Thread Event Loop and Proper Termination

When a thread function starts an event loop, proper thread termination requires specific handling.

QThread::exec() and Event Loop Behavior

The QThread::exec() method causes a thread to enter a event loop:

• Code following exec() cannot execute until the event loop terminates
• The quit() and exit() methods are used to terminate event loops
• quit() is equivalent to exit(0), with exec() returning the value passed to exit()

Important Considerations

• Signals are always queued in the event queue of their associated thread, regardless of whether an event loop is running
• Slot functions are only invoked within their thread when an event loop is active

When to Use Thread Event Loops

Design Principles

• Transactional operations (such as intermittent I/O operations) benefit from thread event loops
• Each operation can be executed in the worker thread by emitting signals that trigger slot functions

File Buffer Concept

File operations are transactional in nature:

• File operations typically use a memory buffer by default
• Data written to files is initially stored in this buffer
• Data is only written to disk when the buffer fills or encounters a newline character
• Buffering reduces low-level disk I/O operations, improving efficiency at the cost of potential data loss risk

Qt Thread Usage Patterns

• No event loop mode: Suitable for long-running background tasks (file copying, network reading)
• Event loop mode: Appropriate for transactional operations (file writing, database transactions)

When using event loop mode, create a thread that runs a event loop, move objects with slot functions to the thread using object affinity transfer, and ensure proper thread termination in destructors.

FileWriter Header Implementation

#ifndef FILEWRITER_H
#define FILEWRITER_H

#include <QObject>
#include <QFile>
#include <QString>
#include <QThread>

class FileWriter : public QObject
{
    Q_OBJECT

private:
    class WorkerThread : public QThread
    {
    protected:
        void run() override;
    };
    
    QFile* m_outputFile;
    WorkerThread m_workerThread;

public:
    explicit FileWriter(const QString& filePath, QObject* parent = nullptr);
    bool openFile();
    void writeData(const char* data);
    void closeFile();
    ~FileWriter();

signals:
    void writeRequest(const char* data);
    void closeRequest();

private slots:
    void handleWrite(const char* data);
    void handleClose();
};

#endif // FILEWRITER_H

FileWriter Implementation

#include "FileWriter.h"
#include <QIODevice>
#include <QDebug>

FileWriter::FileWriter(const QString& filePath, QObject* parent) :
    QObject(parent)
{
    m_outputFile = new QFile(filePath, this);

    connect(this, &FileWriter::writeRequest, this, &FileWriter::handleWrite);
    connect(this, &FileWriter::closeRequest, this, &FileWriter::handleClose);

    moveToThread(&m_workerThread);
    m_workerThread.start();
}

bool FileWriter::openFile()
{
    return m_outputFile->open(QIODevice::WriteOnly | QIODevice::Text);
}

void FileWriter::writeData(const char* data)
{
    emit writeRequest(data);
}

void FileWriter::closeFile()
{
    emit closeRequest();
}

FileWriter::~FileWriter()
{
    m_workerThread.quit();
    m_workerThread.wait();
}

void FileWriter::handleWrite(const char* data)
{
    qDebug() << "Writing data in thread:" << QThread::currentThreadId();
    m_outputFile->write(data);
    m_outputFile->flush();
}

void FileWriter::handleClose()
{
    qDebug() << "Closing file in thread:" << QThread::currentThreadId();
    m_outputFile->close();
}

void FileWriter::WorkerThread::run()
{
    qDebug() << "Worker thread running:" << QThread::currentThreadId();
    exec();
}

Application Entry Point

#include <QtCore/QCoreApplication>
#include "FileWriter.h"

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

    FileWriter writer("C:/temp/output.txt");

    if(writer.openFile())
    {
        writer.writeData("Hello world\n");
        writer.writeData("Qt multithreading\n");
        writer.writeData("Signal-slot across threads\n");
        writer.closeFile();
    }

    return app.exec();
}

Key Points

• QThread::exec() initiates thread event loops
• quit() and exit(0) terminate evant loops and return from exec()
• Transactional operations can utilize thread event loops to distribute work to background threads
• In practice, thread event loops are less commonly used than thread-per-task patterns
• Threads are primarily employed for background processing and long-running operations