker locker(&mutex);// prevent UI thread from accessing objects below while they update with a QMutexLocker
QString time;
time = QDateTime::currentDateTime().toString("hh:mm:ss.zzz");
QThread::usleep(100000); // long operation
setTime(time);
testChild.setTime(time);
}
How can I safely have two threads, one for qml and another for long operations, while qml can access QProperties that update in the long operations thread?

Answer 1

I believe I have solved this using a model so the QML can safely communicate with the another thread.
The model is instantiated in the UI thread. The model receives data using signals and slots from the other thread (which is threadsafe). The model has QProperties that QML can interface with. When the model receives new data from another thread, QProperties are updated and QML receives new data.
The project is here but is also included below
TestQPropertyThread.pro
QT += quick
CONFIG += c++11
DEFINES += QT_DEPRECATED_WARNINGS
QMAKE_CXXFLAGS += -g -Wall -Werror
SOURCES += \
controller.cpp \
controllerchild.cpp \
main.cpp \
model.cpp
RESOURCES += qml.qrc
HEADERS += \
controller.h \
controllerchild.h \
model.h
main.cpp
#include "controller.h"
#include "model.h"
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
int main(int argc, char *argv[])
{
QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
QGuiApplication app(argc, argv);
QQmlApplicationEngine engine;
Controller test;
Model model(test);
engine.rootContext()->setContextProperty(QStringLiteral("model"), &model);
const QUrl url(QStringLiteral("qrc:/main.qml"));
QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
&app, [url](QObject *obj, const QUrl &objUrl) {
if (!obj && url == objUrl)
QCoreApplication::exit(-1);
}, Qt::QueuedConnection);
engine.load(url);
return app.exec();
}
controller.cpp
#include "controller.h"
Controller::Controller(QObject *parent) : QObject(parent), testChild(this), timer(this) // testChild to move to m_workerThread
{
uiThread = QThread::currentThreadId();
// move this object to a separate thread so does not interrupt UI thread
moveToThread(&m_workerThread); // commenting this line will make test::timerExpired() run on UI thread and slow down UI
timer.setInterval(0); // with interval 0, more likely to seg fault due to data race
connect(&timer, SIGNAL(timeout()), this, SLOT(timerExpired()));
connect(&m_workerThread, SIGNAL(started()), &timer, SLOT(start()));
m_workerThread.start();
}
void Controller::timerExpired()
{
if (uiThread == QThread::currentThreadId())
qCritical() << "Controller::timerExpired() Controller thread is same as UI thread";
// prevent UI thread from accessing objects below while they update with a QMutexLocker
QMutexLocker locker(&mutex);
QString time;
time = QDateTime::currentDateTime().toString("hh:mm:ss.zzz");
qDebug() << "Controller::timerExpired()" << time; // this can impact preformance when usleep is commented out
QThread::usleep(100000); // long operation
testChild.setTime(time);
}
controller.h
#ifndef TEST_H
#define TEST_H
#include <QObject>
#include <QTimer>
#include <QThread>
#include <QMutex>
#include <QDebug>
#include <QDateTime>
#include "controllerchild.h"
class Controller : public QObject
{
Q_OBJECT
public:
explicit Controller(QObject *parent = nullptr);
ControllerChild testChild;
public slots:
void timerExpired();
private:
QTimer timer;
QThread m_workerThread;
Qt::HANDLE uiThread;
QMutex mutex;
};
#endif // TEST_H
controllerchild.cpp
#include "controllerchild.h"
ControllerChild::ControllerChild(QObject *parent) : QObject(parent)
{
}
void ControllerChild::setTime(const QString &value)
{
time = value;
emit timeChanged(time);
}
void ControllerChild::onButtonPress(const QString &value)
{
