I have a sever client application and in the beginning client loads data from server. I have a dialog showing status of getting data from server (has progress bar). But when I call the function the dialog appears with no contents in it with white background and suddenly changes to completed status.
void SystemScreen::loadServerData()
{
qDebug() << Q_FUNC_INFO << "Invoked";
if (NULL != mpDataManagerDlg)
{
qDebug() << Q_FUNC_INFO << "show progres screen";
mpDataManagerDlg->showScreen();
}
loadData();
qDebug() << Q_FUNC_INFO << "Exits";
}
void SystemScreen::loadData()
{
qDebug() << Q_FUNC_INFO << "Invoked";
if (NULL != mpDataManager)
{
mpDataManager->loadDataFromServer();
}
qDebug() << Q_FUNC_INFO << "Exits";
}
I feel that dialog is displayed only after loadData() function is completed. Is there any alternative to do this?
I used a timer to start
QTimer::singleShot(100, this, SLOT(loadData()));
But then I have some trouble in getting data. ie data is empty if I read suddenly.
EDIT:
void DataManagerDialog::setDefault()
{
qDebug() << Q_FUNC_INFO << "Invoked";
setProgressBar(0);
setProgressBarColor(false);
ui->deptFailButton->hide();
ui->deptOkButton->hide();
ui->deptLabel->setStyleSheet("color:gray");
ui->subGroupFailButton->hide();
ui->subGroupOkButton->hide();
ui->subGroupLabel->setStyleSheet("color:gray");
ui->itemFailButton->hide();
ui->itemOkButton->hide();
ui->itemLabel->setStyleSheet("color:gray");
ui->salesBtnFailButton->hide();
ui->salesBtnOkButton->hide();
ui->salesBtnLabel->setStyleSheet("color:gray");
qDebug() << Q_FUNC_INFO << "Exits";
}
void DataManagerDialog::alignCenter()
{
qDebug() << Q_FUNC_INFO << "Invoked";
QWidget *par = parentWidget();
if (par)
{
int x = width()/2;
int y = height()/2;
QPoint mid(mapToGlobal(QPoint(x, y)));
int px = par->width()/2;
int py = par->height()/2;
QPoint parMid(mapToGlobal(QPoint(px, py)));
move(parMid.x()-mid.x(), parMid.y()-mid.y());
}
qDebug() << Q_FUNC_INFO << "Exits";
}
void DataManagerDialog::showScreen()
{
setDefault();
alignCenter();
show();
}
You probably do not enter the event loop.
Try to call QCoreApplication::processEvents() from time to time in mpDataManager->loadDataFromServer() to update the GUI.
From the processEvents documentation :
You can call this function occasionally when your program is busy performing a long operation (e.g. copying a file).
Edit added after getting feedback from the comments
A better approach would be to send signals in your loadDataFromServer() method with the status information and have a slot listen to the signal and update the GUI.
Here a prototype illustrating the idea :
void mpDataManagerDlg::loadDataFromServer() {
while(true) {
// do some work
int progress = // some value
emit updateDialogSignal(progress);
}
}
// in your dialog class
public slots:
void DataManagerDialog::updateDialog(int progress) {
// update gui
}
More about signals and slots can be found here.
Related
I'm developing a Qt app which need to read data from weight scale model and can't quite understand how exactly the Bluetooth Low Energy works and how to implement it in Qt.
