A quick overview of what's happening: I am trying to do a GET request using Qt's QNetworkAccessManager, but the callback function on my QObject::connect(..) function is not being called. My questions is can I call QObject::connect from one object, but connect to a slot of another object (given that I have a pointer to both the object and the slot) - See below for more details.
My ultimate goal is to POST the data (seeing as it's a login function), I had POST Request code that was ultimately suffering from the same issue - callback function not being called. So I would like to be able to do a simple GET request first, once I have that, I think I'll be fine on my own from there.
I currently have a QMainWindow LoginWindow, with a button that calls a slot doLogin() in the LoginWindow class. This all works as you would expect. LoginWindow also has a public slots function called loginResponse(QNetworkReply* response).
//---LoginWindow.h
...
public slots:
void doLogin();
void loginResponse(QNetworkReply* response)
...
//---LoginWindow.cpp
LoginWindow::LoginWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::LoginWindow)
{
ui->setupUi(this);
ui->username_le->setFocus();
}
void LoginWindow::doLogin()
{
MyProduct::Network network(this);
qDebug() << "Logging in...";
//Here I call network.login from LoginWindow and pass
//references to the Slot I want to use and the LoginWindow itself
network.login(
ui->username_le->text(), //username
ui->password_le->text(), //password
this, //reference to this object (LoginWindow*)
SLOT(loginResponse(QNetworkReply*)) //loginResponse slot
);
}
void LoginWindow::loginResponse(QNetworkReply* response)
{
qDebug() << "Log in complete";
}
Next I have another class, under the MyProduct namespace, called Network. As you can see above, Network has a function called login. Here it is:
void MyProduct::Network login(QString username, QString password, QObject *receiver, const char *slot)
{
QNetworkRequest request(QUrl(API_ROOT + LOGIN_PATH)); //"http://localhost/basic/login.php"
request.setHeader(QNetworkRequest::ContentTypeHeader,"application/x-www-form-urlencoded");
//nam = QNetworkAccessManager* declared in the constructor
QObject::connect(nam,SIGNAL(finished(QNetworkReply*)), receiver, slot);
qDebug() << "Posting login data...";
nam->get(request);
}
The goal here is to create a login function in my Network class that can be used and connected in any number of windows (as users may log in from multiple places). But I'm getting no response - LoginWindow::loginResponse is not run.
I see "Logging in..." and "Posting login data" output in the console, but not "Log in complete".
Can anyone please point me in the right direction or tell me I'm crazy or that this is a bad idea?
Thanks in advance!
Note that QNetworkAccessManager operates asynchronously. The get() method does not block while the network operation occurs; it returns immediately. (See the Detailed Description section of the documentation for more info.)
This is pretty typical of Qt's network-related APIs, because you usually don't want your application to freeze while waiting for data to move across a network.
What this means is what your instance, nam, isn't alive long enough for the GET request to actually finish. Your instance of the Product::Network class is deleted immediately after the call to login() because it's allocated on the stack. Although I can't see the code, I'm guessing it cleans up the QNetworkAccessManager as well.
If you extend the lifetime of your network object, you may find that your slot will eventually be invoked.
Also, this is more a matter of preference, but I think it would be cleaner to avoid passing a receiver and a slot to your login() function. I'd recommend declaring your own signals in the Network class as part of its API, and to connect to those in the LoginWindow class.
Related
I am trying to setup an infinite loop in another thread. The purpose is to download some data from an URL, send the data to the main thread and sleep for some seconds:
DataFetcher::DataFetcher(QUrl url, int fetchRateSec) :
url {url},
fetchRateSec {fetchRateSec}
{
}
void DataFetcher::run()
{
QNetworkAccessManager* manager = new QNetworkAccessManager();
QObject::connect(manager, &QNetworkAccessManager::finished, this, &DataFetcher::onReply);
while (true) {
QNetworkRequest req;
req.setUrl(url);
manager->get(req);
qDebug() << "run";
sleep(fetchRateSec);
}
}
void DataFetcher::onReply(QNetworkReply* reply)
{
qDebug() << "repl";
emit fetched(reply->readAll());
}
class DataFetcher : public QThread
{
Q_OBJECT
public:
DataFetcher(QUrl, int);
void run() override;
signals:
void fetched(QString);
private:
const QUrl url;
const int fetchRateSec;
private slots:
void onReply(QNetworkReply*);
};
But onReply is never called.
