Qt Signal Slot Different Threads

Signals and slots are used for communication between objects. The signals and slots mechanism is a central feature of Qt and probably the part that differs most from the features provided by other frameworks.

So the GUI class defines slots and signals, the signals are emitted in the implemented interface functions. The signals and slots are connected using: @'connect(this, SIGNAL(xyz), this SLOT(xyz));'@ I think the problem is that the intface functions that emit the signals are running in thread a but the GUI runs in thread b. How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections. Since the thread is blocked, the event will never be processed 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. Signals and Slots Across Threads Qt supports these signal-slot connection types: Auto Connection (default) If the signal is emitted in the thread which the receiving object has affinity then the behavior is the same as the Direct Connection. Otherwise, the behavior is the same as the Queued Connection.'

Introduction

Qt documentation states that signals and slots can be direct, queued and auto. It also stated that if object that owns slot 'lives' in a thread different from object that owns signal, emitting such signal will be like posting message - signal emit will return instantly and slot method will be called in target thread's event loop. How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections This blog is part of a series of blogs explaining the internals of signals and slots. Part 1 - How Qt Signals and Slots Work.

In GUI programming, when we change one widget, we often want another widget to be notified. More generally, we want objects of any kind to be able to communicate with one another. For example, if a user clicks a Close button, we probably want the window's close() function to be called.

Older toolkits achieve this kind of communication using callbacks. A callback is a pointer to a function, so if you want a processing function to notify you about some event you pass a pointer to another function (the callback) to the processing function. The processing function then calls the callback when appropriate. Callbacks have two fundamental flaws: Firstly, they are not type-safe. We can never be certain that the processing function will call the callback with the correct arguments. Secondly, the callback is strongly coupled to the processing function since the processing function must know which callback to call.

Signals and Slots

In Qt, we have an alternative to the callback technique: We use signals and slots. A signal is emitted when a particular event occurs. Qt's widgets have many predefined signals, but we can always subclass widgets to add our own signals to them. A slot is a function that is called in response to a particular signal. Qt's widgets have many pre-defined slots, but it is common practice to subclass widgets and add your own slots so that you can handle the signals that you are interested in.

The signals and slots mechanism is type safe: The signature of a signal must match the signature of the receiving slot. (In fact a slot may have a shorter signature than the signal it receives because it can ignore extra arguments.) Since the signatures are compatible, the compiler can help us detect type mismatches. Signals and slots are loosely coupled: A class which emits a signal neither knows nor cares which slots receive the signal. Qt's signals and slots mechanism ensures that if you connect a signal to a slot, the slot will be called with the signal's parameters at the right time. Signals and slots can take any number of arguments of any type. They are completely type safe.

All classes that inherit from QObject or one of its subclasses (e.g., QWidget) can contain signals and slots. Signals are emitted by objects when they change their state in a way that may be interesting to other objects. This is all the object does to communicate. It does not know or care whether anything is receiving the signals it emits. This is true information encapsulation, and ensures that the object can be used as a software component.

Slots can be used for receiving signals, but they are also normal member functions. Just as an object does not know if anything receives its signals, a slot does not know if it has any signals connected to it. This ensures that truly independent components can be created with Qt.

You can connect as many signals as you want to a single slot, and a signal can be connected to as many slots as you need. It is even possible to connect a signal directly to another signal. (This will emit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programming mechanism.

A Small Example

A minimal C++ class declaration might read:

A small QObject-based class might read:

The QObject-based version has the same internal state, and provides public methods to access the state, but in addition it has support for component programming using signals and slots. This class can tell the outside world that its state has changed by emitting a signal, valueChanged(), and it has a slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT at the top of their declaration. They must also derive (directly or indirectly) from QObject.

Slots are implemented by the application programmer. Here is a possible implementation of the Counter::setValue() slot:

The emit line emits the signal valueChanged() from the object, with the new value as argument.

In the following code snippet, we create two Counter objects and connect the first object's valueChanged() signal to the second object's setValue() slot using QObject::connect():

Calling a.setValue(12) makes a emit a valueChanged(12) signal, which b will receive in its setValue() slot, i.e. b.setValue(12) is called. Then b emits the same valueChanged() signal, but since no slot has been connected to b's valueChanged() signal, the signal is ignored.

