Trying to create 2 objects (clients) living in different threads and communicating with 1 object(server) living on the main thread.
However I'm not seeing any activity on the server object.
The main goal of this code is to see how the signal slot will behave when 2 signals coming from 2 different threads are communicating with a single object on the main thread.
objects.cpp:
#include "objects.h"
Objects::Objects(QObject* parent): QObject (parent)
{
}
void Objects::printData(const QString& data)
{
qDebug() << data;
}
void Objects::process(const QString& data)
{
emit sendData(data+":processed");
}
Q_NORETURN void Objects::run()
{
while (true) {
//qDebug() << "hit" << m_rate;
emit sendData("test"+QString::number(m_rate));
QThread::sleep(m_rate);
}
}
objects.h:
#ifndef OBJECTS_H
#define OBJECTS_H
#include <QObject>
#include <QDebug>
#include <QThread>
class Objects: public QObject
{
Q_OBJECT
public:
explicit Objects(QObject* parent = Q_NULLPTR);
signals:
void sendData(const QString&);
public slots:
void printData(const QString&);
void process(const QString&);
void run();
public:
ulong m_rate;
};
#endif // OBJECTS_H
main.cpp:
#include <QCoreApplication>
#include "objects.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
Objects ser;
Objects cl1;
Objects cl2;
cl1.m_rate = 1;
cl2.m_rate = 2;
QThread cl1_t;
QThread cl2_t;
QObject::connect(&cl1_t, &QThread::started, &cl1, &Objects::run);
QObject::connect(&cl2_t, &QThread::started, &cl2, &Objects::run);
QObject::connect(&cl1, &Objects::sendData, &ser, &Objects::process);
QObject::connect(&cl2, &Objects::sendData, &ser, &Objects::process);
QObject::connect(&ser, &Objects::sendData, &cl1, &Objects::printData);
QObject::connect(&ser, &Objects::sendData, &cl2, &Objects::printData);
cl1.moveToThread(&cl1_t);
cl2.moveToThread(&cl2_t);
cl1_t.start();
cl2_t.start();
return a.exec();
}
Looks like your void run() function-member blocks Qt internal event queue. Just add QCoreApplication::processEvents(); in to your void run() and it will solve your problem.
void Objects::run()
{
while (true) {
qDebug() << "hit" << m_rate;
emit sendData("test" + QString::number(m_rate));
QThread::sleep(m_rate);
QCoreApplication::processEvents();
}
}
UPD:
I will suggest you to read THIS wiki article for more detailed explanation on your problem.
Related
I want to display an error message whenever my independent thread encounters the word "alert1" in a specific .txt file. But I get the above error inside the monitorForAlerts() inside mythread.cpp file. The line expectedly executes if I were to place it inside dialog.cpp. So I guess this is due to non-inheritance of this object. Can you please advise me how to solve this error for the given code?
Here is the code:
dialog.h
#ifndef DIALOG_H
#define DIALOG_H
#include <QDialog>
#include <QtCore>
#include "mythread.h"
namespace Ui {
class Dialog;
}
class Dialog : public QDialog
{
Q_OBJECT
public:
explicit Dialog(QWidget *parent = 0);
~Dialog();
public slots:
private:
Ui::Dialog *ui;
private slots:
void on_pushButton_clicked();
void on_pushButton_2_clicked();
};
#endif // DIALOG_H
mythread.h
#ifndef MYTHREAD_H
#define MYTHREAD_H
#include <QThread>
#include <QtCore>
#include <QDebug>
#include <QFile>
#include <Windows.h>
#include <QMessageBox>
#include <QTimer>
#define ALERTS_MESSAGE_STORAGE_PATH "E:\\QT1\\simpleGUIThread2\\simpleGUIThread2\\usbAlert.txt"
#define TIMER_VALUE 500
class MyThread : public QThread
{
Q_OBJECT
public:
explicit MyThread(QObject *parent = 0);
void run();
QString name;
void monitorForAlerts();
int exec();
public slots:
signals:
void testSignal(QString message);
public slots:
};
#endif // MYTHREAD_H
dialog.cpp
#include "dialog.h"
#include "ui_dialog.h"
Dialog::Dialog(QWidget *parent) :
QDialog(parent),
ui(new Ui::Dialog)
{
ui->setupUi(this);
}
Dialog::~Dialog()
{
delete ui;
}
void Dialog::on_pushButton_clicked()
{
ui->label->show();
}
void Dialog::on_pushButton_2_clicked()
{
ui->label->hide();
}
mythread.cpp
#include "mythread.h"
#include "dialog.h"
MyThread::MyThread(QObject *parent) :
QThread(parent)
{
}
void MyThread::run()
{
exec();
}
int MyThread::exec()
{
while(1)
{
monitorForAlerts();
emit(testSignal("hello world!!"));
sleep(1);
}
}
void MyThread::monitorForAlerts()
{
QString response = ALERTS_MESSAGE_STORAGE_PATH;
QFile resp(response);
resp.open(QIODevice::WriteOnly);
resp.close();
QFile resp1(response);
char buf[121];
char buf1[] = "alert1";
char buf2[] = "alert2";
