Signals: void dispatch(int threadid, int value); Similarly, a receive slot with the same arguments needs to be created to receive the signal. Public slots: void receive(int threadid, int value); Now, this will work very nicely when one is dealing with the predefined primitive or the qt data types. This is an example of threading using QThread and signal/slots of Qt libraries in Python using PySide. The same concepts should also be valid for PyQt bindings. Qt provides thread support in the form of platform-independent threading classes, a thread-safe way of posting events, and signal-slot connections across threads. This makes it easy to develop portable multithreaded Qt applications and take advantage of multiprocessor machines. Home Articles Programming C/C. Qt's meta-object system provides the signals and slots mechanism for inter-object communication, run-time type information (RTTI), and the dynamic property system. Signals and slots is one of the most important concepts in Qt, and it will be discussed in the next chapter. The meta-object system is implemented with a three-part mechanism.
I have a Qt 'signal-slot connection'.
I want to test the class with the signal
The class with the slot is mocked/stubbed.
Now I would like to verify that when I emit signal, the slot is
called.
I wonder if it is possible to use gmock to verify this.
// class Tx contains the signal, which is protected so I inherit
// to get access to it
class Test : public ::testing::Test, public Tx
{ public:
MockRx *rx;
void SetUp()
{ rx = new MockRx;
rx->rxsignal(); // Just to see it compiles
rx->gmock_rxsignal(); // Compiles, gmock generated function
QObject::connect(this, SIGNAL(txsignal(), rx, SLOT(rxsignal()));
}
void TearDown() { delete rx; }
};
TEST_F(Test, Signal)
{
EXPECT_CALL(*rx, rxsignal()).Times(1).WillOnce(Return());
this->txsignal();
}
class MockRx : public QWidget
{ Q_OBJECT
public:
MOCK_METHOD0(rxsignal, void());
};
When running, I get the error:
QObject::connect: No such slot as RxMock::rxsignal()
If I remove Q_OBJECT in class MockRx, I get: No such slot as
QWidget::rxsignal()
I also tried to QObject::conntect(.. SLOT(gmock_rxsignal()));
which gives No such slot as RxMock::rxsignal()
If I replace the MockRx class with a class Rx with rxsignal()
everything works.
I wonder why QObject::connect cannot find RxText::rxsignal() when
- it works if I replace with a regular class Rx instead of class
MockRx
- rx->rxsignal() can be called
Is gmock doing something under the hood that prevents this?
Hope someone can help, thanks a lot
Paul
This blog is part of a series of blogs explaining the internals of signals and slots.
In this article, we will explore the mechanisms powering the Qt queued connections.
Summary from Part 1
In the first part, we saw that signalsare just simple functions, whose body is generated by moc. Lucky eagle casino recent winners. They are just calling QMetaObject::activate, with an array of pointers to arguments on the stack.Here is the code of a signal, as generated by moc: (from part 1)
QMetaObject::activatewill then look in internal data structures to find out what are the slots connected to that signal.As seen in part 1, for each slot, the following code will be executed:
So in this blog post we will see what exactly happens in queued_activateand other parts that were skipped for the BlockingQueuedConnection
Qt Event Loop
A QueuedConnection will post an event to the event loop to eventually be handled.
When posting an event (in QCoreApplication::postEvent),the event will be pushed in a per-thread queue(QThreadData::postEventList).The event queued is protected by a mutex, so there is no race conditions when threadspush events to another thread's event queue.
Other words to describe a poker face. Once the event has been added to the queue, and if the receiver is living in another thread,we notify the event dispatcher of that thread by calling QAbstractEventDispatcher::wakeUp.This will wake up the dispatcher if it was sleeping while waiting for more events.If the receiver is in the same thread, the event will be processed later, as the event loop iterates.
The event will be deleted right after being processed in the thread that processes it.
An event posted using a QueuedConnection is a QMetaCallEvent. When processed, that event will call the slot the same way we call them for direct connections.All the information (slot to call, parameter values, ..) are stored inside the event.
Copying the parameters
The argv coming from the signal is an array of pointers to the arguments. The problem is that these pointers point to the stack of the signal where the arguments are. Once the signal returns, they will not be valid anymore. So we'll have to copy the parameter values of the function on the heap. In order to do that, we just ask QMetaType. We have seen in the QMetaType article that QMetaType::create has the ability to copy any type knowing it's QMetaType ID and a pointer to the type.
To know the QMetaType ID of a particular parameter, we will look in the QMetaObject, which contains the name of all the types. We will then be able to look up the particular type in the QMetaType database.
queued_activate
We can now put it all together and read through the code ofqueued_activate, which is called by QMetaObject::activate to prepare a Qt::QueuedConnection slot call.The code showed here has been slightly simplified and commented:
Upon reception of this event, QObject::event will set the sender and call QMetaCallEvent::placeMetaCall. That later function will dispatch just the same way asQMetaObject::activate would do it for direct connections, as seen in Part 1
BlockingQueuedConnection
BlockingQueuedConnection is a mix between DirectConnection and QueuedConnection. Like with aDirectConnection, the arguments can stay on the stack since the stack is on the thread thatis blocked. No need to copy the arguments.Like with a QueuedConnection, an event is posted to the other thread's event loop. The event also containsa pointer to a QSemaphore. The thread that delivers the event will release thesemaphore right after the slot has been called. Meanwhile, the thread that called the signal will acquirethe semaphore in order to wait until the event is processed.
