Qt Signal Emitted But Slot Not Called

The following sources have been used in making this material:

1. Qt Signal Emitted But Slot Not Called One
2. Qt Signal Emitted But Slot Not Called Two
3. Qt Signal Emitted But Slot Not Called The Most
4. Qt Signal Emitted But Slot Not Called Light

If your knowledge on signals and slots is rusty or they are a completely new concept to you, you can get started with the Programming 2 material on Qt basics.

Signals and slots¶

If the object being registered on the D-Bus is a regular QObject with statically defined slots, it works correctly. The signals are then emitted through QMetaObject::activate, which uses different logic to determine if anyone is connected to the signal, which correctly count the QDBusConnection as being connected.

1. QSignal - A QT Signal/slot for python. This project provides easy to use Signal class for implementing Signal/Slot mechanism in Python. It does not implement it strictly but rather creates the easy and simple alternative. Qsignal.Signal is the main class. To create a signal, just make a sig = qsignal.Signal.
2. 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.
3. If the signal is not getting emitted, obviously the slot will never be called. Once more, a breakpoint on the signal itself (remember that signals are ordinary member functions) or a lambda connected to it, can immediately show whether the signal is being emitted or not. Is the connection queued?

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 (emitclicked()). A slot is a function called as a response to an emitted signal. For example, a slot findClicked() is executed as a response to a button click.

All subclasses of QObject or one of its subclasses can contain signals and slots. In practice, two things are required for the signal and slot mechanism to work. All classes that contain signals or slots must:

• mention the Q_OBJECT macro at the top of their declaration.
• be derived from QObject

Objects emit signals when their state changes in a way that may be interesting to other objects. The objects do not need to know anything about eachother. In fact, the only thing the object does is emit the signal. It does not know, care or need to know, if anyone receives the signals emitted and vice versa, a slot does not know if any signals are connected to it. Qt's signal and slot mechanism makes sure that if the signal is connected to a slot, the slot is automatically called with the signal's parameters.

One signal can be connected to as many slots as needed. If more than one slot is connected, they are called when the signal is emitted one after another in the same order they were connected. Similarly, more than one signal can be connected to the same slot. The slot is called when any of the signals are emitted. A signal can even be connected to another signal to emit the second signal as the first is emitted.

Signals¶

Qt's widgets have predefined signals. New signals can be defined in the class's signals section of the interface.

Signals must not be implemented. The meta-object compiler moc generates them automatically. A signal can never have a return type and is always void.

Slots¶

Slots are C++ functions that can be called normally as any other function. They can also be virtual if needed. In addition, signals can be connected to them. New slots are defined in the class's slots section of the interface.

The programmer implements the slots (slots are C++ functions). Similarly to functions, slots can be either public or private.

Connecting¶

Signals and slots are connected with QObject::connect(). The preferred syntax is functor based i.e.

where sender is a pointer to the QObject object emitting the signal and receiver is a pointer to the QObject object containing the slot. The second and fourth parameters are the signal and the slot respectively. The biggest benefits with this syntax is the compile-time type checking and the possibility to use lambdas as a part of the connect.

The other way to connect a signal to a slot is to use the SIGNAL and SLOT macros. This is typically referred as the old syntax.

where the SIGNAL and SLOT macros convert their parameters to strings. Type checking is left as a run time operation. However, you can connect C++ functions to QML function with this syntax.

Meta-object System¶

The Qt's meta-object system in Qt enables among other things signals and slots. It is based on three things:

# The QObject class as the a base class for objects that can take advantage of the meta-object system.# The Q_OBJECT macro at the beginning of the class declaration is used to enable meta-object features.# The Meta-Object Compiler (moc) supplies each of the QObject subclasses with the necessary code to implement meta-object features. The moc tool reads a C++ source file. For the classes that contain the Q_OBJECTmacro, it produces another C++ source file containing the meta-object code for each class. This generated source file is then typically compiled and linked with the class's implementation.

It is possible to use QObject as a base class without the Q_OBJECT macro. This means that signals, slots and the other meta-object features will not be available. For the meta-object system, a QObject subclass without meta code is equivalent to its closest ancestor with meta-object code. Therefore, all subclasses of QObject should use the Q_OBJECT macro even if they do not use signals, slots, and other meta-object system properties.

