I have created a QThread class to run a function that is in another class, but this another class has a pointer to a QWidget (QwtPlot), and I am receiving this message in the application output:
QCoreApplication::sendPostedEvents: Cannot send posted events for objects in another thread
I already read in another topics that QThreads doesn't work with QWidgets (the UI widgets must be in the main thread), but the output in my application seems to be correct.
Can anyone explain to me why this message appears? And what can happen if I let the code as is?
Note: sorry, I can't post the code.
Thanks in advance
I already read in another topics that QThreads doesn't work with QWidgets [...]
but the output in my application seems to be correct.
It's not correct, otherwise you wouldn't ask, right?
A QWidget must be in the main thread. And most likely it is. But you're invoking its methods from another thread, and the methods you invoke are not thread safe. Don't do that. There are other ways of invoking methods safely across threads. Use them instead. For example, assuming that you wish to call QWidget::resize, you could use postToThread from this answer:
QWidget* widget;
QSize size;
Q_ASSERT_X(widget->thread() == qApp->thread(), "widget",
"The widget must live in the main thread.");
postToThread([=]{ widget->resize(size); }, widget);
If you want to be more verbose, or have to maintain a Qt 4 code base, you could do this instead:
class TSWidgetAdapter : public QObject {
Q_OBJECT
QWidget * widget() const { return qobject_cast<QWidget*>(parent()); }
Q_SLOT void resize_d(const QSize & size) { widget()->resize(size); }
public:
explicit TSWidgetAdapter(QWidget * parent) : QObject(parent) {
Q_ASSERT_X(parent->thread() == qApp->thread(), "TSWidgetAdapter()",
"The widget must live in the main thread.");
connect(this, SIGNAL(resize(QSize)), this, SLOT(resize_d(QSize)));
}
Q_SIGNAL void resize(const QSize & size);
};
QWidget* widget;
QSize size;
TSWidgetAdapter widget_ts(widget);
widget_ts.resize(size);
The _d slots are called in the thread of the widget. That's the beauty of automatic connections: you can call the signal in any thread, and the slot will be called in the thread of the target object only. Since the adapter is a child of the widget, it is in the widget's thread - that's enforced by Qt.
Related
the runnable project is here:
enter link description here
I sincerely glad to have your detail answers to solve this, but I am still confusing on this issue:
case 1: changing socket_session as a member variable of mainwindow
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
SocketThread* socket_session;
private:
...
But this is not the solution to access setFlag, even after I change the `Form1::on_qpushButton__set_white_level_0_clicked()' function like this:
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
socket_session->setThreadFlag(true);
}
Still it doesn't make sense because form1 instance doesn't have "the" instance of socket_thread which has been instantiated from mainwindow.
There's a solution I think is making another class that includes all instances that I want to use from inside of mainwindow but I don't think that is a good one because I am using thread and accessing a global big instance class that includes all of them to be "shared" is not a good idea for someone like me.
#include <form1.h>
#include <ui_form1.h>
#include "socketthread.h"
Form1::Form1(QWidget *parent) :
QWidget(parent),
ui(new Ui::Form1) {
ui->setupUi(this);
}
Form1::~Form1() {
delete ui;
}
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
socket_session->setThreadFlag(true);
}
enter image description here
I know I am lack of understanding about this but, do I wanna make something nobody does...? I think everyone wants to separate all objects and their methods clearly and communicate via signals or calling functions from delivered object instances...
case 2: ... let me try how you suggested make possible first...
I can read C++ code and overall structure, but I don't know why I have to struggle with this, so please help me, dear Guru.
On socketthread.h :
class SocketThread : public QThread {
Q_OBJECT
public:
QTcpSocket *socket_session;
SocketThread();
~SocketThread(){}
bool connectToServer(QString, int);
void sendData(const char*, int, int);
void run(void);
private:
QString message;
volatile bool threadFlag;
signals:
void changedThreadFlag(void);
void changedMessageStr(void);
void setThreadFlag(bool);
void setMessageStr(QString);
private slots:
void setStr(QString);
void setFlag(bool);
void socketError(QAbstractSocket::SocketError);
};
And its implementation is...
SocketThread::SocketThread() {
socket_session = NULL;
threadFlag = false;
message = "NULL";
connect(this, SIGNAL(setThreadFlag(bool)), this, SLOT(setFlag(bool)));
}
...
void SocketThread::setStr(QString str) {
message = str;
}
void SocketThread::setFlag(bool flag) {
threadFlag = flag;
}
void SocketThread::run() {
while(true) {
if(threadFlag) {
QThread::msleep(100);
qDebug() << message;
} else
break;
}
qDebug() << "loop ended";
}
And I have one form which has a button, and I put a clicked() slot of it like this...