I have a UC-352BLE weight scale which uses BLE to send data. What I want to achieve is this:
After initial pairing the scale with my Raspberry Pi on which my app is running, when the scale sends data (it does it automatically when you take a measurement), my app receive it. For example I have a blood pressure monitor which uses normal Bluetooth and here it's easy. In my app, I create a QBluetoothServer and call its listen() method. Then when the (already paired) device sends a measurement, it connects with my app automatically, then I create QBluetoothSocket and read the data. But with the scale and it's BLE it seems that you can't do it that way. I tried to follow the Qt documentation on this and right now I just have to manually press the button which connects to the scale when it's sending the data. And every time it connects to the device it discovers it's characteristics and services etc. Don't know if I can do it so the app automatically receives a connection and reads a data when the scale sends a measurement. And even when I try to connects like that sometimes it connects and sometimes don't (I get Unknown error form QLowEnergyController::Error when connecting). Here is what I already have:
Bletest::Bletest(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::Bletest)
{
ui->setupUi(this);
if (localDevice.isValid()) {
localDevice.powerOn();
localDevice.setHostMode(QBluetoothLocalDevice::HostDiscoverable);
connect(&localDevice, &QBluetoothLocalDevice::deviceConnected, this, &Bletest::deviceConnected);
connect(&localDevice, &QBluetoothLocalDevice::deviceDisconnected, this, &Bletest::deviceDisconnected);
connect(&localDevice, &QBluetoothLocalDevice::pairingFinished, this, &Bletest::pairingFinished);
}
discoveryAgent = new QBluetoothDeviceDiscoveryAgent(this);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::deviceDiscovered, this, &Bletest::addDevice);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::finished, this, &Bletest::scanFinished);
connect(discoveryAgent, &QBluetoothDeviceDiscoveryAgent::canceled, this, &Bletest::scanFinished);
discoveryAgent->start(QBluetoothDeviceDiscoveryAgent::LowEnergyMethod);
}
Bletest::~Bletest()
{
delete ui;
}
// Local device slots
void Bletest::deviceConnected(const QBluetoothAddress &address)
{
qDebug() << address.toString() << " connected";
}
void Bletest::deviceDisconnected(const QBluetoothAddress &address)
{
qDebug() << address.toString() << " disconnected";
}
void Bletest::pairingFinished(const QBluetoothAddress &address, QBluetoothLocalDevice::Pairing pairing)
{
}
// Agent slots
void Bletest::addDevice(const QBluetoothDeviceInfo &device)
{
if (device.coreConfigurations() & QBluetoothDeviceInfo::LowEnergyCoreConfiguration) {
if (device.name().contains("352")) {
bleDevice = device;
qDebug() << "Found: " + device.name() + "\t" + device.address().toString();
}
}
}
void Bletest::scanFinished()
{
qDebug() << "Devices scan finished";
}
///////////
void Bletest::on_connectButton_clicked()
{
controller = QLowEnergyController::createCentral(bleDevice, this);
connect(controller, &QLowEnergyController::serviceDiscovered, this, &Bletest::serviceDiscovered);
connect(controller, &QLowEnergyController::discoveryFinished, this, &Bletest::serviceScanFinished);
connect(controller, static_cast<void (QLowEnergyController::*)(QLowEnergyController::Error)>(&QLowEnergyController::error),
this, [this](QLowEnergyController::Error error) {
qDebug() << "Cannot connect to device: " + QString::number(error);
});
connect(controller, &QLowEnergyController::connected, this, [this]() {
qDebug() << "Connected to device";
controller->discoverServices();
});
connect(controller, &QLowEnergyController::disconnected, this, [this]() {
qDebug() << "Disconnected";
});
controller->connectToDevice();
}
// Controller slots
void Bletest::serviceDiscovered(const QBluetoothUuid &gatt)
{
qDebug() << "Service discovered: " << gatt.toString();
if (gatt.toString().contains("0000181d-0000-1000-8000-00805f9b34fb")) {
service = controller->createServiceObject(QBluetoothUuid(QBluetoothUuid::WeightScale));
if (service) {
qDebug() << "Found weight scale service";
connect(service, &QLowEnergyService::stateChanged, this, &Bletest::serviceStateChanged);
connect(service, &QLowEnergyService::characteristicChanged, this, &Bletest::updateWeight);
connect(service, &QLowEnergyService::characteristicRead, this, &Bletest::updateWeight);
service->discoverDetails();
}
}
}
void Bletest::serviceScanFinished()
{
qDebug() << "Service scan finished";
}
////////////////////////////
// Service slots
void Bletest::serviceStateChanged(QLowEnergyService::ServiceState newState)
{
if (controller->state() == QLowEnergyController::UnconnectedState)
return;
if (newState == QLowEnergyService::DiscoveringServices) {
qDebug() << "Discovering services state";
} else if (QLowEnergyService::ServiceDiscovered) {
qDebug() << "Service discovered.";
const QLowEnergyCharacteristic weightChar = service->characteristic(QBluetoothUuid(QBluetoothUuid::WeightMeasurement));
if (!weightChar.isValid()) {
qDebug() << "Weight data not found";
return;
}
qDebug() << "Weight data found";
//service->readCharacteristic(weightChar);
desc = weightChar.descriptor(QBluetoothUuid::ClientCharacteristicConfiguration);
if (desc.isValid()) {
qDebug() << "Writing descriptor";
service->writeDescriptor(desc, QByteArray::fromHex("0100"));
}
}
}
void Bletest::updateWeight(const QLowEnergyCharacteristic &c, const QByteArray &value)
{
qDebug() << "Updating weight";
if (c.uuid() != QBluetoothUuid(QBluetoothUuid::WeightMeasurement))
return;
double weight = qFromLittleEndian<qint16>(value.mid(1, 2).data()) * RESOLUTION;
qDebug() << "New weight: " << value.mid(1, 2);
qDebug() << "New weight: " + QString::number(weight, 'f', 2);
}
///////////////////////////
Can someone point me in the right direction with this? Is is even possible with BLE to automatically connects to my app and receive data?