resp1.open(QIODevice::ReadOnly);
while(resp1.size() == 0)
{
Sleep(3000);
}
qint64 lineLength = resp1.readLine(buf, sizeof(buf));
resp1.close();
if(strcmp(buf,buf1) == 0)
{
QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
qDebug()<<"warning 1!!";
QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));
}
if(strcmp(buf,buf2) == 0)
{
QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
qDebug()<<"warning 2!!";
QMessageBox::critical(this,tr("ERROR"),tr("The camera position has been moved or an object is obscuring its view.\nPlease check the device.\n"));
}
}
main.cpp
#include "dialog.h"
#include <QApplication>
#include "mythread.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MyThread mThread1;
mThread1.name = "mThread1";
mThread1.start();
Dialog w;
w.show();
return a.exec();
}
LATEST UPDATE*********************************************************************
Hi Zlatomir,
I choose to take your 1st advice. I have created a signal that the thread will emit and connect it to a slot for QDialog. Please let me know if my understanding is correct, because I do not know where to implement the connect(), since the signal is declared in mythread.h and the slot in dialog.h. The connection type argument for connect is Qt::QueuedConnection, so that gui elements from another thread different than main-thread.
are NOT created. Is this statement correct? and where do I place this?
connect( mThread, SIGNAL(alertSignal(QString)), this, SLOT(alertSlot(QString)), Qt::QueuedConnection);
mythread.h
//....
signals:
void alertSignal(QString message);
//....
dialog.h
//....
public slots:
void alertSlot(QString message);
//....
mythread.cpp
//....
if(strcmp(buf,buf1) == 0)
{
QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
qDebug()<<"warning 1!!";
emit(alertSignal("alert1"));
}
else if(strcmp(buf,buf2) == 0)
{
QFile::remove(ALERTS_MESSAGE_STORAGE_PATH);
qDebug()<<"warning 2!!";
emit(alertSignal("alert2"));
}
dialog.cpp
void Dialog::alertSlot(QString message)
{
if(strcmp(message, "alert1"))
QMessageBox::critical(this,tr("ERROR"),tr("Large change in illumination.\nPlease re-capture reference image.\n"));
else if(strcmp(message, "alert2"))
QMessageBox::critical(this,tr("ERROR"),tr("The camera position has been moved or an object is obscuring its view.\nPlease check the device.\n"));
}
Now if this were correct, how do i implement the connect() and in which file?
The first argument is the problem, in your case this is not a good argument, because there this is a pointer to a MyThread instance, and MyThread is not a QWidget (is not derived from QWidget).
To solve this you can show the QMessageBox::critical from a slot in mainwindow (the Dialog class in your code, there you pass the instance of main-window that is a QWidget) and connect that slot with a signal that you emit from your thread, make sure that the connection type argument for connect is Qt::QueuedConnection, so that you don't try to create gui elements from another thread different than main-thread.
Another option would be to validate the data before you start the second thread and to tell
the user that he needs to provide the right files.
LE: Also check the QThread's documentation for the recommended way to use the class, now it's recommended not to derive from QThread.
LE2 - answer to the update
That connect can be made where ever you can have the two instances that you want to connect, in your case main.cpp is a good place to connect those (don't forget to fully qualify the name for connect: QObject::connect):
//...
MyThread mThread1;
mThread1.name = "mThread1";
mThread1.start();
Dialog w;
QObject::connect( &mThread1, SIGNAL(alertSignal(QString)), &w, SLOT(alertSlot(QString)), Qt::QueuedConnection);
w.show();
//...
I'm sending two (or more) same signals, which receives one slot, but it is only called once instead of two times.. What I'm doing wrong?
main.cpp:
#include <QCoreApplication>
#include "app.h"
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
App app;
QMetaObject::invokeMethod(&app, "run", Qt::QueuedConnection);
return a.exec();
}
app.h:
#ifndef APP_H
#define APP_H
#include <QObject>
#include "tcpserver.h"
#include "tcpsocket.h"
class App : public QObject
{
Q_OBJECT
public:
explicit App(QObject *parent = 0);
signals:
public slots:
void run()
{
qDebug() << "run()";
server.server_start(1111);
socket.connectToHost("127.0.0.1", 1111);
socket.write("hello", 5);
socket.write("olleh", 5); // should execute slot two times.
