signal and slot in QT - c++

Question

i have a problem in working signal and slot in QT framework .
my slot is not working .
here is the code .
connect(&th,SIGNAL(change()),this,SLOT(closeWindow()));
this->moveToThread(th);
closeWindow();
th.start();
"th" is a var from a class like this :
class Thread : public QThread
{
public :
Thread();
bool pause,flag;
QString URL;
QFile *mFile;
void run();
void RESUME();
void PAUSE();
bool Check();
bool Check2();
signals:
void change();
QString myTxt;
};
"change" is the signal
and here is the code of my MainWindow :
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
Thread th;
private slots:
void on_pushButton_clicked();
private:
Ui::MainWindow *ui;
public slots:
void closeWindow();
};
and here is the constructor for MainWindow
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
connect(&th,SIGNAL(change()),this,SLOT(closeWindow()));
th.start();
}
the signal is Ok . but the Slot is not Working .
can You guide me ?

Answer 1

I think the main issue is that the receiving object lives in a dead thread.
Your Thread instance lives on the application thread, while your MainWindow lives on the thread created by your Thread instance. However as you have reimplemented QThread::run(), you do not have an event loop on this thread and the slot can never be invoked.
Check Qt documentation for details:
Threading Basics
Threads and QObjects
Also you have some big issues in your code:
Missing Q_OBJECT macro in Thread definition (but you might have removed it while copy/pasting otherwise you wouldn't be able to emit the signal).
You must not call moveToThread() on object derived from QWidget (e.g QMainWindow). QWidgets must live on the application thread.
You generally cannot mix using moveToThread() and reimplementing run().
When using moveToThread() you are using the thread event loop to make QObjects living on this thread (i.e moved to this thread) live.
When reimplementing QThread::run(), people generally want to execute a single function on a thread. This will lead to a thread being created, the function executed and the thread destroyed without running an event loop. And no event loop mean that if a QObject lives on this thread, it will not received inter-thread slot invokations.
NB: Using moveToThread() is the correct/intended way to use QThread. Reimplementing QThread::run() will work and can be found in Qt documentation but isn't generally recommended and you might be better of using QThreadPool.
More readings: http://blog.qt.io/blog/2010/06/17/youre-doing-it-wrong/