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.

QT Signal and slot arguments are not compatible. QT编译错误: Signal and slot arguments are not compatible. 原因如下：信号和插槽参数不兼容。.

Signals and Slots in PySide. From Qt Wiki (Redirected from Signals and slots in PySide) Redirect page. Redirect to: Qt for Python Signals and Slots. The signal/slot argument lists is not compatible (.) The elements marked with a (.) do not fully apply to the new connection syntax, which performs checks at compile-time. When QObject::connect fails it returns false and it emits a warning which gets typically printed on the console, if you have one, or in Creator’s output pane. A frequent question coming up when programming with PyQt is how to pass extra arguments to slots. After all, the signal-slot connection mechanism only specifies how to connect a signal to a slot - the signal's arguments are passed to the slot, but no additional (user-defined) arguments may be directly passed. Connect(protocolb, SIGNAL(RequestUpdatePlot(QVector, QVector)), plotb, SLOT(UpdatePlot(QVector, QVector))); I've tried looking up elsewhere on line how to connect signals to slots when there are arguments in the calls. I tried changing the connect call to the new style offered in Qt 5.4.

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

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

A minimal C++ class declaration might read:

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 t

Qt Signal And Slot Arguments Are Not Compatible