}
private:
TcpServer server;
TcpSocket socket;
};
#endif // APP_H
TcpSocket.h:
#ifndef TCPSOCKET_H
#define TCPSOCKET_H
#include <QTcpSocket>
class TcpSocket : public QTcpSocket
{
Q_OBJECT
public:
explicit TcpSocket(QObject *parent = 0);
signals:
void dataReady(QByteArray data);
public slots:
void readyRead()
{
qDebug() << "Bytes available:" << this.bytesAvailable(); // called only once.
data = this.readAll(); // just for testing.
emit dataReady(data); //
}
void disconnected();
private:
QByteArray data;
};
#endif // TCPSOCKET_H
as you can see, I'm doing two socket.write functions, which should be handled two readyRead slots, but it is called only once. I honestly don't understand what I'm doing wrong.
Regards.
I derived a class from QTextEdit and use it as a "logbook". I equipped it with a slot to receive log-messages.
class CLogbook : public QTextEdit
{
Q_OBJECT;
public:
void log(QString msg) {append(msg)};
public slots:
void recvLogSignal(const QString message)
{
append("hallo");
std::cout << "signal received.\n";
log(message);
}
};
another class then emits a signal like this:
// in the header
signals:
void logMessage(const QString);
// in the implementation
emit logMessage("qt is cute");
std::cout << "if you can read this the logMessage was emitted\n";
and also i connect the signal to the slot
connect(tableeditor, SIGNAL(logMessage(const QString)), logbook, SLOT(recvLogSignal(const QString)));
However the message is never shown in the "logbook". What am i missing here?
SOLVED: The connect method was called after emitting the signal :-(
It is hard to see exactly what is wrong with your implementation without a full example. Sometimes signals or slots will fail if an object goes out of scope if it isn't initialized on the heap.
Another way that it could fail is if your QApplication hasn't reached the exec() call.
I haven't experimented with using const in signal and slot calls, and I haven't seen it in any examples before, so that could be causing the problem; but it seems to work fine in the example below.
Working Example With a Derived Class of QTextEdit
Here is a simple example I put together that does some basic logging with a push button and a line edit.
Here is the header:
#ifndef WIDGET_H
#define WIDGET_H
#include <QtGui/QWidget>
//#include <QTextEdit>
#include "clogbook.h"
#include <QLineEdit>
class Widget : public QWidget
{
Q_OBJECT
public:
Widget(QWidget *parent = 0);
~Widget() {}
public slots:
void on_pushButton();
void on_lineEditReturn();
private:
CLogBook * text_edit_log;
QLineEdit * line_edit;
};
#endif // WIDGET_H
Here is the source:
#include "widget.h"
#include <QBoxLayout>
#include <QPushButton>
#include <QTimer>
Widget::Widget(QWidget *parent)
: QWidget(parent)
{
QBoxLayout * box_layout = new QBoxLayout(QBoxLayout::TopToBottom);
text_edit_log = new CLogBook;
line_edit = new QLineEdit("Type Here and press Enter.");
QPushButton * push_button = new QPushButton("Click to Add To Log");
text_edit_log->setText("My Log Book");
box_layout->addWidget(text_edit_log);
box_layout->addWidget(line_edit);
box_layout->addWidget(push_button);
this->setLayout(box_layout);
QObject::connect(push_button, SIGNAL(clicked()), this, SLOT(on_pushButton()));
QObject::connect(line_edit, SIGNAL(returnPressed()), this, SLOT(on_lineEditReturn()));
}
void Widget::on_pushButton()
{
// text_edit_log->append("Push Button Logging Test");
text_edit_log->recvLogSignal("Push button test.");
}
void Widget::on_lineEditReturn()
{
// text_edit_log->append(line_edit->text());
text_edit_log->recvLogSignal(QString("LineEdit: ") + line_edit->text() );
line_edit->clear();
}
And here is the main:
#include <QtGui/QApplication>
#include "widget.h"
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
Widget w;
w.show();
return a.exec();
}
And here is the CLogBook class:
#ifndef CLOGBOOK_H
#define CLOGBOOK_H
#include <QTextEdit>
#include <iostream>
class CLogBook : public QTextEdit
{
Q_OBJECT
public:
explicit CLogBook(QWidget *parent = 0) : QTextEdit(parent) { }
void log (QString msg) { append(msg); }
public slots:
void recvLogSignal(const QString message)
{
append("hallo");
std::cout << "signal received.\n" << std::endl;
log(message);
}
};
#endif // CLOGBOOK_H
The following codes will crash ,and even hang the linux, any ideas about it ?