Home | All Classes | Main Classes | Annotated | Grouped Classes | Functions

Signals and slots are used for communication between objects. Thesignal/slot mechanism is a central feature of Qt and probably thepart that differs most from other toolkits.

In GUI programming we often want a change in one widget to be notifiedto another widget. More generally, we want objects of any kind to beable to communicate with one another. For example if we were parsingan XML file we might want to notify a list view that we're using torepresent the XML file's structure whenever we encounter a new tag.

Older toolkits achieve this kind of communication using callbacks. Acallback is a pointer to a function, so if you want a processingfunction to notify you about some event you pass a pointer to anotherfunction (the callback) to the processing function. The processingfunction then calls the callback when appropriate. Callbacks have twofundamental flaws. Firstly they are not type safe. We can never becertain that the processing function will call the callback with thecorrect arguments. Secondly the callback is strongly coupled to theprocessing function since the processing function must know whichcallback to call.

An abstract view of some signals and slots connections

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

The signals and slots mechanism is type safe: the signature of asignal must match the signature of the receiving slot. (In fact a slotmay have a shorter signature than the signal it receives because itcan ignore extra arguments.) Since the signatures are compatible, thecompiler can help us detect type mismatches. Signals and slots areloosely coupled: a class which emits a signal neither knows nor careswhich slots receive the signal. Qt's signals and slots mechanismensures that if you connect a signal to a slot, the slot will becalled with the signal's parameters at the right time. Signals andslots can take any number of arguments of any type. They arecompletely typesafe: no more callback core dumps!

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

An example of signals and slots connections

Qt Signal Emitted But Slot Not Called One

Slots can be used for receiving signals, but they are also normalmember functions. Just as an object does not know if anything receivesits signals, a slot does not know if it has any signals connected toit. This ensures that truly independent components can be created withQt.

You can connect as many signals as you want to a single slot, and asignal can be connected to as many slots as you desire. It is evenpossible to connect a signal directly to another signal. (This willemit the second signal immediately whenever the first is emitted.)

Together, signals and slots make up a powerful component programmingmechanism.

A Small Example

Qt Signal Emitted But Slot Not Called Two

A minimal C++ class declaration might read:

Qt Signal Emitted But Slot Not Called The Most

A small Qt class might read:

This class has the same internal state, and public methods to access thestate, but in addition it has support for component programming usingsignals and slots: this class can tell the outside world that its statehas changed by emitting a signal, valueChanged(), and it hasa slot which other objects can send signals to.

All classes that contain signals or slots must mention Q_OBJECT intheir declaration.

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

The line emit valueChanged(v) emits the signalvalueChanged from the object. As you can see, you emit asignal by using emit signal(arguments).

Here is one way to connect two of these objects together:

Calling a.setValue(79) will make a emit a valueChanged()signal, which b will receive in its setValue() slot,i.e. b.setValue(79) is called. b will then, in turn,emit the same valueChanged() signal, but since no slot has beenconnected to b's valueChanged() signal, nothing happens (thesignal is ignored).

Note that the setValue() function sets the value and emitsthe signal only if v != val. This prevents infinite loopingin the case of cyclic connections (e.g. if b.valueChanged()were connected to a.setValue()).

A signal is emitted for every connection you make, so if youduplicate a connection, two signals will be emitted. You can alwaysbreak a connection using QObject::disconnect().

This example illustrates that objects can work together without knowingabout each other, as long as there is someone around to set up aconnection between them initially.

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

Run the moc on class definitions that containsignals or slots. This produces a C++ source file which should be compiledand linked with the other object files for the application. If you useqmake, the makefile rules toautomatically invoke the moc will be added toyour makefile for you.

Signals

Signals are emitted by an object when its internal state has changedin 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 thesignal.

A list box, for example, emits both clicked() andcurrentChanged() signals. Most objects will probably only beinterested in currentChanged() which gives the current list itemwhether the user clicked it or used the arrow keys to move to it. Butsome objects may only want to know which item was clicked. If thesignal is interesting to two different objects you just connect thesignal to slots in both objects.