Note that the setValue() function sets the value and emits the signal only if value != m_value. This prevents infinite looping in the case of cyclic connections (e.g., if b.valueChanged() were connected to a.setValue()).

By default, for every connection you make, a signal is emitted; two signals are emitted for duplicate connections. You can break all of these connections with a single disconnect() call. If you pass the Qt::UniqueConnectiontype, the connection will only be made if it is not a duplicate. If there is already a duplicate (exact same signal to the exact same slot on the same objects), the connection will fail and connect will return false

This example illustrates that objects can work together without needing to know any information about each other. To enable this, the objects only need to be connected together, and this can be achieved with some simple QObject::connect() function calls, or with uic's automatic connections feature.

Building the Example

The C++ preprocessor changes or removes the signals, slots, and emit keywords so that the compiler is presented with standard C++.

By running the moc on class definitions that contain signals or slots, a C++ source file is produced which should be compiled and linked with the other object files for the application. If you use qmake, the makefile rules to automatically invoke moc will be added to your project's makefile.

Signals

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Only the class that defines a signal and its subclasses can emit the signal.

When a signal is emitted, the slots connected to it are usually executed immediately, just like a normal function call. When this happens, the signals and slots mechanism is totally independent of any GUI event loop. Execution of the code following the emit statement will occur once all slots have returned. The situation is slightly different when using queued connections; in such a case, the code following the emit keyword will continue immediately, and the slots will be executed later.

If several slots are connected to one signal, the slots will be executed one after the other, in the order they have been connected, when the signal is emitted.

Signals are automatically generated by the moc and must not be implemented in the .cpp file. They can never have return types (i.e. use void).

A note about arguments: Our experience shows that signals and slots are more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as the hypothetical QScrollBar::Range, it could only be connected to slots designed specifically for QScrollBar. Connecting different input widgets together would be impossible.

Slots

Qt Signal Slot Different Threads

A slot is called when a signal connected to it is emitted. Slots are normal C++ functions and can be called normally; their only special feature is that signals can be connected to them.

Since slots are normal member functions, they follow the normal C++ rules when called directly. However, as slots, they can be invoked by any component, regardless of its access level, via a signal-slot connection. This means that a signal emitted from an instance of an arbitrary class can cause a private slot to be invoked in an instance of an unrelated class.

You can also define slots to be virtual, which we have found quite useful in practice.

Compared to callbacks, signals and slots are slightly slower because of the increased flexibility they provide, although the difference for real applications is insignificant. In general, emitting a signal that is connected to some slots, is approximately ten times slower than calling the receivers directly, with non-virtual function calls. This is the overhead required to locate the connection object, to safely iterate over all connections (i.e. checking that subsequent receivers have not been destroyed during the emission), and to marshall any parameters in a generic fashion. While ten non-virtual function calls may sound like a lot, it's much less overhead than any new or delete operation, for example. As soon as you perform a string, vector or list operation that behind the scene requires new or delete, the signals and slots overhead is only responsible for a very small proportion of the complete function call costs.

The same is true whenever you do a system call in a slot; or indirectly call more than ten functions. On an i586-500, you can emit around 2,000,000 signals per second connected to one receiver, or around 1,200,000 per second connected to two receivers. The simplicity and flexibility of the signals and slots mechanism is well worth the overhead, which your users won't even notice.

Note that other libraries that define variables called signals or slots may cause compiler warnings and errors when compiled alongside a Qt-based application. To solve this problem, #undef the offending preprocessor symbol.

Meta-Object Information

The meta-object compiler (moc) parses the class declaration in a C++ file and generates C++ code that initializes the meta-object. The meta-object contains the names of all the signal and slot members, as well as pointers to these functions.

The meta-object contains additional information such as the object's class name. You can also check if an object inherits a specific class, for example:

The meta-object information is also used by qobject_cast<T>(), which is similar to QObject::inherits() but is less error-prone:

See Meta-Object System for more information.

A Real Example

Qt Signals Slots Threads Example

Here is a simple commented example of a widget.

LcdNumber inherits QObject, which has most of the signal-slot knowledge, via QFrame and QWidget. It is somewhat similar to the built-in QLCDNumber widget.