It is interesting because the qDebug in the while loop is executed as it should.
Im a bit nooby with QT so i might have missed something regarding how to connect the slots / signals but i think i got it right following some other examples.
It looks about right from the accepted answer in How do I write a Qt HTTP GET request?
what could be the problem here?
Per the discussion in the comments, it seems like the whole business of threading is not needed.
One of Qt's strongest attributes is that it is event-driven. You generally create a program by describing what you want to happen (slots) in response to certain events (signals). Explicitly waiting or sleeping is very rare in Qt-based applications (usually only testing), and is generally considered a no-no in event-driven development.
For your specific problem, a solution might look like this. You can create a QTimer in the main thread and connect its timeout signal to a function to make your HTTP request. You can then connect a slot to the QNetworkAccessManager::finished signal, which will run when your response completes. All of this can take place in the main thread, relying on the thread's event loop to manage the callbacks. No need to manage a separate thread yourself, and no looping, sleeping, blocking, or anything like that.
I have this code:
QNetworkAccessManager man;
QNetworkRequest req(QUrl("URL"));
QString ua("HttpRequestDemo/0.1 (Win64) Qt/5.14.0");
req.setHeader(QNetworkRequest::UserAgentHeader, QVariant(ua));
QNetworkReply* reply = man.get(req);
QObject::connect(reply){
QByteArray read = reply->readLine();
QFile out("file.txt");
out.open(QIODevice::WriteOnly|QIODevice::Text);
out.write(read);
out.close();
})
This works on the main.cpp file, using the QCoreApplication, but I want to use the QApplication and download a specific data while pressing a button.
I put the same code on the on_pushButton_clicked() in the mainwindow.cpp file and it didn't even generate the file from the url.
The problem is that man and req go out of scope and are destroyed as soon as your on_pushButton_clicked() function returns, at which point the request probably hasn't even been sent yet.
You need to make sure that these objects outlive the current scope, either by making them members of the window class, or by allocating them on the heap and setting some QObject (maybe also the window class) as the parent.
The problem is that if you put the same code in a method like X you make QNetworkAccessManager a local variable that will be removed instantly that the connection is asynchronous. The solution is to make QNetworkAccessManager an attribute of the class.
*.h
private:
QNetworkAccessManager man;
*.cpp
void Klass::on_pushButton_clicked(){
QNetworkRequest req(QUrl("URL"));
QString ua("HttpRequestDemo/0.1 (Win64) Qt/5.14.2");
req.setHeader(QNetworkRequest::UserAgentHeader, QVariant(ua));
QNetworkReply* reply = man.get(req);
connect(reply, &QNetworkReply::finished, [&]() {
QByteArray read = reply->readAll();
QFile out("file.txt");
out.open(QIODevice::WriteOnly|QIODevice::Text);
out.write(read);
out.close();
reply->close();
reply->deleteLater();
})
}
If you are planning on potentially queuing very many downloads, I strongly recommend using libcurl in your Qt app. I was using QNetworkAccessManager to down 100+ financial quote files, and it would fail downloading ~ 1/3 of the time, and take a while to download. I switched to libcurl, and after figuring out how to get my crypto root certificates setup for https, it runs much faster, and almost never fails. I run it as a dll.
And yes, you will need to make sure the network manager, whether QNetworkManager or curl, doesn't go out of scope upon exiting the button handler. A more conventional pattern, although not necessarily better, is to either have a pointer to e.g. QNetworkManager in your parent class, and new it, or use a std::unique_ptr and std::make_unique (purportedly safer). Creating large objects on the stack can cause problems (in the old days, dare I say, stack overflows), and so is usually done on the heap. In this case, it's not very big, so it doesn't really matter. Alternatively, a form creating big objects might itself be created on the heap.
