Correct usage of QThread

[Patrik]

Hi, I'm developing a qt application that interfaces with an external hardware device. My application consists of just 2 start/stop buttons and a listwidget used to display data received from the device. Since the data acquisition from the device requires me to call a blocking function, I've decided to put the call in a separated thread because data acquisition can be stopped only by calling another function from another thread. So I've created this QThread subclass:


class InventoryThread : public QThread
{
Q_OBJECT

private:
int handle;
RFID_18K6C_INVENTORY_PARMS * inventoryParms;
RFID_18K6C_INVENTORY_PARMS * SetInventoryParams(const RFID_RADIO_HANDLE handle, const int cycles);

public:
explicit InventoryThread(QObject *parent = 0, int h = 0);

protected:
void run();

signals:

public slots:

};

The relevant implementation is this:


InventoryThread::InventoryThread(QObject *parent, int h) :
QThread(parent)
{
handle = h;
}

void InventoryThread::run()
{
RFID_18K6CTagInventory(handle, SetInventoryParams(handle, 5), 0);
}

The main window code will call InventoryThread::start() when I click on the start button, then RFID_18K6CTagInventory will be called (which is a blocking function). This function can be stopped only by calling RFID_RadioCancelOperation in the main thread. Now, I'm not sure if my approach is correct, the documentation about QThread its main loop il pretty confusing.

What should I do?

[MarekR22]

http://labs.qt.nokia.com/2010/06/17/youre-doing-it-wrong/

[Patrik]

Ok, that clarified some things. I'm still having a little issue. My classes look as follows:


class InventoryThread : public QObject
{
Q_OBJECT

private:
int handle;
QListWidget * list;
RFID_18K6C_INVENTORY_PARMS * inventoryParms;
RFID_18K6C_INVENTORY_PARMS * SetInventoryParams(const RFID_RADIO_HANDLE handle, const int cycles);
QTimer * timer;

public:
explicit InventoryThread(QObject *parent = 0, int h = 0, QListWidget *lst = 0);

signals:

public slots:
void DoInventory();
};

InventoryThread::InventoryThread(QObject *parent, int h, QListWidget * lst) : QObject(parent)
{
handle = h;
list = lst;
timer = new QTimer(this);
list->connect(timer,SIGNAL(timeout()),SLOT(clear()));
//connect(timer,SIGNAL(timeout()),list,SLOT(clear()) );
timer->setSingleShot(false);
timer->start(1500);
}

void InventoryThread::DoInventory()
{
printf("Doing inventory\n");
RFID_18K6C_INVENTORY_PARMS * parms = SetInventoryParams(handle, 5);
printf("Tag Inventory returned %d\n", RFID_18K6CTagInventory(handle, parms, 0));
free(parms);
}



In my main class I have


class QThreadEx : public QThread
{
protected:
void run() { exec(); }
};

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QObject::connect(this->ui->start,SIGNAL(clicked()),this,SLOT(StartInventory() ));
invThread = new InventoryThread(0, handle, this->ui->listWidget);
thr = new QThreadEx();
invThread->moveToThread(thr);
invThread->connect(thr,SIGNAL(started()),SLOT(DoInventory())) ;
}


Finally, when I click on my start button I call thr->start() and when I click on the end button I call thr->quit(). This scheme works only for the first time. If I click on the start button again the thread will not launch (I can't see the printf's output on my terminal). Obviously I'm still missing something. How can I launch again my thread after quitting it (or after DoInventory() slot ended its work)?

[MarekR22]

Rename InventoryThread to InventoryWorker or something similar.
You really don't need QThreadEx.


MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);

invThread = new QThread(this);
worker = new InventoryWorker; // parent is forbidden here
worker->moveToThread(invThread);
connect(ui->start,SIGNAL(clicked()),
invThread, SIGNAL(startTimer())); // add slot startTimer in InventoryWorker
invThread->start();
}

MainWindow::~MainWindow() {
worker->deleteLater();
invThread->terminate();
invThread->waitFor(3000);
}

InventoryWorker::InventoryWorker(QObject *parent, int h, QListWidget * lst) : QObject(parent)
{
handle = h;
list = lst;
timer = new QTimer(this);
list->connect(timer,SIGNAL(timeout()),SLOT(clear()));
connect(timer,SIGNAL(timeout()),list,SLOT(clear()) );
timer->setSingleShot(false);
}

InventoryWorker::startTimer() {
timer->start(1500);
}

[Patrik]

How does your code call InventoryWorker:: DoInventory()? To put it simply what I need is:

every time ui->start emits clicked() then run InventoryWorker:: DoInventory() in a different thread

[MarekR22]

This code I gave you is not complete. I wrote it just to show you a pattern. I don't know what you code do I don't need to.

[Patrik]

Ok, after reading it twice I got it. Tank you all, you've been really helpful