#include <QCoreApplication>
#include <QString>
#include <QMap>
#include <QList>
#include <QDebug>
#include <QThread>
#include <QTest>
long long emited=0;
long long handled=0;
int flag=0;
class A:public QThread
{
Q_OBJECT
public:
A(QString n):n_(n){moveToThread(this);}
void run() {
QMetaObject::invokeMethod(this, "g",Qt::QueuedConnection);
exec();
}
QString n_;
signals:
void as();
public slots:
void g(){
while(1) {
++emited;
emit as();
}
}
};
class Main:public QObject
{
Q_OBJECT
public slots:
void s0(){}
void s1(){
++flag;
++handled;
A *obj = qobject_cast<A*>(sender());
int nothandle=emited-handled;
--flag;
if(obj) {
qDebug()<<"s1"<<obj->n_<<"not handled:"<<nothandle<<flag;
}
}
};
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
QThread th1,th2;
A a1("a1"),a2("a2");
Main m;
QObject::connect(&a1,SIGNAL(as()),&m,SLOT(s1()),Qt::QueuedConnection);
QObject::connect(&a2,SIGNAL(as()),&m,SLOT(s1()),Qt::QueuedConnection);
a1.start();
a2.start();
return a.exec();
}
It crashes because of this:
while(1) {
++emited;
emit as();
}
The Qt signal queue keeps growing but you are not letting Qt process the signals, so it will keep on going until it crashes. Use a QTimer to avoid freezing your application and to let Qt process your signals.
I'm a new bit in Qt...
I have a Qt GUI application (written by me), let's call it QtAPP.exe
When QtAPP.exe running, I will use a QThread and QProcess to execute some external file,
such as player.exe (written in native C).
Here's my question:
In QtAPP.exe, there are 2 classes,
1. QMainWindow - Core of QtAPP.exe
2. QThread - A thread class to execute external things
For now, if I got a finished() signal in that QThread,
how do I to force the QMainWindow to repaint itself ?
Hope somebody can show me some tips, maybe sample code :)
Any suggestion are welcome~
One solution would be to simply connect the finished() signal to a slot in MainWindow whose implementation calls update(). Note that delivery of this signal will be asynchronous because the sender and receiver objects are in different threads.
Here is a working example:
main.cpp
#include <QtGui/QApplication>
#include "stuff.h"
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MainWindow w;
w.show();
return app.exec();
}
stuff.h
#ifndef STUFF_H
#define STUFF_H
#include <QtGui/QMainWindow>
#include <QtCore/QThread>
class QLabel;
class Thread : public QThread
{
Q_OBJECT
public:
Thread(QObject *parent);
void run();
private:
void startWork();
signals:
void workFinished();
};
class MainWindow : public QWidget
{
Q_OBJECT
public:
MainWindow();
public slots:
void startWork();
void workFinished();
private:
QLabel* m_label;
Thread* m_thread;
};
#endif
stuff.cpp
#include <QtCore/QTimer>
#include <QtCore/QMutex>
#include <QtCore/QWaitCondition>
#include <QtGui/QVBoxLayout>
#include <QtGui/QPushButton>
#include <QtGui/QLabel>
#include "stuff.h"
#include <QDebug>
// Global variables used for ITC
QWaitCondition buttonPressed;
QMutex mutex;
Thread::Thread(QObject *parent)
: QThread(parent)
{
}
void Thread::run()
{
qDebug() << "Thread::run" << QThread::currentThreadId();
while (1) {
mutex.lock();
buttonPressed.wait(&mutex);
mutex.unlock();
startWork();
}
}
void Thread::startWork()
{
qDebug() << "Thread::startWork" << QThread::currentThreadId();
// Simulate some long-running task
sleep(3);
// Emit a signal, which will be received in the main thread
emit workFinished();
}
MainWindow::MainWindow()
: m_label(new QLabel(this))
, m_thread(new Thread(this))
{
QPushButton *button = new QPushButton("Start", this);
connect(button, SIGNAL(pressed()), this, SLOT(startWork()));
QVBoxLayout *layout = new QVBoxLayout(this);
layout->addWidget(button);
layout->addWidget(m_label);
setLayout(layout);
// Create connection across thread boundary
connect(m_thread, SIGNAL(workFinished()), this, SLOT(workFinished()));
m_thread->start();
}
void MainWindow::startWork()
{
// Signal the thread to tell it that the button has been pressed
mutex.lock();
m_label->setText("Started");
buttonPressed.wakeAll();
mutex.unlock();
}
void MainWindow::workFinished()
{
qDebug() << "MainWindow::workFinished" << QThread::currentThreadId();
m_label->setText("Finished");
}