I have something like this:
class Thing : public QObject {
...
public slots:
void doSomething ();
...
};
I then have an object that manages Things, like this:
class ManyThings : public QObject {
...
public:
void makeThingDoSomething (int thingIndex);
private:
QVector<Thing *> things_;
...
};
My question is this: The Things in ManyThing's collection are scattered among a few different threads. I'd like makeThingDoSomething(int) to call the things_[thingIndex]->doSomething() slot as if the slot was called from a signal connected with Qt::AutoConnection. Essentially this, but using Qt's queuing mechanism if the Thing is on a different thread than the caller:
void ManyThings::makeThingDoSomething (int thingIndex) {
// i want to do this AutoConnection style, not direct:
things_[thingIndex]->doSomething();
// doesn't *need* to block for completion
}
What's the simplest way to set this up? I could make a signal in ManyThings and connect it to each of the Thing's slots, but then emitting that signal would call the slot on every Thing, not just a specific one. Is there some way to easily set up connections so that I can connect the same signal to different object's slots depending on an index parameter passed to the signal, or something? Or some way to call the slot using Qt's signal/slot mechanism without actually having to create a signal?
Try using QMetaObject::invokeMethod:
void ManyThings::makeThingDoSomething(int thingIndex) {
QMetaObject::invokeMethod(things_[thingIndex], "doSomething",
Qt::AutoConnection);
}
Note that doSomething will likely have to remain a slot if you use this approach.
Related
Declaration part of Controller class:
class ProtocolPacket: public QQuickItem
{
private:
Q_OBJECT
...
QThread objWorkerThread;
public:
ProtocolPacket();
~ProtocolPacket();
...
The constructor of controller class:
ProtocolPacket::ProtocolPacket()
{
ReadWriteSerialPort *objReadWriteSerialPort = new ReadWriteSerialPort;
objReadWriteSerialPort->moveToThread(&objWorkerThread);
connect(&objWorkerThread, &QThread::finished,
objReadWriteSerialPort, &QObject::deleteLater);
/// Following signals of ProtocolPacket class are connected to the corresponding
/// slots of thread class ReadWriteSerialPort.
connect(this, &ProtocolPacket::cameraValueChanged,
objReadWriteSerialPort, &ReadWriteSerialPort::receiveAndSendUpdatedCameraValue);
...
...
objWorkerThread.start();
}
The constructor of worker thread class:
ReadWriteSerialPort::ReadWriteSerialPort()
{
...
serialPort.setPortName ("/dev/ttyUSB0");
serialPort.setBaudRate (QSerialPort::Baud19200);
if(serialPort.open (QIODevice::ReadWrite))
{
serialPort.setDataBits(QSerialPort::Data8);
serialPort.setParity(QSerialPort::NoParity);
serialPort.setStopBits(QSerialPort::OneStop);
}
objWriter = new SerialPortWriter(&serialPort);
objReader = new SerialPortReader(&serialPort);
timerSendVelocityPackets = new QTimer(this);
connect (timerSendVelocityPackets, SIGNAL (timeout()),
this, SLOT (sendVelocityPacket()));
timerSendHealthFeedbackPackets = new QTimer(this);
connect (timerSendHealthFeedbackPackets, SIGNAL (timeout()),
this, SLOT (formAndSendHealthFeedbackPackets()));
timerReceivePackets = new QTimer (this);
connect (timerReceivePackets, SIGNAL (timeout()),
this, SLOT (receiveAndUnpackPacket()));
timerReceivePackets->start (300);
}
I am receiving the error:
QObject::startTimer: Timers cannot be started from another thread
What am I doing wrong?
I wish to start the timers from the worker class: ReadWriteSerialPort
Every object constructed in the object that you move to another thread must have its parent set:
class ReadWriteSerialPort : public QObject {
QSerialPort serialPort { this }; // important
QTimer timerSendVelocityPackets { this }; // etc.
...
};
You also must ensure that there are no direct calls into the thread-unsafe methods of ReadWriteSerialPort from other threads.
Furthermore, unless you're polling for data, the timer-driven packet receive is incorrect. You should act on the readyRead signal issued by the serial port.
I also question the separate reader and writer: Usually these tasks both need to be aware of the expected state of the target device, so separating them might be unhelpful. It's hard to say without seeing more of the code.
My question relates to using volatile for stop flag in order to stop method of some threaded server.
In some open-source projects I see declaring stop as volatile:
class ThreadedServer {
public:
//...