The Q_OBJECT macro is expanded by the preprocessor to declare several member functions that are implemented by the moc; if you get compiler errors along the lines of 'undefined reference to vtable for LcdNumber', you have probably forgotten to run the moc or to include the moc output in the link command.

It's not obviously relevant to the moc, but if you inherit QWidget you almost certainly want to have the parent argument in your constructor and pass it to the base class's constructor.

Some destructors and member functions are omitted here; the moc ignores member functions.

LcdNumber emits a signal when it is asked to show an impossible value.

If you don't care about overflow, or you know that overflow cannot occur, you can ignore the overflow() signal, i.e. don't connect it to any slot.

If on the other hand you want to call two different error functions when the number overflows, simply connect the signal to two different slots. Qt will call both (in the order they were connected).

A slot is a receiving function used to get information about state changes in other widgets. LcdNumber uses it, as the code above indicates, to set the displayed number. Since display() is part of the class's interface with the rest of the program, the slot is public.

Several of the example programs connect the valueChanged() signal of a QScrollBar to the display() slot, so the LCD number continuously shows the value of the scroll bar.

Note that display() is overloaded; Qt will select the appropriate version when you connect a signal to the slot. With callbacks, you'd have to find five different names and keep track of the types yourself.

Some irrelevant member functions have been omitted from this example.

Signals And Slots With Default Arguments

The signatures of signals and slots may contain arguments, and the arguments can have default values. Consider QObject::destroyed():

When a QObject is deleted, it emits this QObject::destroyed() signal. We want to catch this signal, wherever we might have a dangling reference to the deleted QObject, so we can clean it up. A suitable slot signature might be:

To connect the signal to the slot, we use QObject::connect() and the SIGNAL() and SLOT() macros. The rule about whether to include arguments or not in the SIGNAL() and SLOT() macros, if the arguments have default values, is that the signature passed to the SIGNAL() macro must not have fewer arguments than the signature passed to the SLOT() macro.

All of these would work:

But this one won't work:

...because the slot will be expecting a QObject that the signal will not send. This connection will report a runtime error.

Advanced Signals and Slots Usage

For cases where you may require information on the sender of the signal, Qt provides the QObject::sender() function, which returns a pointer to the object that sent the signal.

The QSignalMapper class is provided for situations where many signals are connected to the same slot and the slot needs to handle each signal differently.

Suppose you have three push buttons that determine which file you will open: 'Tax File', 'Accounts File', or 'Report File'.

In order to open the correct file, you use QSignalMapper::setMapping() to map all the clicked() signals to a QSignalMapper object. Then you connect the file's QPushButton::clicked() signal to the QSignalMapper::map() slot.

Then, you connect the mapped() signal to readFile() where a different file will be opened, depending on which push button is pressed.

Qt Signal Slot Different Threads

Note: The following code will compile and run, but due to signature normalization, the code will be slower.

Using Qt with 3rd Party Signals and Slots

It is possible to use Qt with a 3rd party signal/slot mechanism. You can even use both mechanisms in the same project. Just add the following line to your qmake project (.pro) file.

It tells Qt not to define the moc keywords signals, slots, and emit, because these names will be used by a 3rd party library, e.g. Boost. Then to continue using Qt signals and slots with the no_keywords flag, simply replace all uses of the Qt moc keywords in your sources with the corresponding Qt macros Q_SIGNALS (or Q_SIGNAL), Q_SLOTS (or Q_SLOT), and Q_EMIT.

See also Meta-Object System and Qt's Property System.

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A short history

Long long ago, subclass QThread and reimplement its run() function is the only recommended way of using QThread. This is rather intuitive and easy to used. But when SLOTS and Qt event loop are used in the worker thread, some users do it wrong. So Bradley T. Hughes, one of the Qt core developers, recommend that use worker objects by moving them to the thread using QObject::moveToThread . Unfortunately, some users went on a crusade against the former usage. So Olivier Goffart, one of the former Qt core developers, tell the subclass users: You were not doing so wrong. Finally, we can find both usages in the documentation of QThread.

