Proper Use of QProcess

• Platform: Qt 4.8.2, Win 7

Please consider the following logical stream:

1. App started
2. functionA() triggered
3. the app periodically capture some images to external files
4. end of functionA()
5. the app create a video from captured images, using ffmpeg as external program
6. [step 2 -> step 5] may be repeated.
7. App quit

To achieve the thread, I use QProcess to run an external program so that I can join the images, but I am confused about the correct use of QProcess. (I do not need ffmpeg console messages, I determine the success of step 5 by checking if the video file is created.)

Attempt 1

void MyWidget::createAVI()
{
    checkAndDeleteAVI();
    process = new QProcess(this); // process_ defined as class member;
    process->start("ffmpeg -f images2 ....");
    process->waitForFinished(-1);  // (a)
    // (b)
}

In (a), I read the documentation that this call might freeze the main GUI, so what should I call from QThread / QRunnable?

On (b) did I miss something? as when trying to close the application (step 7 in the stream), the application crashes, and I thought that the generated QProcess was not correctly released.

Attempt 2

I wrote the QProcess shell class as follows:

Launcher.h

class Launcher : public QObject
{
    Q_OBJECT
public:
    /** constructor */
    explicit Launcher(QObject *parent = 0);
    /** destructor */
    ~Launcher() {
        if (started_ && process_->state() != QProcess::NotRunning)
            process_->kill();
    } // end_dtor(Launcher)
Q_SIGNALS:
    void feedbackLog(QString log);
public Q_SLOTS:
    void launch(QString program, QStringList argList);
private:
    QProcess * process_;
    bool started_;
private Q_SLOTS:
    void error(QProcess::ProcessError error);
    void finished(int exitCode, QProcess::ExitStatus status);
    void stateChanged(QProcess::ProcessState state);
}; // end_class(Launcher)

#endif // LAUNCHER_H

Launcher.cpp

#include "launcher.h"
#include <QCoreApplication>
#include <QtDebug>

Launcher::Launcher(QObject *parent) : QObject(parent), started_(false)
{
    process_ = new QProcess(this);
    connect(process_,
            SIGNAL(error(QProcess::ProcessError)),
            SLOT(error(QProcess::ProcessError)));
    connect(process_,
            SIGNAL(finished(int, QProcess::ExitStatus)),
            SLOT(finished(int, QProcess::ExitStatus)));
    connect(process_,
            SIGNAL(stateChanged(QProcess::ProcessState)),
            SLOT(stateChanged(QProcess::ProcessState)));
} // end_ctor(ExternalLauncher)

void Launcher::launch(QString program, QStringList argList)
{
    started_ = true;
    process_->start(program, argList);
    process_->waitForFinished(-1); // (c)
    Q_EMIT feedbackLog(process_->readAllStandardOutput());
    process_->close();
} // end Launcher::launch()

void Launcher::error(QProcess::ProcessError error)
{
    /* just feedback some text about the error */
} // end_slot(Launcher::error)

void Launcher::finished(int exitCode, QProcess::ExitStatus status)
{
    started_ = false;
    /* feedback some text about finished */
} // end_slot (Launcher::finished)

void Launcher::stateChanged(QProcess::ProcessState state)
{
    qDebug() << "Luancher::stateChanged" << state;
}

How I use Launcher:

void MyWidget::createAVI()
{
    checkAndDeleteAVI();
    launcher_.launch("ffmpeg", "argsList"); // launcher_ defined as class member;
}

, (c), ForFinished()? ( , waitForXXX() / QProcess)

, -, Launcher, .

: , QProcess:: terminate()/QProcess:: kill() QProcess?