Thanks.
NOTE: C++98
Hi, I'm a little new to c++ and I am writing a databaes program and am attempting to start a timer using the boost::asio package using pthread. The aim of the timer is to start after sql queries have been placed inside a buffer, of which will run an execute function if nothing has been received for a period of time. I have managed to get it to compile, but it doesn't look like the pthread instance is starting.
I have called the pthread inside my getInstance method, and the boost::asio alarm has been set up accordingly. What I will show below is that by calling io_run() directly starts the timer falls into a loop within the alarm.
database.h
void *run_io(void *arg);
class Database
{
private:
static Database *dbInstance; //= NULL;
public:
boost::asio::io_service io_service;
boost::posix_time::millisec interval;
boost::asio::deadline_timer timer;
pthread_t timerThread;
public:
static Database &getInstance()
{
if (!dbInstance)
{
dbInstance = new Database();
// pthread_create(&dbInstance->timerThread,NULL,run_io,&dbInstance->io_service);
std::cout << " INSTANCE CREATED " << std::endl;
pthread_create(&dbInstance->timerThread, NULL, run_io, (void *)&dbInstance->io_service);
// pthread_join(&dbInstance->timerThread, NULL);
}
return *dbInstance;
}
};
database.cpp
Database *Database::dbInstance = NULL;
Database::Database()
: interval(2000), timer(io_service, interval) {}
Database::~Database()
{
sqlite3_close(db);
}
void Database::setAlarm(const boost::system::error_code& /* e */)
{
std::cout << "[TEST] WE ARE IN SET ALARM " << std::endl;
DB_WRITE_TIME = 500;
boost::posix_time::milliseconds interval(DB_WRITE_TIME);
// Reschedule the timer for 1 second in the future:
timer.expires_at(timer.expires_at() + interval);
// Posts the timer event
timer.async_wait(boost::bind(&Database::setAlarm, this, _1));
}
int Database::buffer()
{
// DO BUFFER STUFF
timer.async_wait(boost::bind(&Database::setAlarm, this, _1));
// io_service.run() <-- uncommenting this results in the loop
return rc ;
}
void *run_io(void *arg)
{
boost::asio::io_service *io_service = (boost::asio::io_service *)arg;
io_service->run();
}
So I don't feel like the pthread is even starting. I tried putting a print statement in there to see if it came out, and nothing appeared in my terminal.