Disclaimer: I am relatively new to Qt and any type of programming that revolves around Threads and Networking. I have also adopted a lot of code from Qt Examples, API, and other online examples.
All code can be found on GitHub. This code is relatively as simple as it can get minus striping out GUI. I figure supplying it this way would help as well versus just pasting the code below.
I want to use and believe I need to use Threads as I need multiple clients send a request to the server, the server run some SQL code, then spit out the results back to the client (basically deriving a MySQL Server, but specific to what I am doing). Right now though, I am just working on learning the workings of it all.
With all that being said, as the Title states.. My client can connect to the server, the server sets up the thread, and will receive data (a String) through the readReady. After the data is read in, for right now I am just trying to echo it back to the client. It will do this, but only once. Then it spits out:
QObject: Cannot create children for a parent that is in a different thread.
(Parent is QNativeSocketEngine(0x266cca92ea0), parent's thread is serverThread(0x266cca9ed60), current thread is QThread(0x266cac772e0)
I cannot send any further data to the server unless I have the client reconnect, then after the data is sent, it will do its job but then spit out the same error and cease functioning. I have tried quite a bit of different things, but cannot seem to fix the issue. I even tried setting up a SIGNAL/SLOT for this as suggested in API:
It is important to remember that a QThread instance lives in the old thread that instantiated it, not in the new thread that calls run(). This means that all of QThread's queued slots will execute in the old thread. Thus, a developer who wishes to invoke slots in the new thread must use the worker-object approach; new slots should not be implemented directly into a subclassed QThread.
Anyway, any help would be greatly appreciated! My Code is below..
Server
ServerThread.cpp
// Project
#include "ServerDialog.h"
#include "ServerThread.h"
ServerThread::ServerThread(qintptr _socketDiscriptor, QObject *parent /*= 0*/)
: QThread(parent)
{
socketDiscriptor = _socketDiscriptor;
}
void ServerThread::run()
{
emit threadStarted(socketDiscriptor);
// Start Thread
clientSocket = new QTcpSocket;
// Set SocketDisc
if (!clientSocket->setSocketDescriptor(socketDiscriptor))
{
emit error(clientSocket->error());
return;
}
// Connect Socket and Signal
connect(clientSocket, SIGNAL(readyRead()), this, SLOT(readyRead()));
connect(clientSocket, SIGNAL(disconnected()), this, SLOT(disconnected()));
//// Loop Thread to Stay Alive for Signals and Slots
exec();
}
void ServerThread::readyRead()
{
QDataStream in(clientSocket);
in.setVersion(QDataStream::Qt_5_7);
in.startTransaction();
QString dataReceived;
in >> dataReceived;
if (!in.commitTransaction())
{
emit readyReadError(socketDiscriptor);
return;
}
emit readyReadMessage(socketDiscriptor, dataReceived);
echoData(dataReceived);
}
void ServerThread::disconnected()
{
emit threadStopped(socketDiscriptor);
clientSocket->disconnect();
clientSocket->deleteLater();
this->exit(0);
}
void ServerThread::echoData(QString &data)
{
QByteArray block;
QDataStream out(&block, QIODevice::WriteOnly);
out.setVersion(QDataStream::Qt_5_7);
out << data;
clientSocket->write(block);
}
So in ServerThread.cpp when echoData is called, that is when the error shows up and the Socket ceases functioning.
Any and all help will be appreciated. I know there are a few other posts regarding "Cannot create children for..." in regards to Threads. But I did not find any of them helpful. The one thing that I did find interesting but did not understand was maybe using moveToThread() but a lot of mixed comments on that.
I learn best through code examples along with explanation versus just an explanation or pointer to API. Thank you!