qDebug() << "ControllerChild::onButtonPress()" << value << QThread::currentThreadId() << QDateTime::currentDateTime().toString("hh:mm:ss.zzz");
emit onBoolChanged(value); // send value back to UI label
}
controllerchild.h
#ifndef TESTCHILD_H
#define TESTCHILD_H
#include <QObject>
#include <QDebug>
#include <QThread>
#include <QDateTime>
class ControllerChild : public QObject
{
Q_OBJECT
public:
explicit ControllerChild(QObject *parent = nullptr);
void setTime(const QString &value);
public slots:
void onButtonPress(const QString &value);
signals:
void timeChanged(const QString &value);
void onBoolChanged(QString value);
private:
QString time;
};
#endif // TESTCHILD_H
model.cpp
#include "model.h"
Model::Model(Controller &test, QObject *parent) : QObject(parent), test(test)
{
connect(&test.testChild, &ControllerChild::timeChanged, this, [=](const QString &time){ this->setTime(time); });
connect(this, &Model::onButtonPressChanged, &test.testChild, &ControllerChild::onButtonPress);
connect(&test.testChild, &ControllerChild::onBoolChanged, this, [=](const QString &value){ this->setBoolValue(value); });
}
QString Model::getTime() const
{
// return QString("test Child %1").arg(time);
return time;
}
void Model::setTime(const QString &value)
{
time = value;
emit timeChanged();
}
void Model::onButtonPress(const QString &value)
{
qDebug() << "Model::onButtonPress()" << value << QThread::currentThreadId() << QDateTime::currentDateTime().toString("hh:mm:ss.zzz");
emit onButtonPressChanged(value);
}
void Model::onBool(const QString &value)
{
qDebug() << "Model::onBool()" << value << QThread::currentThreadId() << QDateTime::currentDateTime().toString("hh:mm:ss.zzz");
emit onBoolChanged(value);
}
void Model::setBoolValue(const QString &value)
{
boolValue = value;
qDebug() << "Model::setBoolValue()" << value << QThread::currentThreadId() << QDateTime::currentDateTime().toString("hh:mm:ss.zzz");
emit onBoolChanged(value);
}
model.h
#ifndef MODEL_H
#define MODEL_H
#include <QObject>
#include "controller.h"
class Model : public QObject
{
Q_OBJECT
Q_PROPERTY(QString time READ getTime WRITE setTime NOTIFY timeChanged)
Q_PROPERTY(QString boolValue MEMBER boolValue NOTIFY onBoolChanged)
public:
explicit Model(Controller &test, QObject *parent = nullptr);
QString getTime() const;
void setTime(const QString &value);
void setBoolValue(const QString &value);
public slots:
void onButtonPress(const QString &value);
void onBool(const QString &value);
signals:
void timeChanged();
void onButtonPressChanged(const QString &value);
void onBoolChanged(const QString &value);
private:
QString time;
Controller &test;
QString boolValue;
};
#endif // MODEL_H
main.qml
import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.12
Window {
visible: true
width: 640
height: 480
title: qsTr("Hello World")
Label {
id: time1
text: model.time
}
BusyIndicator {
id: busyIndicator
x: 0
y: 23
}
PathView {
id: pathView
x: 0
y: 135
width: 640
height: 130
delegate: Column {
spacing: 5
Rectangle {
width: 40
height: 40
color: colorCode
anchors.horizontalCenter: parent.horizontalCenter
}
Text {
x: 5
text: name
anchors.horizontalCenter: parent.horizontalCenter
font.bold: true
}
}
path: Path {
startY: 100
startX: 120
PathQuad {
x: 120
y: 25
controlY: 75
controlX: 260
}
PathQuad {
x: 120
y: 100
controlY: 75
controlX: -20
}
}
model: ListModel {
ListElement {
name: "Grey"
colorCode: "grey"
}
ListElement {
name: "Red"
colorCode: "red"
}
ListElement {
name: "Blue"
colorCode: "blue"
}
ListElement {
name: "Green"
colorCode: "green"
}
}
}
Button {
id: button
x: 0
y: 271
text: qsTr("Press me!!")
checkable: true
onToggled: model.onButtonPress(button.checked)
}
Label {
id: label
x: 106
y: 294
text: model.boolValue
}
}