// this method will be launched in separate worker thread
void run() {
while (!stop_) {
//some useful work...
}
if (stop_) {
//cleanup
}
}
void stop() {
stop_ = true;
}
private:
volatile bool stop_;
};
In my server I declared a stop field WITHOUT volatile modifier, and everything works. So why to make class variable volatile in such concrete situation?
volatile signals that the value of stop_ has to be read from the main memory every time it is evaluated.
What could happen if you didn't do that, in the worst case, is to run without stopping, because thread1 sets stop_ to false, but thread2 does only look at his buffered stop on the CPU cache.
But keep in mind that it must not result in a problem, but it could.
I'm new in QT...and i haven't time. I have a GUI with 3 labels that must be update from 3 different threads (permanents, that invoke 3 different methods) every 10 secs. What is the best way to make this? thanks in advance!
You should use Qt signaling mechanism.
class QWindow : QMainWindow
{
//this macro is important for QMake to let the meta programming mechanism know
//this class uses Qt signalling
Q_OBJECT
slots:
void updateLabel(QString withWhat)
...
};
And now You just need to connect this slot to some signal
class SomeThreadClass : QObject
{
Q_OBJECT
...
signals:
void labelUpdateRequest(QString withwhat);
};
in the constructor of the window
QWindow::QWindow(QWidget* parent) : QMainWindow(parent)
{
m_someThread = new SomeThreadClass();
//in old style Qt, now there's a mechanism for compile time check
//don't use pointers, you need to free them at some point and there might be many receivers
//that might use it
connect(m_someThread, SIGNAL(labelUpdateRequest(QString)), this, SLOT(updateLabel(QString));
...
}
now just at some point in the thread:
SomeThreadClass::someMethod()
{
//do something...
emit labelUpdateRequest(QString("Welcome to cartmanland!"));
//this will be received by all classes that call connect to this class.
}
Hope that helps :)
I've subclassed my Qthread so I can implement my code in run() method. I have to pass it some parameters,
I tried it like this, so what's wrong in here?
class QMyThread :
public QThread
{
public:
QMyThread();
~QMyThread(void);
virtual void start(FILE *data, int sock, int bits);
protected:
virtual void run(FILE *data, int sock, int bits);
};
run method;
void QMyThread::run(FILE *data, int sock, int bits)
{
//do stuff
}
start the thread:
QMyThread *thread;
thread->start(datafile, sockint, bitsint);
first it says the thread might not be initialized and then it crashes in the start() method with SIGSEGV error. Anyone can help me?
You shouldn't be subclassing the QThread class as this is no longer the recommended way of using QThread.
For more information http://qt-project.org/doc/qt-4.8/qthread.html
To answer your question, couldn't you just make those parameters members of your class and assign their values through setters or its contructor?
You should do this instead:
QMyThread thread;
thread.start(...)
You created a pointer to a thread and did not new it. I frankly see no reason for a pointer here, you can just create a normal variable and call a method on it.
If you do want a pointer, then use std::unique_ptr in C++11 or boost::unique_ptr
std::unique_ptr<QMyThread> thread;
thread->start(...);
EDIT:
You should really just create a QThread * thread = new QThread(this); as per the documentation.
How about using the QMetaObject class to pass the parameters to worker class. You can try like this:
QMetaObject::invokeMethod(worker, "methodName", Q_ARG(QString, "ParameterQStringValue");
Note this method will work if methodName is a slot and you use the new way of creating threads: https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/
You can specify different parameters using Q_ARG macro up to 9 (http://doc.qt.io/qt-5/qmetaobject.html#details). If you need more parameters, then I suggest you to create the QVector with a structure and pass it to QMetaObject::invokeMethod as the parameter.
The QFuture class has methods such as cancel(), progressValue(), etc. These can apparently be monitored via a QFutureWatcher. However, the documentation for QtConcurrent::run() reads:
Note that the QFuture returned by
QtConcurrent::run() does not support
canceling, pausing, or progress
reporting. The QFuture returned can
only be used to query for the
running/finished status and the return
value of the function.
I have looked in vain for what method actually can create a QFuture that can be cancelled and report progress for a single long-running operation. (It looks like maybe QtConcurrent::map() and similar functions can, but I just have a single, long-running method.)
(For those familiar with .Net, something like the BackgroundWorker class.)
What options are available?
