I must send a file through serial port, so I use a separate thread, named serialWorker.
In the main thread I have this:
ui->progressBar->setMaximum(file.size());
QTextStream in(&file);
while(!in.atEnd()){
serialWorker.send(line);
ui->progressBar->setValue(in.pos());
}
The problem is that the progress bar is not "in real time", namely, as serialWorker doesn't block main thread, the loop runs without waiting. How I should approach this better?
You are updating progress bar in a loop which blocks main GUI thread thus your progressbar does not get updated.
You should use a signal in your serialWorker to notify the progress bar in your user interface of the value for the progress. In your send function you should emit the signal with the appropriate value. The signal is like:
void progressChanged(int val);
You should also connect the progressChanged(int) signal to the setValue(int value) slot of the QProgressBar.
And the last point is that you should not call send directly when it is in an other thread. The correct way is defining send as a slot and connecting a signal to that slot and emitting the signal when you want to call send.
Related
If I have a progressbar in the ui thread (window), which shall run endless until a method finishes it's work, will the ui thread block and so the progress bar if I'm waiting for a second QThread to finish? If the ui thread blocks waiting, then i would not to wait for the second thread. I thought to implement a callback method which will be called when the second thread finished, but then: How can i connect to the callback method?
What do I want to do?
I have a window, this window has a progressbar which is first not visible. When the user presses a certain button to request data, a method will be called which returns a RequestPointer which contains a method which returns the request status.
When the user presses the button, i want to make the progress bar visible, running infinitely till the request is finished and i can print the data to the window.
To the Worker i want to pass this pointer and the worker checks in a while (flag) loop, if the status is still running and sleep if so. When the worker finishes, i want to stop the progressbar and make it unvisible again. Do i have to pass the progress bar to the thread, or can i wait for the thread without blocking the ui?
I'm not quite a Qt pro. Really new to it. I tried to get some info from the https://doc.qt.io/Qt-5/qthread.html website, but it's a bit hard for me to understand the code example.
Method in my worker class:
void Worker::watchRequest(RequestPtr r_ptr)
{
bool exit = true;
while (!exit)
{
ErrorCode errorCode = r_ptr->Test();
switch (errorCode)
{
case Request_RUNNING:
QThread::msleep(10);
break;
case Request_ABORTED:
exit = true;
break;
case Request_SUCCESS:
exit = true;
break;
}
}
QThread has a finished signal. Connect this one to some appropriate slot, which will trigger any action necessary on thread completion.
I suppose best candidate to know how far the progress went is the worker thread itself. You might create your own signal that sends the current progress to some slot that will do the update of the progress bar appropriately.
Alternatively, you might use a QTimer to read the current progress from time to time (this resembles closer to what you have now, but does not block the UI).
If you don't want to block the user interface, all you have to do is to call QApplication::processEvents(); in your while-loop.
I have some code which uses std::future instead of QThread and my code looks like this:
while (!progressIndicator->UserBreak()
&& (future.wait_for(std::chrono::seconds(0)) != std::future_status::ready))
{
QApplication::processEvents();
}
This works well.
To update the UI Thread's progress bar while the operation is running, use a QTimer object to increment the progress bar's value (Max value will be :One less than Progress bar's value when the operation completes). Also connect the QThread to a slot by Signal/Slot method to signal the UI thread when the operation has ended. When the QThread finishes the operation, send a signal to a Slot in the UI Thread, which will set the final value of the progress bar and also stop the QTimer.
I have a Qt C++ program. I have a main driver MainWindow and a TCPClient class. The TCPClient class is used to communicate with a remote server, transmit some data over TCP, request for processing of the data and receive processed data from server. In my TCPClient class, I am using QAbstractSocket signal disconnected. This is emitted when the connection with the server is disconnected. In the function (slot) which handles this disconnect signal (ifDisconnected), onCompletionCallback function of the MainWindow is called. Now my question is how do I prevent the transmission of execution back to TCPClient after the said onCompletionCallback finishes executing. What's following is incomplete code describing the issue;
mainwindow.cpp
void MainWindow::on_connectButton_clicked()
{
std::function<void(void)> callback std::bind(&MainWindow::onCompletetionCallback, this);
tcpClient_ = new TCPClient(callback)->connectToServer(someData);
}
void MainWindow::onCompletetionCallback()
{
if(tcpClient_->isRequestSuccess())
{
QJsonDocument responseJson = tcpClient_->getResponse();
return; //When this finishes executing, I want to prevent the execution control to go back to TCPClient
}
}
TCPClient.cpp
void TCPClient::connectToServer(QJsonDocument requestJson)
{
// Removed code of other connect signals
connect(tcpSocket_, &QTcpSocket::disconnected, this, &TCPClient::ifDisconnected);
}
void TCPClient::ifDisconnected()
{
// Here the callback is called. After the callback finishes executing, I don't want execution to return to `TCPClient`.
onCompletionCallback_();
return;
}
How do I solve this problem. I need to use the signal disconnected because QAbstractSocket doesn't provide any utility function to check if the connection is available.