Most of Qt network functions are asynchronous; they do not block the calling thread. There is no need to mess up with threads if you are using QTcpSockets. In fact, creating a thread for every socket is an overkill, since that thread will spend most of its time just waiting for some network operation to finish. Here is how I would implement a single-threaded echo server in Qt:
#include <QtNetwork>
#include <QtCore>
//separate class for the protocol's implementation
class EchoSocket : public QTcpSocket{
Q_OBJECT
public:
explicit EchoSocket(QObject* parent=nullptr):QTcpSocket(parent){
connect(this, &EchoSocket::readyRead, this, &EchoSocket::EchoBack);
connect(this, &EchoSocket::disconnected, this, &EchoSocket::deleteLater);
}
~EchoSocket() = default;
Q_SLOT void EchoBack(){
QByteArray receivedByteArray= readAll();
write(receivedByteArray);
disconnectFromHost();
}
};
class EchoServer : public QTcpServer{
public:
explicit EchoServer(QObject* parent= nullptr):QTcpServer(parent){}
~EchoServer() = default;
//override incomingConnection() and nextPendingConnection()
//to make them deal with EchoSockets instead of QTcpSockets
void incomingConnection(qintptr socketDescriptor){
EchoSocket* socket= new EchoSocket(this);
socket->setSocketDescriptor(socketDescriptor);
addPendingConnection(qobject_cast<QTcpSocket*>(socket));
}
EchoSocket* nextPendingConnection(){
QTcpSocket* ts= QTcpServer::nextPendingConnection();
return qobject_cast<EchoSocket*>(ts);
}
};
int main(int argc, char* argv[]){
QCoreApplication a(argc, argv);
EchoServer echoServer;
echoServer.listen(QHostAddress::Any, 9999);
QObject::connect(&echoServer, &EchoServer::newConnection, [&](){
EchoSocket* socket= echoServer.nextPendingConnection();
qDebug() << "Got new connection from: " << socket->peerAddress().toString();
});
return a.exec();
}
#include "main.moc"
Notes:
This server has the ability to handle more than one client at the same time, since there is no blocking. The thread will just respond to the event that happens with the appropriate action; So, if that event was a new connection, it will create a new EchoSocket object to handle it and prints a statement out to qDebug(), and if that event was receiving something on a previously created socket, the same thread will echo received data back and close the connection. It will never block on a single connection waiting for data to arrive nor it will block waiting for a new connection to arrive.
Since you mention using some SQL queries in response for some connections later in your project. Please avoid threading since an SQL database connection in Qt can be used only from the thread that created it, see docs here. So, You'll have to either create a new database connection for each thread (and thus for each connection) in your application (and this is beyond just overkill), or switch later to a single threaded design.
In this section, I am explaining why threading does not work for you the way you are doing it:
You should not be declaring slots in your QThread subclass, Instead, use worker QObjects and move them to QThreads as needed.
The quote you have provided in your question is the exact explanation for why you get this warning. The ServerThread instance you create will be living in the main thread (or whatever thread that created it). Now let's consider this line from your code:
connect(clientSocket, SIGNAL(readyRead()), this, SLOT(readyRead()));
The signal readyRead() will be emitted from the current ServerThread instance (since the clientSocket object that emits it lives there), However, the receiver object is the current ServerThread instance, But that lives in the main thread. Here is what the documentation says:
If the receiver lives in the thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used.
Now, the main point of Qt::QueuedConnection is executing the slot in the receiver object's thread. This means that, your slots ServerThread::readyRead() and ServerThread::disconnected will get executed in the main thread. This is most likely not what you meant to do, since you'll end up accessing clientSocket from the main thread. After that, any call on clientSocket that results in child QObjects being created will result in the warning you get (you can see that QTcpSocket::write() does this here).
Mixed comments of movetothread are linked mostly to usage of it to move thread object to itself.