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
--how can I emit the signal of the one of socketthread from here??
}
Now, the mainwindow is like this:
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow) {
QString addr_server = "223.194.32.106";
int port = 11000;
SocketThread* socket_session = new SocketThread();
socket_session->connectToServer(addr_server, port);
ui->setupUi(this);
Form1* form1;
form1 = new Form1();
ui->stackedWidget_mainwindow->addWidget(form1);
ui->stackedWidget_mainwindow->setCurrentWidget(form1);
socket_session->run();
...
I just simply want to emit the signal setThreadFlag of the socketthread from inside of QPushbutton_clicked() slot.
Once the socket_session->run() started, I need to change the threadFlag by clicking the button by emitting setThreadFlag() of one's from the running thread. And I just stuck in here.
Does it possible even?
Or am I doing this all wrong from the beginning?
As mentioned in this post:
"Emitting a signal" == "calling a function"
So all you really have to do is call the signal function, and all connected slots should be called.
This of course means that the Form1 object needs a pointer to the thread object, i.e. it needs a copy of socket_session. Then you can simply call the signal on the object
socket_session->setThreadFlag(your_flag);
Of course, if the Form1 have a copy of the socket_session pointer, it might as well call setFlag directly, if it was public.
I just simply want to emit the signal setThreadFlag of the socketthread from inside of QPushbutton_clicked() slot.
No signal is needed – just call the function.
void Form1::on_qpushButton__set_white_level_0_clicked() {
qDebug() <<"clicked()";
// --how can I emit the signal of the one of socketthread from here??
// E.g. this way:
socket_session->setThreadFlag(true);
}
To make this possible, another fix is needed:
socket_session is a local variable in OP's exposed code.
To make it "persistent", it has to become e.g. a member variable.
So, the constructor MainWindow::MainWindow() has to be changed:
// Nope: SocketThread* socket_session = new SocketThread();
// Instead:
socket_session = new SocketThread();
and SocketThread* socket_session; has to be added to member variables of class MainWindow.
To make it accessible in Form1, it has to be passed to Form1 as well.
This could be done e.g. by making it a member variable in Form1 also which is initialized with a constructor argument (or set from MainWindow afterwards).
(I must admit that I never have used the Qt UI builder QtDesigner but build all my UIs by C++ code exclusively.)
But, now, another fix is necessary:
volatile doesn't make a variable suitable for interthread communication.
(This was used in ancient times before multi-threading started to be supported by C++11.)
However, this is wrong: Is volatile useful with threads?
An appropriate fix would be to use std::atomic instead:
// Wrong for interthread-com.
//volatile bool threadFlag;
// Correct:
std::atomic<bool> threadFlag; // #include <atomic> needed
FYI: SO: Multithreading program stuck in optimized mode but runs normally in -O0
And, finally, in SocketThread::SocketThread():
connect(this, SIGNAL(setThreadFlag(bool)), this, SLOT(setFlag(bool)));
is not necessary in this case.
SocketThread::setThreadFlag() could call SocketThread::setFlag() directly, or even write threadFlag itself:
void setThreadFlag(bool flag) { threadFlag = flag; }
As I (recommended to) make threadFlag atomic, it can be accessed from any thread without causing a data race.
Update:
After OP has updated the question:
I just simply want to emit the signal setThreadFlag of the socketthread from inside of QPushbutton_clicked() slot.
The button (created from UI Form1) can be connected in the MainWindow as well (without using any method of Form1):
QObject::connect(form1->button1, &QPushButton::clicked,
socket_session, &SocketThread::setThreadFlag,
Qt::QueuedConnection);
Notes:
About form1->button1, I'm not quite sure.
I noticed that widgets in UI generated forms can be accessed this way but I don't know the exact details (as I never used the Qt UI builder on my own).
I used the Qt5 style of QObject::connect().
This is what I would recommend in any case.
The Qt5 style is verified at compile time. –
Wrong connections are detected by the C++ type checking.
Additionally, any function with matching signature can be used – no explicit exposure of slots is anymore necessary.
Even conversion of non-matching signature or adding additional parameters becomes possible by using C++ lambdas which are supported as well.
Qt: Differences between String-Based and Functor-Based Connections
It is possible to connect signals and slots of distinct threads.
I used Qt::QueuedConnection to remark this as interthread communication.
(However, I roughly remember that Qt might be able to detect it itself.
See the doc. for Qt::AutoConnection which is the default.
Further reading: Qt: Signals & Slots
Btw. using the Qt signals for inter-thread communication would exclude the necissity to make SocketThread::threadFlag() atomic. It could become a simple plain bool threadFlag; instead. The slot SocketThread::setThreadFlag() is called in the Qt event loop of QThread, in this case.
