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. Signals and slots are made possible by Qt's meta-object system .

Signals and slots¶. In Qt, signals and slots are a way for objects to communicate with eachother. A signal is emitted when some event occurs in the program. For example, when a button on the UI is pushed, the button (a QPushButton widget) notifys the other widgets and objects about this by emitting a signal (emit clicked).A slot is a function called as a response to an emitted signal. If the signal is a Qt signal, the type names must be the C type names, such as int and QString. C type names can be rather complex, with each type name possibly including one or more of const,., and &. When we write them as signal (or slot) signatures we can drop any consts and &s, but must keep any.s. Slots are assigned and overloaded using the decorator QtCore.Slot. Again, to define a signature just pass the types like the QtCore.Signal class. Unlike the Signal class, to overload a function, you don't pass every variation as tuple or list. Instead, you have to define a new decorator for every different signature. Old syntax Qt 5 continues to support the old string-based syntax for connecting signals and slots defined in a QObject or any class that inherits from QObject (including QWidget) connect (sender, SIGNAL (valueChanged (QString, QString)), receiver, SLOT (updateValue (QString))); New: connecting to QObject member.

Introduction

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.

Other 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. While successful frameworks using this method do exist, callbacks can be unintuitive and may suffer from problems in ensuring the type-correctness of callback arguments.

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.

Signals and slots is a language construct introduced in Qt for communication between objects which makes it easy to implement the observer pattern while avoiding boilerplate code.The concept is that GUI widgets can send signals containing event information which can be received by other widgets / controls using special functions known as slots. This is similar to C/C function pointers, but.

Signals and slots in Qt

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 when using the function pointer-based syntax. The string-based SIGNAL and SLOT syntax will detect type mismatches at runtime. 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.

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. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

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

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. 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.

Connecting the signal to the slot

Prior to the fifth version of Qt to connect the signal to the slot through the recorded macros, whereas in the fifth version of the recording has been applied, based on the signs.

Writing with macros:

Writing on the basis of indicators:

The advantage of the second option is that it is possible to determine the mismatch of signatures and the wrong slot or signal name of another project compilation stage, not in the process of testing applications.

An example of using signals and slots

For example, the use of signals and slots project was created, which in the main window contains three buttons, each of which is connected to the slot and these slots already transmit a signal in a single slot with the pressed button number.

Project Structure

Project Structure

According to the tradition of conducting lessons enclosing structure of the project, which is absolutely trivial and defaulted to the disgrace that will not even describe members of her classes and files.

mainwindow.h

Thus, the following three buttons - three slots, one signal at all three buttons, which is fed into the slot button and transmits the number buttons into a single slot that displays a message with the number buttons.

mainwindow.cpp

A file in this logic is configured as described in the preceding paragraphs. Just check the code and go to the video page, there is shown in detail the whole process, demonstrated the application, and also shows what happens if we make coding a variety of errors.

Video

The QThread class provides a platform-independent way to managethreads. More...

Inherits QObject.

Types

• enum Priority { IdlePriority, LowestPriority, LowPriority, NormalPriority, ..., InheritPriority }

Methods

Qt Signals And Slots Tutorial

• int exec_ (self)
• bool isFinished (self)
• Priority priority (self)
• run (self)
• setStackSize (self, int stackSize)
• start (self, Priority priority = QThread.InheritPriority)
• bool wait (self, int msecs = ULONG_MAX)

Static Methods

• int currentThreadId ()
• msleep (int)
• sleep (int)
• yieldCurrentThread ()

Qt Signals

• void started ()

Detailed Description

The QThread class provides a platform-independent way to managethreads.

A QThread object manages one thread of control within theprogram. QThreads begin executing in run(). By default, run() starts the event loop by callingexec_() and runs a Qt event loopinside the thread.

You can use worker objects by moving them to the thread usingQObject.moveToThread().

The code inside the Worker's slot would then execute in aseparate thread. However, you are free to connect the Worker'sslots to any signal, from any object, in any thread. It is safe toconnect signals and slots across different threads, thanks to amechanism called queuedconnections.