You cannot and you should not prevent that the signal handler returns to caller. Otherwise, you would corrupt your call stack.
The actual question (for me) is: What is the caller of signal handler?
To understand what I mean, please, read the Qt doc. about QObject::connect() with special attention to Qt::ConnectionType.
The default is Qt::AutoConnection which means:
If the receiver lives in the thread that emits the signal, Qt::DirectConnection is used. Otherwise, Qt::QueuedConnection is used. The connection type is determined when the signal is emitted.
Qt::DirectConnection:
The slot is invoked immediately when the signal is emitted. The slot is executed in the signalling thread.
The most common case (for me) are signal handlers called for modifications of GUI objects (i.e. widgets, etc.) which modify data or other widgets in response (in strict single-threading manner). In this case, it is Qt::DirectConnection i.e. the widget signal emitter is the caller of my signal handler.
A possible error (I did once) is to delete the widget which emitted the signal (e.g. handling a close button event of a dialog) – bad idea: I destroyed the widget with a pending method call on call stack. After returning from my signal handler it ended in a crash. The caller method (the signal emitter) had no instance anymore, or in other words: its this was invalidated. (It's like sawing the limb you sit on.) (Btw. deleteLater could be one solution for this. I found SO: How delete and deleteLater works with regards to signals and slots in Qt? concerning this.)
Considering your code sample
connect(tcpSocket_, &QTcpSocket::disconnected, this, &TCPClient::ifDisconnected);
I suspect this is a Qt::DirectConnection.
The other aspect: calling a main window function out of the TCP client thread is something which needs special attention as well. The caller is something in the TCP client thread but addresses an object (the main window) which resides in the (different) GUI thread. Phew. Everything (exept local variables) what is accessed in this called function must be mutex guarded if the GUI thread itself uses this as well.
So, what about the other options:
Qt::QueuedConnection:
The slot is invoked when control returns to the event loop of the receiver's thread. The slot is executed in the receiver's thread.
For communication between threads, IMHO, the Qt::QueuedConnection is the safer way: The TCP client emits a signal which results in a respective entry in the event loop of the GUI thread (assuming the main window was given as receiver object). The GUI thread will pick up this entry while processing its event loop. In this case, the event loop of GUI thread is the caller of the signal handler. The TCP client thread didn't wait after sending the signal request but continued its processing. If this is not desired the third option comes into play:
Qt::BlockingQueuedConnection:
Same as Qt::QueuedConnection, except that the signalling thread blocks until the slot returns. This connection must not be used if the receiver lives in the signalling thread, or else the application will deadlock.
The Qt::BlockingQueuedConnection lets the signal emitter (the TCP client) until the GUI thread has processed the signal handler. (The warning about the dead-lock is not effective here as the TCP client thread is the signaling thread where the GUI thread is the receiver.)
I'm a little bit uncertain what to recommend. I'm afraid your application needs a little bit re-design but for this the code sample is a little bit to incomplete.
A possible solution:
Introduce a Qt signal emitted when completion is required. The MainWindow::onCompletetionCallback() may be connected as signal handler to this TCP client signal using a Qt::BlockingQueuedConnection.
If the end of transmission is recognized it may perhaps destroy the TCP client thread. However, a thread which kills another thread is no good idea in general (and I'm not sure how Qt handles this "under the hood"). Therefore, a better concept would be: If the end of transmission is recognized the main thread flags the TCP client thread to leave it's main loop. Flagging could be done e.g. with a std::atomic<bool> (or you stay in Qt which has its own pendant QAtomicInt. The TCP client checks this flag in its main loop or at least after emitting a signal and exits in case.
A last hint:
If you are uncertain whether you understood the all the signal stuff correctly – I checked my understanding by putting a break point into the signal handler and inspecting the call stack when execution stopped at that break point. This is easy and straight forward (except you are dealing with mouse or drag & drop events).