The quote hints that the members of QThread aren't designed to be called from worker. Strictly proper way to call signal would be by using worker object model, that was shown in Qt examples and explained a few times on QT-related blogs:
class Worker : public QObject
{
Q_OBJECT
private slots:
void onTimeout()
{
qDebug()<<"Worker::onTimeout get called from?: "<<QThread::currentThreadId();
}
};
class Thread : public QThread
{
Q_OBJECT
private:
void run()
{
qDebug()<<"From work thread: "<<currentThreadId();
QTimer timer;
Worker worker;
connect(&timer, SIGNAL(timeout()), &worker, SLOT(onTimeout()));
timer.start(1000);
exec();
}
};
worker constructed inside run() is "property" of the thread it created, so figuratively speaking, it is slaved to its context. The same effect maybe achieved if you create worker in other thread, then move it to this thread before connection was made. When you connect signal to slot of the QThread itself, you connect child thread to thread it was created by.
Use of
connect(&timer, SIGNAL(timeout()), this, SLOT(onTimeout()), Qt::DirectConnection);
or creating connection from your thread sometimes seems to achieve proper result, but not in this case, where you try use objects constructed in different threads together. Calling moveToThread(this) in constructor is a thing not recommended to do.
I need to update the content of a field on my QWidget via a JSON file (updated in real time). I've read about functions readLine() and readAll() of QFile, but when I try a loop like :
while(true):
jsfile.readLine()
creation of objects, update of values, display etc ...
I lost the focus on my window. But I want to keep the control of the application with my buttons and obviously to watch the evolution of the JSON values.
I have thought that Qt manages itself the events and keeps the focus on the current window, but like I've said, it's not the case.
Is there a good solution (multi threads maybe) to use my window while the application reads the file (with new informations in real time)?
(With the constraint "real time" I can't read the whole file every time and I've no choice about the format of this file)
Update
I tried the thread method.
So, I choose to create my thread instance into the main (with my main window) and connect here. But, when I run the program, I've this error :
no matching member function for call to 'connect'
Reader reader;
QObject::connect(controler, SIGNAL(ready()),
reader, SLOT(received()));
According to this error, I've thought that the reason was main don't inherits of Object, and so, I've move the connection ans the creation of thread instance into my main window.
Reader reader;
QObject::connect(reader, SIGNAL(newobject(QJsonObject)),
this, SLOT(displayJSON(QJsonObject)));
With this one, I've the same error while I've already connect lot of widget into this class without any error.
What can be the problem ?
Update 2
I've a solution when I give as argument my main window (controler) in reader's constructor and connect into this one but, if possible, I would an explanation for the previous problem.
The current problem that I have is that signals are emit well but slots are executed after the end the application (so after the end of the thread's execution and not during)
This isn't really the subject of this topic so we can close this one.
You can use QThread (Qt documentation: QThread) class to create a thread, which will read your file. The main thread will execute your GUI application and it will be available during file reading.
You can find a simple example in documentation for creating your thread:
class WorkerThread : public QThread
{
Q_OBJECT
void run() Q_DECL_OVERRIDE {
QString result;
/* ... here is the expensive or blocking operation ... */
emit resultReady(result);
}
signals:
void resultReady(const QString &s);
};
void MyObject::startWorkInAThread()
{
WorkerThread *workerThread = new WorkerThread(this);
connect(workerThread, &WorkerThread::resultReady, this, &MyObject::handleResults);
connect(workerThread, &WorkerThread::finished, workerThread, &QObject::deleteLater);
workerThread->start();
}
You can modify this example for your purpose. For example, WorkerThread for your task may be something like this:
class WorkerThread : public QThread
{
Q_OBJECT
void run() Q_DECL_OVERRIDE {
while(!stopFlag)
{
// read JSON file to QByteArray. Use QFile and QTextStream
// use QJsonDocument to read JSON content
// find what is new in JSON
emit signalSomethingNew(/*parameters*/);
QThread::currentThread()->msleep(/*timeout*/);
}
}
signals:
void signalSomethingNew(/*parameters*/);
};
At the end you must implement slot on your QWidget for signalSomethingNew(/*parameters*/) and make connection:
connect(yourThread, &WorkingThread::signalSomethingNew, youWidget, &YouWidget::yourSlot);
For working with JSON data: QJsonDocument
I'm interpreting your question as "my application is unresponsive whilst doing work" rather than "my focus jumped to another window" - please comment if you meant something different.