Another way to make code run in a separate thread, is tosubclass QThread and reimplement run(). For example:

In that example, the thread will exit after the run function hasreturned. There will not be any event loop running in the threadunless you call exec_().

It is important to remember that a QThread instance lives in the old thread thatinstantiated it, not in the new thread that calls run(). This means that all of QThread'squeued slots will execute in the old thread. Thus, a developer whowishes to invoke slots in the new thread must use the worker-objectapproach; new slots should not be implemented directly into asubclassed QThread.

When subclassing QThread, keep in mind that the constructorexecutes in the old thread while run() executes in the new thread. If amember variable is accessed from both functions, then the variableis accessed from two different threads. Check that it is safe to doso.

Note: Care must be taken when interacting with objectsacross different threads. See Synchronizing Threads fordetails.

Managing threads

QThread will notifiy you via a signal when the thread isstarted(), finished(), and terminated(), or you can use isFinished() and isRunning() to query the state of thethread.

You can stop the thread by calling exit() or quit(). In extreme cases, you may want toforcibly terminate() anexecuting thread. However, doing so is dangerous and discouraged.Please read the documentation for terminate() and setTerminationEnabled()for detailed information.

From Qt 4.8 onwards, it is possible to deallocate objects thatlive in a thread that has just ended, by connecting the finished() signal to QObject.deleteLater().

Use wait() to block the callingthread, until the other thread has finished execution (or until aspecified time has passed).

The static functions currentThreadId() and currentThread() return identifiersfor the currently executing thread. The former returns a platformspecific ID for the thread; the latter returns a QThreadpointer.

To choose the name that your thread will be given (as identifiedby the command ps -L on Linux, for example), you can callsetObjectName() beforestarting the thread. If you don't call setObjectName(), the name givento your thread will be the class name of the runtime type of yourthread object (for example, 'RenderThread' in the case ofthe Mandelbrot Example, asthat is the name of the QThread subclass). Note that this iscurrently not available with release builds on Windows.

QThread also provides static, platform independent sleepfunctions: sleep(), msleep(), and usleep() allow full second, millisecond,and microsecond resolution respectively.

Note:wait() and thesleep() functions should beunnecessary in general, since Qt is an event-driven framework.Instead of wait(), considerlistening for the finished()signal. Instead of the sleep()functions, consider using QTimer.

{Mandelbrot Example}, {Semaphores Example}, {Wait ConditionsExample}

Type Documentation

QThread.Priority

Method Documentation

QThread.__init__ (self, QObjectparent = None)

The parent argument, if not None, causes self to be owned by Qt instead of PyQt.

Constructs a new QThread to manage anew thread. The parent takes ownership of the QThread. The thread does not begin executinguntil start() is called.

See alsostart().

QThread QThread.currentThread ()

Returns a pointer to a QThread whichmanages the currently executing thread.

int QThread.currentThreadId ()

int QThread.exec_ (self)

Enters the event loop and waits until exit() is called, returning the value thatwas passed to exit(). The valuereturned is 0 if exit() is calledvia quit().

This function is meant to be called from within run(). It is necessary to call this functionto start event handling.

See alsoquit() andexit().

QThread.exit (self, int returnCode = 0)

After calling this function, the thread leaves the event loopand returns from the call to QEventLoop.exec(). The QEventLoop.exec() function returnsreturnCode.

By convention, a returnCode of 0 means success, anynon-zero value indicates an error.

Note that unlike the C library function of the same name, thisfunction does return to the caller -- it is event processingthat stops.

No QEventLoops will be started anymore in this thread untilQThread.exec() has been calledagain. If the eventloop in QThread.exec() is not running then thenext call to QThread.exec() willalso return immediately.

See alsoquit() andQEventLoop.

int QThread.idealThreadCount ()

bool QThread.isFinished (self)

See alsoisRunning().

bool QThread.isRunning (self)

See alsoisFinished().

QThread.msleep (int)

See alsosleep() andusleep().

Priority QThread.priority (self)

Returns the priority for a running thread. If the thread is notrunning, this function returns InheritPriority.

Qt Signal Slot Different Classifications

This function was introduced in Qt 4.1.