Is it possible to hide qt widget window from other thread?
For example if using ptr->window->hide();
from other thread it crashes with error:
Cannot send events to objects owned by a different thread...
Should signals and slots be used in this case or there are easier. alternatives?
Is it possible to hide Qt widget window from other thread?
Absolutely, all you need is to connect the signal on your worker thread with the slot on UI thread. And luckily QWidget::hide is a slot already (not even needed to wrap that in own slot).
// in WorkerQObject.h file:
class WorkerQObject : public QObject
{
Q_OBJECT
public:
///
signals:
void hideUI();
private:
///
};
// in WorkerQObject.cpp file:
WorkerQObject::WorkerQObject()
{
// thread initialization; move to thread etc.
connect(this, SIGNAL(hideUI()), pWidget, SLOT(hide()));
}
void WorkerQObject::methodOnWorkerThread()
{
emit hideUI();
}
I'm fighing since last week with problem caused by update of QPlainTextEdit. I'm trying to create separate from QMainWindow Dialog window with QPlainTextEdit inside. The problem begins when I try to use appendHtml signal (also tried with appendText), text that is placed is not visible unless marked by by mouse. Repainting or updating cause in program crash or no visible action.
Simplified code of QDialog with QPlainTextEdit header:
namespace Ui {
class LogWindow;
}
class LogWriter: public QDialog
{
Q_OBJECT
QMutex print_lock;
public:
class Log{
Q_OBJECT
const static int MAX_SIZE = 100;
bool to_terminal;
QString color;
QMutex *print_lock;
QPlainTextEdit *text_place;
QVector< QPair<QString,time_t> > history;
LogWriter * obj;
public:
bool print;
Log(bool _print,QString _color,LogWriter *obj_ = NULL)
{print = _print; color = _color; obj = obj_;}
void setLock(QMutex *print_lock_){print_lock = print_lock_;}
void setTextField(QPlainTextEdit *_text) {text_place = _text;}
Log& operator<<(QString &a);
Log& operator<<(const char* a);
};
static LogWriter* getInstance()
{
static LogWriter instance; // Guaranteed to be destroyed.
// Instantiated on first use.
return &instance;
}
~LogWriter();
Log LOW,MEDIUM,HIGH;
Ui::LogWindow *ui;
signals:
void signalLogAppend(QString);
};
Simplified code of methods definitions:
LogWriter::LogWriter(QWidget * parent): QDialog(parent) {
ui = new Ui::LogWindow;
ui->setupUi(this);
LOW.setLock(&print_lock);
MEDIUM.setLock(&print_lock);
HIGH.setLock(&print_lock);
connect(this,SIGNAL(signalLogAppend(QString)),ui->plainTextEdit,
SLOT(appendHtml(QString)),Qt::DirectConnection);
}
LogWriter::Log& LogWriter::Log::operator<< (QString &s){
history.push_front(qMakePair(s,time(NULL)));
if(history.size() > MAX_SIZE) history.pop_back();
if(print){
//print_lock->lock();
QString text = "<font color=\"";
text += color + "\">";
text += s + "</font>";
//cout << text.toStdString() << endl;
//text_place->appendHtml(text);
//text_place->repaint();
emit (obj)->signalLogAppend(text);
//print_lock->unlock();
}
return *this;
}
I have two separate ui files (first for main window, second for log window).
I have to use log window all across my program (something about 10 threads), and I stucked on this issue. My question is - is it possible to force GUI update without using main thread and if not - what else possibilities I have. If possible I would rather avoid reconstructing all my code - it would take me some time to do it. Right now logging is super easy - I ony need:
LogWindow *log = LogWindow::getInstance();
log->MEDIUM << "something";
As additional info I add QTCreator warning:
QObject::connect: Cannot queue arguments of type 'QTextBlock'
(Make sure 'QTextBlock' is registered using qRegisterMetaType().)
QObject::connect: Cannot queue arguments of type 'QTextCursor'
(Make sure 'QTextCursor' is registered using qRegisterMetaType().)
If I understand your code correctly, you're trying to log from a background thread and are using a direct connection to pass the signal to the GUI thread? That's not going to work, you have to send the signal via the default connection so Qt can figure out that it's a cross-thread signal and pass it across threads accordingly (ie, via the message loop on the foreground thread).
In Qt, any GUI interaction has to happen in the Main/foreground thread otherwise bad things happen as you discovered. You can certainly send a signal from a background thread to trigger a GUI update - I do this all the time - but you need to ensure that you're using the correct connection for it. The direct connection results in a direct function call and is not going to work for you in this case.