Though it's been a while since this question was posted and answered I decided to add my way of solving this problem because it is rather different from what was discussed here and I think may be useful to someone else. First, motivation of my approach is that I usually don't like to invent own APIs when framework already has some mature analogs. So the problem is: we have a nice API for controlling background computations represented by the QFuture<>, but we have no object that supports some of the operations. Well, let's do it. Looking on what's going on inside QtConcurrent::run makes things much clearer: a functor is made, wrapped into QRunnable and run in the global ThreadPool.
So I created generic interface for my "controllable tasks":
class TaskControl
{
public:
TaskControl(QFutureInterfaceBase *f) : fu(f) { }
bool shouldRun() const { return !fu->isCanceled(); }
private:
QFutureInterfaceBase *fu;
};
template <class T>
class ControllableTask
{
public:
virtual ~ControllableTask() {}
virtual T run(TaskControl& control) = 0;
};
Then, following what is made in qtconcurrentrunbase.h I made q-runnable for running this kind of tasks (this code is mostly from qtconcurrentrunbase.h, but slightly modified):
template <typename T>
class RunControllableTask : public QFutureInterface<T> , public QRunnable
{
public:
RunControllableTask(ControllableTask<T>* tsk) : task(tsk) { }
virtial ~RunControllableTask() { delete task; }
QFuture<T> start()
{
this->setRunnable(this);
this->reportStarted();
QFuture<T> future = this->future();
QThreadPool::globalInstance()->start(this, /*m_priority*/ 0);
return future;
}
void run()
{
if (this->isCanceled()) {
this->reportFinished();
return;
}
TaskControl control(this);
result = this->task->run(control);
if (!this->isCanceled()) {
this->reportResult(result);
}
this->reportFinished();
}
T result;
ControllableTask<T> *task;
};
And finally the missing runner class that will return us controllable QFututre<>s:
class TaskExecutor {
public:
template <class T>
static QFuture<T> run(ControllableTask<T>* task) {
return (new RunControllableTask<T>(task))->start();
}
};
The user should sublass ControllableTask, implement background routine which checks sometimes method shouldRun() of TaskControl instance passed to run(TaskControl&) and then use it like:
QFututre<int> futureValue = TaskExecutor::run(new SomeControllableTask(inputForThatTask));
Then she may cancel it by calling futureValue.cancel(), bearing in mind that cancellation is graceful and not immediate.
I tackled this precise problem a while ago, and made something called "Thinker-Qt"...it provides something called a QPresent and a QPresentWatcher:
http://hostilefork.com/thinker-qt/
It's still fairly alpha and I've been meaning to go back and tinker with it (and will need to do so soon). There's a slide deck and such on my site. I also documented how one would change Mandelbrot to use it.
It's open source and LGPL if you'd like to take a look and/or contribute. :)
Yan's statement is inaccurate. Using moveToThread is one way of achieving the proper behavior, but it not the only method.
The alternative is to override the run method and create your objects that are to be owned by the thread there. Next you call exec(). The QThread can have signals, but make sure the connections are all Queued. Also all calls into the Thread object should be through slots that are also connected over a Queued connection. Alternatively function calls (which will run in the callers thread of execution) can trigger signals to objects that are owned by the thread (created in the run method), again the connections need to be Queued.
One thing to note here, is that the constructor and destructor are running in the main thread of execution. Construction and cleanup need to be performed in run. Here is an example of what your run method should look like:
void MythreadDerrivedClass::run()
{
constructObjectsOnThread();
exec();
destructObjectsOnThread();
m_waitForStopped.wakeAll();
}
Here the constructObjectsOnThread will contain the code one would feel belongs in the constructor. The objects will be deallocated in destructObjectsOnThread. The actual class constructor will call the exit() method, causing the exec() to exit. Typically you will use a wait condition to sit in the destructor till the run has returned.
MythreadDerivedClass::~MythreadDerivedClass()
{
QMutexLocker locker(&m_stopMutex);
exit();
m_waitForStopped.wait(locker.mutex(), 1000);
}
So again, the constructor and destructor are running in the parent thread. The objects owned by the thread must be created in the run() method and destroyed before exiting run. The class destructor should only tell the thread to exit and use a QWaitCondition to wait for the thread to actually finish execution. Note when done this way the QThread derived class does have the Q_OBJECT macro in the header, and does contain signals and slots.
Another option, if you are open to leveraging a KDE library, is KDE's Thread Weaver. It's a more complete task based multitasking implementation similar QtConcurrentRun in that it leverages a thread pool. It should be familiar for anyone from a Qt background.