See alsoPriority, setPriority(), and start().

QThread.quit (self)

See alsoexit() andQEventLoop.

QThread.run (self)

The starting point for the thread. After calling start(), the newly created thread callsthis function. The default implementation simply calls exec_().

You can reimplement this function to facilitate advanced threadmanagement. Returning from this method will end the execution ofthe thread.

See alsostart() andwait().

QThread.setPriority (self, Prioritypriority)

This function sets the priority for a running thread. Ifthe thread is not running, this function does nothing and returnsimmediately. Use start() to starta thread with a specific priority.

The priority argument can be any value in theQThread.Priority enum except forInheritPriorty.

The effect of the priority parameter is dependent on theoperating system's scheduling policy. In particular, thepriority will be ignored on systems that do not supportthread priorities (such as on Linux, seehttp://linux.die.net/man/2/sched_setscheduler for moredetails).

Qt Signal Slot Not Working

This function was introduced in Qt 4.1.

See alsoPriority, priority(), and start().

QThread.setStackSize (self, int stackSize)

See alsostackSize().

QThread.setTerminationEnabled (bool enabled = True)

Enables or disables termination of the current thread based onthe enabled parameter. The thread must have been started byQThread.

When enabled is false, termination is disabled. Futurecalls to QThread.terminate()will return immediately without effect. Instead, the termination isdeferred until termination is enabled.

When enabled is true, termination is enabled. Futurecalls to QThread.terminate()will terminate the thread normally. If termination has beendeferred (i.e. QThread.terminate() was called withtermination disabled), this function will terminate the callingthread immediately. Note that this function will not returnin this case.

See alsoterminate().

QThread.sleep (int)

How Qt Signal And Slots Works

See alsomsleep() andusleep().

int QThread.stackSize (self)

Returns the maximum stack size for the thread (if set withsetStackSize()); otherwisereturns zero.

See alsosetStackSize().

Qt Signal Slot Different Classification

QThread.start (self, Prioritypriority = QThread.InheritPriority)

This method is also a Qt slot with the C++ signature void start(QThread::Priority = QThread.InheritPriority).

Begins execution of the thread by calling run(). The operating system will schedulethe thread according to the priority parameter. If thethread is already running, this function does nothing.

The effect of the priority parameter is dependent on theoperating system's scheduling policy. In particular, thepriority will be ignored on systems that do not supportthread priorities (such as on Linux, seehttp://linux.die.net/man/2/sched_setscheduler for moredetails).

See alsorun() andterminate().

QThread.terminate (self)

Terminates the execution of the thread. The thread may or maynot be terminated immediately, depending on the operating system'sscheduling policies. Listen for the terminated() signal, or use QThread.wait() after terminate(), to besure.

When the thread is terminated, all threads waiting for thethread to finish will be woken up.

Warning: This function is dangerous and its use isdiscouraged. The thread can be terminated at any point in its codepath. Threads can be terminated while modifying data. There is nochance for the thread to clean up after itself, unlock any heldmutexes, etc. In short, use this function only if absolutelynecessary.

Termination can be explicitly enabled or disabled by callingQThread.setTerminationEnabled().Calling this function while termination is disabled results in thetermination being deferred, until termination is re-enabled. Seethe documentation of QThread.setTerminationEnabled()for more information.

Qt Signal Slot Parameter

See alsosetTerminationEnabled().

QThread.usleep (int)

See alsosleep() andmsleep().

bool QThread.wait (self, int msecs = ULONG_MAX)

• The thread associated with this QThread object has finished execution (i.e. whenit returns from run()). Thisfunction will return true if the thread has finished. It alsoreturns true if the thread has not been started yet.
• time milliseconds has elapsed. If time isULONG_MAX (the default), then the wait will never timeout (thethread must return from run()). Thisfunction will return false if the wait timed out.

This provides similar functionality to the POSIXpthread_join() function.

See alsosleep() andterminate().

QThread.yieldCurrentThread ()

Qt Signal Documentation

void finished ()

See alsostarted() andterminated().

void started ()

See alsofinished()and terminated().

void terminated ()

See alsostarted() andfinished().