I have a question about Qt and its signals/slots mechanism.
I have created a custom widget and in that I have created a custom SIGNAL (ShowMessage). This signal is connected to a SLOT, which displays the message (along with the specified timeout) in my main window's status bar.
Now, I have an operation in my class that takes a long time to execute, and it's blocking the UI. I was hoping to emit my signal before starting the operation and when it's finished, emit it again to update the status bar; something like this:
emit ShowMessage(message, timeout);
// Do the long operation
emit ShowMessage(newMessage, timeout);
But my problem is that it seems that Qt waits until the whole operation is finished, and only updates the status bar with newMessage.
Is there a way to somehow "force" immediate processing of my signal, because if I want to resort to threads, then my life will get much more complicated!
Is there a way to somehow "force" immediate processing of my signal
Yes, there is. :-).
After you show your first message, call QCoreApplication::processEvents(). This forces all pending events to be processed at the point of call. For example,
emit ShowMessage(message, timeout);
QCoreApplication::processEvents();
// Do the long operation
emit ShowMessage(newMessage, timeout);
I have simple application that in the main view I have QListview . my flow going like this I need to know if it right
1.App starts and start single thread , also see signal/slot connect between the thread object and the main app
2.Thread gets data from remote server as xml format its and sets the data into object container (class that represent the data )
3.when the data is ready in the object it trigger SIGNAL back to the main app (the signal/slot from section 1)
4.The SIGNAL invoking update function that sets the formatted data into the QListView via its model (QAbstractListModel)
The problem is when stage 4 is happening I see some frize in the application for 2-3 seconds that makes me wonder what is wrong here .
UPDATE:
after profiling the app with sleepy
it looks like the delay in the app im not sure but is shows in the Exclusive column
very high number 322.35s.
in my Thread that calls the http request inside the run method i have this code that couse the thread to pause.
void RequestThread::run()
{
m_RequestThreadTimer = new QTimer();
connect(m_RequestThreadTimer, SIGNAL(timeout()),
this,SLOT(fire(),Qt::DirectConnection));
QVariant val(GetValFromConfig());
int interval = val.toInt();
m_RequestThreadTimer->setInterval(interval);
m_RequestThreadTimer->start();
QThread::exec();
}
but now is the question how to improve it ?
I suspect that since you create the timer in the QThread::run() method the slot the timer connects to is being called in the context of the main thread.
You don't need to subclass QThread to run code in its own thread.
Just subclass a QObject, add the functionality you want, create a QThread instance, start it and use the QObject::moveToThread() method to set the QObject's thread affinity to the new thread.
worker = new WorkerClass;
connect(worker,SIGNAL(response(QString)),this,SLOT(response(QString)));
QThread *t = new QThread;
t->start();
worker->moveToThread(t);
//Start it either like this or by emitting a signal connected to the startWorking slot
QMetaObject::invokeMethod(worker,"startWorking",Qt::QueuedConnection);
I suggest you to use the QEventloop in the case of the thread.
Start the event loop in the main
//start the function to get data from remote server
GetData::getInstance()->StratReading();
QEventLoop loop; //loop to continue the reading.
loop.connect(GetData::getInstance(),SIGNAL(ReadingFinished()),SLOT(quit()));
loop.exec();
GetData::StratReading()
{
//sets the data into object container
//the data is ready in the object it trigger SIGNAL to main function to update to Ui
emit ReadingFinished(); //this will quit the loop
}
in my program, I am subclassing QThread, and I implemented the virtual method run() like so:
void ManagerThread::run() {
// do a bunch of stuff,
// create some objects that should be handled by this thread
// connect a few signals/slots on the objects using QueuedConnection
this->exec(); // start event loop
}
Now, in another thread (let's call it MainThread), I start the ManagerThread and wait for its started() signal, after which I proceed to use the signals and slots that should be handled by ManagerThread. However, the started() signal is essentially emmitted right before run() is called, so depending on thread scheduling I lose some signals from MainThread, because the event loop hasn't started yet!
(EDIT: turns out that's not the problem, it's just the signals are not connected in time, but for the same reason)
I could emit a signal right before calling exec(), but that's also asking for trouble.
Is there any definitive/simple way of knowing that the event loop has started?
Thanks!
EDIT2:(SOLUTION)
Alright, so it turns out the problem isn't exactly what I phrased. The fact that the event loop hasn't started isn't the problem, since signals should get queued up until it does start. The problem is, some of the signals would not get connected in time to be called- since the started() signal is emitted before run() is called.