---- EDIT ----
I have made changes as per Sehe's advice, however it still does not look like I am able to call the alarm handler (setAlarm()). I had to slightly modify it to be compatible with the whole program, but essentially it is this (I gave the interval time a value of 5000 to give it enough time for the tests):
database.h
class Database
{
private:
static boost::shared_ptr<Database> dbInstance;
private:
typedef boost::asio::io_service io_service;
io_service io;
boost::scoped_ptr<io_service::work> work;
boost::posix_time::millisec interval;
boost::asio::deadline_timer timer;
boost::thread timerThread;
void run_io()
{
std::cout << "ENTER IO THREAD" << std::endl;
io.run();
std::cout << "LEAVE IO THREAD" << std::endl;
}
public:
static Database &getInstance()
{
if (!dbInstance)
{
std::cout << " INSTANCE CREATED " << std::endl;
dbInstance.reset(new Database());
dbInstance->timerThread = boost::thread(boost::bind(&Database::run_io,dbInstance));
}
return *dbInstance;
}
Database(); // <-- default constructor (doesn't take any args)
~Database();
database.cpp
boost::shared_ptr<Database> Database::dbInstance;
static const int DB_WRITE_TIME = 5000;
Database::Database()
: work(new io_service::work(io)), interval(5000), timer(io, interval)
{
// std::cout << " CONSTRUCTED " << std::endl;
}
Database::~Database()
{
// std::cout << " DESTROYED " << std::endl;
// sqlite3_close(db);
}
void Database::setAlarm(const boost::system::error_code& ec)
{
std::cout << "[TEST] WE ARE IN SET ALARM - ec message = " << ec.message() << std::endl;
executeSqlInBuffer(); // once timer expire, call the execute function
if(!ec)
{
boost::posix_time::milliseconds interval(DB_WRITE_TIME);
timer.expires_from_now(interval);
timer.async_wait(boost::bind(&Database::setAlarm, this, _1));
}
}
void Database::teardown()
{
// std::cout << " INSTANCE SHUTTING DOWN " << std::endl;
timer.cancel(); // stop timer loop
work.reset(); // allows io.run() to exit
if(timerThread.joinable())
{
std::cout << " JOINED " << std::endl;
timerThread.join(); // releasing bound of shared_ptr
}
else std::cout << " NOT JOINED " << std::endl;
dbInstance.reset(); // releasing instance
}
int Database::buffer()
{
// do buffering
if(buffer.size() == max_size)
{
executeSqlInBuffer();
}
std::cout << timer.expires_from_now(interval) << std::endl;
// std::cout << " ~ BEFORE TIMER ~ " << std::endl;
timer.async_wait(boost::bind(&Database::setAlarm, this, _1));
return 1;
}
main.cpp
int main()
{
pthread_t thread1; // a few pthreads in main that handle other areas of the program.
pthread_create(&thread1,NULL,thread1Arg,NULL);
pthread_t dbThread; // my pthread for the database
pthread_create(&dbThread,NULL,dbThreadArg,NULL);
Database& database = Database::getInstance();
database.teardown();
pthread_join(thread1,NULL);
pthread_join(dbThread,NULL);
return 0;
}
You can also see here that it enters and leaves the IO thread, and creates an instance, plus the debug output for timer.expires_from_now(interval):
INSTANCE CREATED
JOINED
ENTER IO THREAD
LEAVE IO THREAD
...
...
0 ---> first cycle
1 ---> second cycle
...
1 ---> nth cycle
I'm very ccnfused why anyone who uses Boost or C++11 (or both...) would ever use raw pthread threads (see e.g. C++ boost asynchronous timer to run in parallel with program for a good juxtaposition).
The real problem is likely that you have io_service running out of work (see e.g. https://www.boost.org/doc/libs/1_57_0/doc/html/boost_asio/reference/io_service__work.html).
If you have no pending async operations the thread just exits.
Another problem is accuracy issues with
timer.expires_at(timer.expires_at() + interval);
It's possible that some handlers take so much time that by the time you schedule your next alarm, the deadline has already expired. It's probably better to use
timer.expires_from_now(interval);
Note this also matches the comment better. The comment suffers from comment already because it says "1 second" but it is actually some defined constant DB_WRITE_TIME
or separate your timer from the other handlers in some other way to guarantee accurate scheduling.
Finally, you had UB due to the absense of any shutdown. The static instance never gets destroyed, but what's worth the non-detached thread never is joined, creating undefined behaviour at shutdown.