When a signal is emitted, the slots connected to it are executedimmediately, just like a normal function call. The signal/slotmechanism is totally independent of any GUI event loop. Theemit will return when all slots have returned.

If several slots are connected to one signal, the slots will beexecuted one after the other, in an arbitrary order, when the signalis emitted.

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

A note about arguments. Our experience shows that signals and slotsare more reusable if they do not use special types. If QScrollBar::valueChanged() were to use a special type such as thehypothetical QRangeControl::Range, it could only be connected toslots designed specifically for QRangeControl. Something as simple asthe program in Tutorial #1 part 5would be impossible.

Slots

A slot is called when a signal connected to it is emitted. Slots arenormal C++ functions and can be called normally; their only specialfeature is that signals can be connected to them. A slot's argumentscannot have default values, and, like signals, it is rarely wise touse your own custom types for slot arguments.

Since slots are normal member functions with just a little extraspice, they have access rights like ordinary member functions. Aslot's access right determines who can connect to it:

A public slots section contains slots that anyone can connectsignals to. This is very useful for component programming: you createobjects that know nothing about each other, connect their signals andslots so that information is passed correctly, and, like a modelrailway, turn it on and leave it running.

A protected slots section contains slots that this class and itssubclasses may connect signals to. This is intended for slots that arepart of the class's implementation rather than its interface to therest of the world.

A private slots section contains slots that only the class itselfmay connect signals to. This is intended for very tightly connectedclasses, where even subclasses aren't trusted to get the connectionsright.

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

The signals and slots mechanism is efficient, but not quite as fast as'real' callbacks. Signals and slots are slightly slower because of theincreased flexibility they provide, although the difference for realapplications is insignificant. In general, emitting a signal that isconnected to some slots, is approximately ten times slower thancalling the receivers directly, with non-virtual function calls. Thisis the overhead required to locate the connection object, to safelyiterate over all connections (i.e. checking that subsequent receivershave not been destroyed during the emission) and to marshall anyparameters in a generic fashion. While ten non-virtual function callsmay 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 avery small proportion of the complete function call costs. The same istrue whenever you do a system call in a slot; or indirectly call morethan ten functions. On an i586-500, you can emit around 2,000,000signals per second connected to one receiver, or around 1,200,000 persecond connected to two receivers. The simplicity and flexibility ofthe signals and slots mechanism is well worth the overhead, which yourusers won't even notice.

Meta Object Information

The meta object compiler (moc) parses the classdeclaration in a C++ file and generates C++ code that initializes themeta object. The meta object contains the names of all the signal andslot members, as well as pointers to these functions. (For moreinformation on Qt's Meta Object System, see Whydoesn't Qt use templates for signals and slots?.)

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

A Real Example

Here is a simple commented example (code fragments from qlcdnumber.h ).

QLCDNumber inherits QObject, which has most of the signal/slotknowledge, via QFrame and QWidget, and #include's the relevantdeclarations.

Q_OBJECT is expanded by the preprocessor to declare several memberfunctions that are implemented by the moc; if you get compiler errorsalong the lines of 'virtual function QButton::className not defined'you have probably forgotten to run the moc or toinclude the moc output in the link command.

Qt Signal Emitted But Slot Not Called Light

It's not obviously relevant to the moc, but if you inherit QWidget youalmost certainly want to have the parent and namearguments in your constructors, and pass them to the parentconstructor.

Some destructors and member functions are omitted here; the mocignores member functions.

QLCDNumber emits a signal when it is asked to show an impossiblevalue.

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

If, on the other hand, you want to call two different error functionswhen the number overflows, simply connect the signal to two differentslots. Qt will call both (in arbitrary order).

A slot is a receiving function, used to get information about statechanges in other widgets. QLCDNumber uses it, as the code aboveindicates, to set the displayed number. Since display() is partof the class's interface with the rest of the program, the slot ispublic.

Several of the example programs connect the newValue() signal of aQScrollBar to the display() slot, so the LCD number continuously showsthe value of the scroll bar.

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

Some irrelevant member functions have been omitted from thisexample.