QFileSystemWatcher

myclass.cpp

void Myclass::start()
{
    /* Read the file once */
    tmpFileProcess("/tmp/testi");

    /* Watch the file */
    tmpFileNotifier = new QFileSystemWatcher(QStringList() << "/tmp/testi", this);
   
    /* Connect fileChanged to our slot */
    connect(tmpFileNotifier, SIGNAL(fileChanged(QString)), this, SLOT(tmpFileProcess(QString)));
}

void Myclass::tmpFileProcess(QString name)
{
    qDebug() << "file" << name << "changed";

    QFile fn(name);

    if (!fn.open(QIODevice::ReadOnly | QIODevice::Text))
    {
        /* If file is not found, say so */
        _fileContents = "File not found";
        /* Tell QML that the property has changed */
        emit fileContentsChanged();
        return;
    }

    /* Kinda stupid, read just 100 last characters from file */
    _fileContents = QString(fn.readAll().right(100));

    fn.close();
    /* Tell QML that the property has changed */
    emit fileContentsChanged();
}

myclass.h

#include <QFileSystemWatcher>
#include <QFile>
...

  Q_OBJECT
  ...
  Q_PROPERTY(QString fileContents READ readFileContents NOTIFY fileContentsChanged())

public:
....
  QString readFileContents() { return _fileContents; }
  Q_INVOKABLE void start();

private slots:
...
    void tmpFileProcess(QString name);

signals:
...
  void fileContentsChanged();

private:
...
  QFileSystemWatcher *tmpFileNotifier;
  QString _fileContents;
};

some.qml

            Button
            {
                anchors.horizontalCenter: parent.horizontalCenter
                text: "start"
                onClicked: myclass.start()
            }

            Label
            {
                x: Theme.paddingLarge
                width: parent.width - 2 * Theme.paddingLarge
                text: myclass.fileContents
                color: Theme.primaryColor
                wrapMode: Text.Wrap
            }