You have a choice of options:
Create and run a background QThread to do the work. Have it emit signals (connected to your widgets using Qt::QueuedConnection - the default) when it has results to display.
This is a good solution when the worker has a lot of computation to do, or needs all the input to be read before it can start. It works very well when the target system has processors available with no other work to do.
Use a QSocketNotifier to signal your GUI thread when some of the input becomes available (note that the name is misleading - it actually works on all kinds of file descriptor, not just sockets).
This is appropriate when the algorithm is simple and incremental - i.e. if a small chunk of input can be read and processed quickly.
Incorporate periodic calls to processEvents() in your algorithm:
auto *const dispatcher = QThread::currentThread()->eventDispatcher;
while (line = json.readLine()) {
doSomethingWith(line);
if (dispatcher)
dispatcher->processEvents();
}
This won't work unless you can modify the algorithm like this - if the loop is in somebody else's (closed) code, then you'll need one of the other solutions.
EDIT: Not related to signals/slots/connect. Problem was constructor calling constructor.
There might be a better way to do this - I'd be interested in hearing those...
I have MyClass that is derived from a QLabel. I want to pass more data about the derived class back in the signal than what the base signal does. So I made a slot to intercept the customContextMenuRequested signal and emit a revised one that has more data.
When I try to connect up this signal in the constructor, then my slot never gets called. But if I move the Policy and connect lines out to the parent widget(not class hierarchy parent) so they execute after MyClass is fully constructed, then my slot will get called. But I always want that to be connected for this class and it seems like something I would want in it's constructor rather than counting on the parent class to remember to do it.
Is there something I'm doing wrong? Or a better way to add data to a signal?
MyClass::MyClass() : QLabel()
{
QFont currFont = font();
currFont.setPointSize(15);
setFont(currFont);
setBackgroundRole(QPalette::Mid);
std::cout << "connecting customContextMenuRequested" << std::endl;
/** PROBLEM START */
setContextMenuPolicy(Qt::CustomContextMenu);
// Is there anything wrong with connecting from "this" to "this" in a constructor?
QObject::connect(this, SIGNAL(customContextMenuRequested(const QPoint&)),
this, SLOT(addCellDataToMenuContextRequest(const QPoint&)));
/* PROBLEM END **/
}
MyClass::MyClass(QString &cellString, int row, int col)
: QLabel(cellString)
{
MyClass();
setRow(row);
setCol(col);
}
// This one is a slot
void MyClass::addCellDataToMenuContextRequest(const QPoint& pos)
{
// This doesn't get printed if I connect in my constructor,
// but it does print if I do the same connect from a parent widget.
std::cout << "called addCellDataToMenuContextRequest" << std::endl;
emit customContextMenuRequestedForCell(pos, _row, _col);
}
So I would like the parent widget to just look for customContextMenuRequestedForCell but right now, the parent widget seems to need to be responsible for customContextMenuRequested as well.
Actually, you CAN call (sort of) another constructor if you are using C++11. It's called delegating constructor. But I don't think that will make the problem go away. Your issue seems to be that meta object is not fully constructed when connect() is called. Also, you'll probably need to move to Qt 5 for C++11 to work.
The solution is to delay the connection until the object is fully constructed. You can start a timer with an interval of zero. It will trigger on the next event loop processing which will certainly be after your object is fully constructed.
Then in your timerEvent, make the connection and kill the timer.
EDIT: Didn't see your edit. Looks like you find the solution. Ignore this then. :)
BTW. You didn't call another constructor. You created a temporary MyClass object.
A way to make this "cleaner" would be to reimplement QWidget::mouseReleaseEvent() within MyClass. The way to implement it would be, if the QMouseEvent type passed to mouseReleaseEvent is not a right-click mouse release, call QLabel::mouseReleaseEvent(event). If it is a right-click mouse release event, you can emit your custom signal. This gives the benefit of using the existing mouse button release handling code given by QLabel/QWidget, while allowing you to intercept the one case where you want to emit the custom signal.
EDIT Oh, and be sure to call event->accept() after your mouseReleaseEvent handles the custom case.