The solution is to emit another custom signal after all the connections and right before exec. That way all signals/slots are ensured to be connected.
This is the solution to my problem, but not really an answer to the thread title. I have accepted the answer that does answer the title.
I have left all my code below for those curious, with the solution being, to wait for another signal in the instance() method.
CODE:
Many of you are saying that I cannot lose signals, so here is my whole class implementation. I will simplify it to just the bare necessities.
Here is the interface to ManagerThread:
// singleton class
class ManagerThread: public QThread {
Q_OBJECT
// trivial private constructor/destructor
public:
static ManagerThread* instance();
// called from another thread
public:
void doSomething(QString const& text);
// emitted by doSomething,
// connected to JobHandler whose affinity is this thread.
signals:
void requestSomething(QString const& text);
// reimplemented virtual functions of QThread
public:
void run();
private:
static QMutex s_creationMutex;
static ManagerThread* s_instance;
JobHandler* m_handler; // actually handles the requests
};
Some relevant implementations. Creating the singleton instance of the thread:
ManagerThread* ManagerThread::instance() {
QMutexLocker locker(&s_creationMutex);
if (!s_instance) {
// start socket manager thread, and wait for it to finish starting
s_instance = new ManagerThread();
// SignalWaiter essentially does what is outlined here:
// http://stackoverflow.com/questions/3052192/waiting-for-a-signal
SignalWaiter waiter(s_instance, SIGNAL(started()));
s_instance->start(QThread::LowPriority);
qDebug() << "Waiting for ManagerThread to start";
waiter.wait();
qDebug() << "Finished waiting for ManagerThread thread to start.";
}
return s_instance;
}
Reimplementation of run that sets up signals/slots and starts event loop:
void ManagerThread::run() {
// we are now in the ManagerThread thread, so create the handler
m_handler = new JobHandler();
// connect signals/slots
QObject::connect(this,
SIGNAL(requestSomething(QString const&)),
m_handler,
SLOT(handleSomething(QString const&)),
Qt::QueuedConnection);
qDebug() << "Starting Event Loop in ManagerThread";
// SOLUTION: Emit signal here and wait for this one instead of started()
this->exec(); // start event loop
}
Function that delegates the handling to the correct thread. This is where
I emit the signal that is lost:
void ManagerThread::doSomething(QString const& text) {
qDebug() << "ManagerThread attempting to do something";
// if calling from another thread, have to emit signal
if (QThread::currentThread() != this) {
// I put this sleep here to demonstrate the problem
// If it is removed there is a large chance the event loop
// will not start up in time to handle the subsequent signal
QThread::msleep(2000);
emit(requestSomething(text));
} else {
// just call directly if we are already in the correct thread
m_handler->handleSomething(text);
}
}
Finally, here is the code from MainThread that will fail if the event loop doesn't start in time:
ManagerThread::instance()->doSomething("BLAM!");
Assuming that the handler just prints out its text, here is what gets printed out on a successful run:
Waiting for ManagerThread to start
Finished waiting for ManagerThread thread to start.
Starting Event Loop in ManagerThread
ManagerThread attempting to do something
BLAM!
And here is what happens on an unsuccessful run:
Waiting for ManagerThread to start
Finished waiting for ManagerThread thread to start.
ManagerThread attempting to do something
Starting Event Loop in ManagerThread
Clearly the event loop started after the signal was emitted, and BLAM never prints.
There is a race condition here, that requires the knowledge of when the event loop starts,
in order to fix it.
Maybe I'm missing something, and the problem is something different...
Thanks so much if you actually read all that! Phew!
If you setup the connections right, you shouldn't be losing the signals. But if you really want to get a notice on the start of the thread's event loop, you can try QTimer::singleShot() in your run() right before calling exec(). It will be delivered when the event loop starts and only delivered once.
You could look at QSemaphore to signal between threads. Slots and signals are better for ui events and callbacks on the same thread.
Edit: Alternately you could combine QMutex with QWaitCondition if a semaphore is not applicable. More example code to see how you are using the ManagerThread in conjunction with the MainThread would be helpful.
This is a non-issue. Signals between threads are queued (more specifically, you need to set them up to be queued in the connect() call because direct connections between threads aren't safe).
http://doc.qt.io/qt-5/threads-qobject.html#signals-and-slots-across-threads
You could create the signal/slots connections in the constructor of the ManagerThread. In that way, they are certainly connected even before run() is called.