In your code, the problem is the call to connect() - you explicitly specify the connection mode for the signal to slot connection when you should just use the default setting. If you set the connection explicitly to Qt::DirectConnection, the underlying code will execute a direct call to the specified slot, which means that you end up calling the slot in the thread context of the signal. You don't want that because the signal is triggered in a background thread.
You can't pass arbitrary types/classes to signals and slots. The list is limited and not all Qt classes are in the list. To add types/classes to the list of those that can be passed to a signal/slot, you must call qRegisterMetaType for that class. I recommend calling it in the constructor of the class you're trying to pass to a signal like this:
MyClass::MyClass() : MyParentClass()
{
static int reg = qRegisterMetaType<MyClass>("MyClass");
}
The static int ensures the registration is only called once and before any instance of MyClass could ever be used.
I would like to affect a QDialog from a separate thread, I have to do two things:
dialog->show();
dialog->raise();
dialog->activateWindow();
As far as I understand, these are events and thus MUST be done in the main thread event loop. I think I achieve this with QApplication::postEvent within in the QThread::run(). Could anyone confirm?
Second, I would like to display an image which was processed in that separate thread. I think I need to subclass my dialog class and write a thread safe setImage() function, which is then called by paintEvent()... However, this seems like it's impossible. I can't block the paintEvent with a QMutex::unlock()? Could anyone offer some suggestions?
QApplication::postEvent(dialog, new QShowEvent()); did not work.
This is the solution for calling slots (as functions) from a seperate thread:
QMetaObject::invokeMethod(dialog, "show", Qt::QueuedConnection);
QMetaObject::invokeMethod(dialog, "raise", Qt::QueuedConnection);
... Still working on activateWindow() and QThread safe functions.
So for the QImage, it is a QPaintDevice. Supposedly it is thread safe. My approach would've been to have a class member QImage that is set within the thread. Then painted later.
Note, that the below approach is as good, if not better though.
I think this is a clearer way to do what you want:
class Dialog : public QDialog
{
...
public slots:
void showImage(QImage img);
...
}
void Dialog::showImage(QImage img);
{
setImage(img);
show();
raise();
activateWindow();
}
class Thread : public QThread
{
...
signals:
void imageReady(QImage);
}
void Thread::run()
{
QImage img;
/// image processing stuff
emit imageReady(img);
...
}
Thread *thread = new Thread;
Dialog *dialog = new Dialog;
connect(thread, SIGNAL(imageReady(QImage)), dialog, SLOT(showImage(QImage)));
thread->start();
I have an application in which each thread (except the main thread) needs to create its own window. I tried creating a thread and then calling this->exec() in the run function. However, I get an error before I even get to that call: ASSERT failure in QWidget: "Widgets must be created in the GUI thread."
I want to popup a message window. The problem is that the source has multiple threads each of which may need to popup its own message.
If you need to create QWidget(or some other gui component(s)) in different(non-main) thread(s) you can implement it in such way:
Create simple wrapper which holds gui component:
// gui component holder which will be moved to main thread
class gui_launcher : public QObject
{
QWidget *w;
// other components
//..
public:
virtual bool event( QEvent *ev )
{
if( ev->type() == QEvent::User )
{
w = new QWidget;
w->show();
return true;
}
return false;
}
};
create QApplication object in main thread
another thread body:
..
// create holder
gui_launcher gl;
// move it to main thread
gl.moveToThread( QApplication::instance()->thread() );
// send it event which will be posted from main thread
QCoreApplication::postEvent( &gl, new QEvent( QEvent::User ) );
..
be happy, :)
Qt will only let you create GUI elements in the GUI thread - what is it that you need to display from the other threads? See something like This answer for an example of updating a progress bar with data from a non-GUI thread.
Update:
If you want to show a message for each window, you can have a class like this:
class MyWorkerThread : public QThread
{
Q_OBJECT
signals:
void sendMessage(QString msg);
private:
void run()
{
/* do stuff */
emit sendMessage(QString("This thread is doing stuff!"));
/* do more stuff */
}
};
Then connect it up to your GUI via the signal-slot mechanism with something like:
connect(workerThread, SIGNAL(sendMessage(QString)),
guiController, SLOT(showMessageBox(QString)));
Where the showMessageBox function does what you need it to do.
I don't believe this is possible. Other non-GUI components can run in other threads and will usually communicate via the signal/slot mechanisms.
The above answers can be combined with a QAction object (or custom class objects) to transfer any action to the main GUI thread to be executed, not just creating widgets or displaying a message box. (e.g. by emitting sendAction(QAction*), or implementing a custom QEvent class embodying a QAction*.)