QThread::run() is the thread entry point

From the Qt Documentation, we can see that

A QThread instance represents a thread and provides the means to start() a thread, which will then execute the reimplementation of QThread::run(). The run() implementation is for a thread what the main() entry point is for the application.

As QThread::run() is the thread entry point, it is rather intuitive to use the Usage 1.

Usage 1-0

To run some code in a new thread, subclass QThread and reimplement its run() function.

For example

The output more or less look like:

Usage 1-1

As QThread::run() is the thread entry point, so it easy to undersand that, all the codes that are not get called in the run() function directly won't be executed in the worker thread.

In the following example, the member variable m_stop will be accessed by both stop() and run(). Consider that the former will be executed in main thread while the latter is executed in worker thread, mutex or other facility is needed.

The output is more or less like

You can see that the Thread::stop() is executed in the main thread.

Usage 1-2 (Wrong Usage)

Though above examples are easy to understand, but it's not so intuitive when event system(or queued-connection) is introduced in worker thread.

For example, what should we do if we want to do something periodly in the worker thread?

  • Create a QTimer in the Thread::run()
  • Connect the timeout signal to the slot of Thread

At first glance, the code seems fine. When the thread starts executing, we setup a QTimer thats going to run in the current thread's event queue. We connect the onTimeout() to the timeout signal. Then we except it works in the worker thread?

But, the result of the example is

Oh, No!!! They get called in the main thread instead of the work thread.

Very interesting, isn't it? (We will discuss what happened behined this in next blog)

How to solve this problem

In order to make the this SLOT works in the worker thread, some one pass the Qt::DirectConnection to the connect() function,

and some other add following line to the thread constructor.

Both of them work as expected. But ...

The second usage is wrong,

Even though this seems to work, it’s confusing, and not how QThread was designed to be used(all of the functions in QThread were written and intended to be called from the creating thread, not the thread that QThread starts)

In fact, according to above statements, the first workaround is wrong too. As onTimeout() which is a member of our Thread object, get called from the creating thread too.

Both of them are bad uasge?! what should we do?

Usage 1-3

As none of the member of QThread object are designed to be called from the worker thread. So we must create an independent worker object if we want to use SLOTS.

Qt Signals And Slots Tutorial

The result of the application is

Problem solved now!

Though this works perfect, but you may have notice that, when event loop QThread::exec() is used in the worker thread, the code in the QThread::run() seems has nothing to do with QThread itself.

So can we move the object creation out of the QThread::run(), and at the same time, the slots of they will still be called by the QThread::run()?

Usage 2-0

Qt Signal Slot Threads

If we only want to make use of QThread::exec(), which has been called by QThread::run() by default, there will be no need to subclass the QThread any more.

  • Create a Worker object
  • Do signal and slot connections
  • Move the Worker object to a sub-thread
  • Start thread

The result is:

As expected, the slot doesn't run in the main thread.

In this example, both of the QTimer and Worker are moved to the sub-thread. In fact, moving QTimer to sub-thread is not required.

Simply remove the line timer.moveToThread(&t); from above example will work as expected too.

The difference is that:

In last example,

  • The signal timeout() is emitted from sub-thread
  • As timer and worker live in the same thread, their connection type is direct connection.
  • The slot get called in the same thead in which signal get emitted.

While in this example,

  • The signal timeout() emitted from main thread,
  • As timer and worker live in different threads, their connection type is queued connection.
  • The slot get called in its living thread, which is the sub-thread.

Qt Signal Slot Performance

Thanks to a mechanism called queued connections, it is safe to connect signals and slots across different threads. If all the across threads communication are done though queued connections, the usual multithreading precautions such as QMutex will no longer need to be taken.

In short

  • Subclass QThread and reimplement its run() function is intuitive and there are still many perfectly valid reasons to subclass QThread, but when event loop is used in worker thread, it's not easy to do it in the right way.
  • Use worker objects by moving them to the thread is easy to use when event loop exists, as it has hidden the details of event loop and queued connection.

Reference

  • http://blog.qt.digia.com/blog/2010/06/17/youre-doing-it-wrong/
  • http://woboq.com/blog/qthread-you-were-not-doing-so-wrong.html
  • http://ilearnstuff.blogspot.com/2012/08/when-qthread-isnt-thread.html