This problem is actually almost identical to the one recently discussed here, where I also explains the way work guards work in more detail: asio::io_service is ending immediately with work
Here's a c++11 rewrite with the necessary fix:
Since I now noticed you're that person stuck in c++03 land for some weird reason, a Boost Thread version:
C++03 DEMO/Boost Thread
Live On Coliru
#include <boost/asio.hpp>
#include <boost/make_shared.hpp>
#include <boost/scoped_ptr.hpp>
#include <boost/thread.hpp>
#include <iostream>
static const int DB_WRITE_TIME = 500;
class Database
{
private:
static boost::shared_ptr<Database> dbInstance;
Database()
: work(new io_service::work(io)),
interval(750),
timer(io, interval)
{
std::cout << "INSTANCE CREATED" << std::endl;
}
void on_timer_completed(const boost::system::error_code& ec) {
std::cout << "[on_timer_completed] " << ec.message() << std::endl;
if (!ec) {
boost::posix_time::milliseconds interval(DB_WRITE_TIME);
// Reschedule the timer
timer.expires_from_now(interval);
timer.async_wait(boost::bind(&Database::on_timer_completed, this, _1));
}
}
int buffer()
{
// DO BUFFER STUFF
timer.expires_from_now(interval);
timer.async_wait(boost::bind(&Database::on_timer_completed, this, _1));
// io_service.run() <-- uncommenting this results in the loop
return 1; // rc ;
}
public:
void do_stuff() {
buffer(); // whatever it does
}
void teardown() {
std::cout << "INSTANCE SHUTTING DOWN\n";
timer.cancel(); // stop timer loop
work.reset(); // allows io.run() to exit
if (timerThread.joinable()) {
timerThread.join(); // releasing the bound shared_ptr
}
dbInstance.reset(); // releasing the instance
}
~Database() {
//sqlite3_close(db);
std::cout << "INSTANCE DESTROYED\n";
}
private:
typedef boost::asio::io_service io_service;
io_service io;
boost::scoped_ptr<io_service::work> work;
boost::posix_time::millisec interval;
boost::asio::deadline_timer timer;
boost::thread timerThread;
void run_io() {
std::cout << "ENTER IO THREAD" << std::endl;
io.run();
std::cout << "LEAVE IO THREAD" << std::endl;
}
public:
static Database &getInstance()
{
if (!dbInstance)
{
dbInstance.reset(new Database());
dbInstance->timerThread =
boost::thread(boost::bind(&Database::run_io, dbInstance));
}
return *dbInstance;
}
};
boost::shared_ptr<Database> Database::dbInstance;
int main() {
Database& db = Database::getInstance();
boost::this_thread::sleep_for(boost::chrono::seconds(1));
db.do_stuff();
boost::this_thread::sleep_for(boost::chrono::seconds(3));
// ....
db.teardown();
}
Prints
INSTANCE CREATED
ENTER IO THREAD
[on_timer_completed] Success
[on_timer_completed] Success
[on_timer_completed] Success
[on_timer_completed] Success
[on_timer_completed] Success
INSTANCE SHUTTING DOWN
[on_timer_completed] Operation canceled
LEAVE IO THREAD
INSTANCE DESTROYED
I am working on test application for printing in Ubuntu 14.04.3 LTS with CUPS 1.7.2 and Qt 5.5.1 Opensource 64bit. I've added printer with its driver in CUPS and test page is printed ok. Now, my app consists of QTextEdit and QPushButton, whose pressed() signal triggers following slot:
void UeCentralWidget::ueSendTextToPrinter()
{
// QStringList availablePrintersNames=QPrinterInfo::availablePrinterNames();
// for(int indexPrinterName=0; indexPrinterName<availablePrintersNames.size(); indexPrinterName++)
// {
// qDebug() << Q_FUNC_INFO
// << availablePrintersNames.at(indexPrinterName);
// } // for
qDebug() << Q_FUNC_INFO
<< "Text to print: "
<< this->uePrinterTextEditor()->document()->toPlainText();
QList<QPrinterInfo> availablePrinters=QPrinterInfo::availablePrinters();
for(int indexPrinter=0; indexPrinter<availablePrinters.size(); indexPrinter++)
{
// qDebug() << Q_FUNC_INFO
// << availablePrinters.at(indexPrinter).makeAndModel();
if(availablePrinters.at(indexPrinter).makeAndModel().contains("POS58"))
{
qDebug() << Q_FUNC_INFO
<< "Printer found";
QPrinter printer(availablePrinters.at(indexPrinter));
qDebug() << Q_FUNC_INFO
<< printer.pageLayout().fullRect();
QPainter printerPainter(&printer);
qDebug() << Q_FUNC_INFO
<< printerPainter.fontInfo().family()
<< printerPainter.fontInfo().pixelSize()
<< printerPainter.fontInfo().pointSize();
if(printerPainter.begin(&printer))
{
printerPainter.drawText(100,
100,
this->uePrinterTextEditor()->document()->toPlainText());
}
printerPainter.end();
} // if
} // for
}
When this slot executes, printer job is added to CUPS queue, which means printer is found, however, printer just feeds printer and no text is printed. What am I still missing?