That said, if you are open to a c++11 method of doing the same thing, I would look at std::async. For one thing, you will no longer have any dependance on Qt, but the api also makes more clear what is going on. With MythreadDerivedClass class inheriting from QThread, the reader gets the impression that MythreadDerivedClass is a thread (since it has an inheritance relationship), and that all its functions run on a thread. However, only the run() method actually runs on a thread. std::async is easier to use correctly, and has fewer gotcha's. All our code is eventually maintained by someone else, and these sorta things matter in the long run.
C++11 /w QT Example:
class MyThreadManager {
Q_OBJECT
public:
void sndProgress(int percent)
void startThread();
void stopThread();
void cancel() { m_cancelled = true; }
private:
void workToDo();
std::atomic<bool> m_cancelled;
future<void> m_threadFuture;
};
MyThreadedManger::startThread() {
m_cancelled = false;
std::async(std::launch::async, std::bind(&MyThreadedManger::workToDo, this));
}
MyThreadedManger::stopThread() {
m_cancelled = true;
m_threadfuture.wait_for(std::chrono::seconds(3))); // Wait for 3s
}
MyThreadedManger::workToDo() {
while(!m_cancelled) {
... // doWork
QMetaInvoke::invokeMethod(this, SIGNAL(sndProgress(int)),
Qt::QueuedConnection, percentDone); // send progress
}
}
Basically, what I've got here isn't that different from how your code would look like with QThread, however, it is more clear that only workToDo() is running on the thread and that MyThreadManager is only managing the thread and not the thread itself. I'm also using MetaInvoke to send a queued signal for sending our progress updates with takes care of the progress reporting requirement. Using MetaInvoke is more explicit and always does the right thing (doesn't matter how you connect signals from your thread managers to other class's slots). You can see that the loop in my thread checks an atomic variable to see when the process is cancelled, so that handles the cancellation requirement.
Improve #Hatter answer to support Functor.
#include <QFutureInterfaceBase>
#include <QtConcurrent>
class CancellationToken
{
public:
CancellationToken(QFutureInterfaceBase* f = NULL) : m_f(f){ }
bool isCancellationRequested() const { return m_f != NULL && m_f->isCanceled(); }
private:
QFutureInterfaceBase* m_f;
};
/*== functor task ==*/
template <typename T, typename Functor>
class RunCancelableFunctorTask : public QtConcurrent::RunFunctionTask<T>
{
public:
RunCancelableFunctorTask(Functor func) : m_func(func) { }
void runFunctor() override
{
CancellationToken token(this);
this->result = m_func(token);
}
private:
Functor m_func;
};
template <typename Functor>
class RunCancelableFunctorTask<void, Functor> : public QtConcurrent::RunFunctionTask<void>
{
public:
RunCancelableFunctorTask(Functor func) : m_func(func) { }
void runFunctor() override
{
CancellationToken token(this);
m_func(token);
}
private:
Functor m_func;
};
template <class T>
class HasResultType
{
typedef char Yes;
typedef void *No;
template<typename U> static Yes test(int, const typename U::result_type * = 0);
template<typename U> static No test(double);
public:
enum { Value = (sizeof(test<T>(0)) == sizeof(Yes)) };
};
class CancelableTaskExecutor
{
public:
//function<T or void (const CancellationToken& token)>
template <typename Functor>
static auto run(Functor functor)
-> typename std::enable_if<!HasResultType<Functor>::Value,
QFuture<decltype(functor(std::declval<const CancellationToken&>()))>>::type
{
typedef decltype(functor(std::declval<const CancellationToken&>())) result_type;
return (new RunCancelableFunctorTask<result_type, Functor>(functor))->start();
}
};
User example:
#include <QDateTime>
#include <QDebug>
#include <QTimer>
#include <QFuture>
void testDemoTask()
{
QFuture<void> future = CancelableTaskExecutor::run([](const CancellationToken& token){
//long time task..
while(!token.isCancellationRequested())
{
qDebug() << QDateTime::currentDateTime();
QThread::msleep(100);
}
qDebug() << "cancel demo task!";
});
QTimer::singleShot(500, [=]() mutable { future.cancel(); });
}
For a long running single task, QThread is probably your best bet. It doesn't have build-in progress reporting or canceling features so you will have to roll your own. But for simple progress update it's not that hard. To cancel the task, check for a flag that can be set from calling thread in your task's loop.
One thing to note is if you override QThread::run() and put your task there, you can't emit signal from there since the QThread object is not created within the thread it runs in and you can't pull the QObject from the running thread. There is a good writeup on this issue.