It is the destructor of QMetaCallEvent which will release the semaphore. This is good becausethe event will be deleted right after it is delivered (i.e. the slot has been called) but also whenthe event is not delivered (e.g. because the receiving object was deleted).
A BlockingQueuedConnection can be useful to do thread communication when you want to invoke afunction in another thread and wait for the answer before it is finished. However, it must be donewith care.
Summary from Part 1
In the first part, we saw that signalsare just simple functions, whose body is generated by moc. Lucky eagle casino recent winners. They are just calling QMetaObject::activate, with an array of pointers to arguments on the stack.Here is the code of a signal, as generated by moc: (from part 1)
QMetaObject::activatewill then look in internal data structures to find out what are the slots connected to that signal.As seen in part 1, for each slot, the following code will be executed:
So in this blog post we will see what exactly happens in queued_activateand other parts that were skipped for the BlockingQueuedConnection
Qt Event Loop
A QueuedConnection will post an event to the event loop to eventually be handled.
When posting an event (in QCoreApplication::postEvent),the event will be pushed in a per-thread queue(QThreadData::postEventList).The event queued is protected by a mutex, so there is no race conditions when threadspush events to another thread's event queue.
Other words to describe a poker face. Once the event has been added to the queue, and if the receiver is living in another thread,we notify the event dispatcher of that thread by calling QAbstractEventDispatcher::wakeUp.This will wake up the dispatcher if it was sleeping while waiting for more events.If the receiver is in the same thread, the event will be processed later, as the event loop iterates.
The event will be deleted right after being processed in the thread that processes it.
An event posted using a QueuedConnection is a QMetaCallEvent. When processed, that event will call the slot the same way we call them for direct connections.All the information (slot to call, parameter values, ..) are stored inside the event.
Copying the parameters
The argv coming from the signal is an array of pointers to the arguments. The problem is that these pointers point to the stack of the signal where the arguments are. Once the signal returns, they will not be valid anymore. So we'll have to copy the parameter values of the function on the heap. In order to do that, we just ask QMetaType. We have seen in the QMetaType article that QMetaType::create has the ability to copy any type knowing it's QMetaType ID and a pointer to the type.
To know the QMetaType ID of a particular parameter, we will look in the QMetaObject, which contains the name of all the types. We will then be able to look up the particular type in the QMetaType database.
queued_activate
We can now put it all together and read through the code ofqueued_activate, which is called by QMetaObject::activate to prepare a Qt::QueuedConnection slot call.The code showed here has been slightly simplified and commented:
Upon reception of this event, QObject::event will set the sender and call QMetaCallEvent::placeMetaCall. That later function will dispatch just the same way asQMetaObject::activate would do it for direct connections, as seen in Part 1
BlockingQueuedConnection
BlockingQueuedConnection is a mix between DirectConnection and QueuedConnection. Like with aDirectConnection, the arguments can stay on the stack since the stack is on the thread thatis blocked. No need to copy the arguments.Like with a QueuedConnection, an event is posted to the other thread's event loop. The event also containsa pointer to a QSemaphore. The thread that delivers the event will release thesemaphore right after the slot has been called. Meanwhile, the thread that called the signal will acquirethe semaphore in order to wait until the event is processed.
It is the destructor of QMetaCallEvent which will release the semaphore. This is good becausethe event will be deleted right after it is delivered (i.e. the slot has been called) but also whenthe event is not delivered (e.g. because the receiving object was deleted).
A BlockingQueuedConnection can be useful to do thread communication when you want to invoke afunction in another thread and wait for the answer before it is finished. However, it must be donewith care.
The dangers of BlockingQueuedConnection
You must be careful in order to avoid deadlocks.
Obviously, if you connect two objects using BlockingQueuedConnection living on the same thread,you will deadlock immediately. You are sending an event to the sender's own thread and then are locking thethread waiting for the event to be processed. Since the thread is blocked, the event will never beprocessed and the thread will be blocked forever. Qt detects this at run time and prints a warning,but does not attempt to fix the problem for you.It has been suggested that Qt could then just do a normal DirectConnection if both objects are inthe same thread. But we choose not to because BlockingQueuedConnection is something that can only beused if you know what you are doing: You must know from which thread to what other thread theevent will be sent.
The real danger is that you must keep your design such that if in your application, you do aBlockingQueuedConnection from thread A to thread B, thread B must never wait for thread A, or you willhave a deadlock again.
When emitting the signal or calling QMetaObject::invokeMethod(), you must not have any mutex lockedthat thread B might also try locking.
A problem will typically appear when you need to terminate a thread using a BlockingQueuedConnection, for example in thispseudo code:
You cannot just call wait here because the child thread might have already emitted, or is about to emitthe signal that will wait for the parent thread, which won't go back to its event loop. Indiana grand casino blackjack. All the thread cleanup information transfer must only happen withevents posted between threads, without using wait(). A better way to do it would be:
The downside is that MyOperation::cleanup() is now called asynchronously, which may complicate the design.
Conclusion
Qt Thread Slot Signal Tester
This article should conclude the series. I hope these articles have demystified signals and slots,and that knowing a bit how this works under the hood will help you make better use of them in yourapplications.