You don't need the printerPainter.begin(&printer) or the printerPainter.end().
I don't know why that fixes it for you, but if I was to guess it might be because explicitly calling printerPainter.end() prematurely completes the paint operation before the drawing is actually done.
The following code is from an example shows how to use QNetworkAccessManager to download things.
void Downloader::replyFinished (QNetworkReply *reply)
{
if(reply->error())
{
qDebug() << "ERROR!";
qDebug() << reply->errorString();
}
else
{
qDebug() << reply->header(QNetworkRequest::ContentTypeHeader).toString();
qDebug() << reply->header(QNetworkRequest::LastModifiedHeader).toDateTime().toString();
qDebug() << reply->header(QNetworkRequest::ContentLengthHeader).toULongLong();
qDebug() << reply->attribute(QNetworkRequest::HttpStatusCodeAttribute).toInt();
qDebug() << reply->attribute(QNetworkRequest::HttpReasonPhraseAttribute).toString();
QFile *file = new QFile("C:/Qt/Dummy/downloaded.txt");
if(file->open(QFile::Append))
{
file->write(reply->readAll());
file->flush();
file->close();
}
delete file;
}
reply->deleteLater();
}
My question is do we have to call reply->deleteLater(); here? If we don't call it, when we perform QNetworkAccessManager::get() call the second time, will the QNetworkReply* in the slot be the same QNetworkReply* ?
If you don't call deleteLater(), the QNetworkReply object will be leaked and its memory not freed. A second get() call will create a new QNetworkReply object.
My OpenGL|ES 2.0 glClear command freezes until the window state changes (eg. the window gets hidden or shown).
The target platform is ARM7 with a Mali 400 GPU.
All code is mostly copied from the Qt OpenGL ES Cube example.
What am I forgetting?
Leon
Source:
#include "streamplayer.h"
#include <QtOpenGL>
#include <QGLFunctions>
StreamPlayer::StreamPlayer(QWidget *parent) :
QGLWidget(QGLFormat(QGL::SampleBuffers), parent)
{
program = new QGLShaderProgram();
}
StreamPlayer::~StreamPlayer()
{
}
void StreamPlayer::initializeGL()
{
qDebug() << "Initializing GL";
initShaders();
glClearColor(0.5f, 0.5f, 0.7f, 1.0f);
return;
}
void StreamPlayer::paintGL()
{
qDebug() << "Paint GL";
qDebug() << "Clearing buffers";
glClear(GL_COLOR_BUFFER_BIT);
qDebug() << "Never comes here until a window state change";
}
void StreamPlayer::resizeGL(int width, int height)
{
qDebug() << "Resizing GL to " << width << "x" << height;
glViewport(0, 0, width, height);
qDebug() << "Done resizing";
}
void StreamPlayer::initShaders()
{
qDebug() << "Initializing shaders";
setlocale(LC_NUMERIC, "C");
if(!program->addShaderFromSourceFile(QGLShader::Vertex, ":/shaders/vshader.glsl")) {
qDebug() << "Failed to create vertex shader";
}
if(!program->addShaderFromSourceFile(QGLShader::Fragment, ":/shaders/fshader.glsl")) {
qDebug() << "Failed to create fragment shader";
}
if(!program->link()) {
qDebug() << "Failed to link";
}
_gl_vertex = program->attributeLocation("vertex");
_gl_texCoord = program->attributeLocation("texCoord");
_gl_matrix = program->attributeLocation("matrix");
_gl_texture = program->attributeLocation("tex");
if(!program->bind()) {
qDebug() << "Failed to bind";
}
setlocale(LC_ALL, "");
qDebug() << "Shaders ready";
}
Have you kept the timer asking for frame updates ? It is this timer that is asking for openGL redraw by calling updateGL() on the glwidget, that ask for a (delayed) paintGL(). Otherwise paintGL will only be called when Qt estimates necessary (for example window shown).
QTimer *timer = new QTimer(this);
timer->setInterval(10);
QObject::connect(timer, SIGNAL(timeout()), glwidget, SLOT(updateGL()));
//And at the end of MainWindow initialization
timer->start();
See this SO thread for related question.