Emit a signal from the main thread or the QThread thread

main.cpp

#include <QGuiApplication>
#include <QQuickView>
#include "MyClass.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQuickView *view = new QQuickView();

    qmlRegisterType<MyClass>("MyClass", 1, 0, "MyClass");

    view->setSource((QUrl(QStringLiteral("qrc:/main.qml"))));
    view->create();
    view->show();

    return app.exec();
}

main.qml

import QtQuick 2.12
import QtQuick.Controls 2.5
import MyClass 1.0

Rectangle {
    id: root

    width: 800
    height: 600
    focus: true

    MyClass {
        id: tester
        onErrorHappened: {
            var s
            switch (errorCode) {
            case MyClass.ErrorA:
                s = "Error A happened"
                break
            }
            console.log(s)
        }
    }

    Row {
        spacing: 30

        Button {
            id: mainThreadButton

            enabled: !tester.testRunning
            text: "Test on main thread"
            onClicked: tester.mainThreadError()
        }

        Button {
            id: otherThreadButton

            enabled: !tester.testRunning
            text: "Test on other thread"
            onClicked: tester.otherThreadError()
        }
    }
}

MyClass.cpp

#include "MyClass.h"

MyClass::MyClass(QObject *parent)
    : QThread(parent)
{
    start();
}

MyClass::~MyClass()
{
    stop();
}

void MyClass::mainThreadError()
{
    emit errorHappened(ErrorCode::ErrorA);
}

void MyClass::otherThreadError()
{
    m_signalError = true;
}

void MyClass::run()
{
    m_running = true;

    while (!m_signalStop) {
        if (m_signalError) {
            emit errorHappened(ErrorCode::ErrorA);
            m_signalError = false;
        }
        msleep(1);
    }

    m_running = false;
    m_signalStop = false;
}

void MyClass::stop()
{
    if (m_running) {
        m_signalStop = true;
        wait();
    }
}

MyClass.h

#pragma once

#include <QThread>
#include <atomic>

class MyClass : public QThread
{
    Q_OBJECT

public:
    explicit MyClass(QObject *parent = Q_NULLPTR);
    virtual ~MyClass() override;

    enum ErrorCode {
        ErrorA,
        ErrorB,
        ErrorC
    };
    Q_ENUM(ErrorCode)

public slots:
    void mainThreadError();
    void otherThreadError();

signals:
    void errorHappened(ErrorCode errorCode);

private:
    std::atomic<bool> m_running;
    std::atomic<bool> m_signalStop;
    std::atomic<bool> m_signalError;

    void run() override;
    void stop();
};

qml.qrc

<RCC>
    <qresource prefix="/">
        <file>main.qml</file>
    </qresource>
</RCC>

Qt_QThread_Enum_Test.pro

TEMPLATE = app
QT += qml quick

HEADERS += \
    $$PWD/MyClass.h \

SOURCES += \
    $$PWD/main.cpp \
    $$PWD/MyClass.cpp \

RESOURCES += \
    $$PWD/qml.qrc \