Qt Plugin Signal Slot
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In this example, we simply extend the Custom Widget Plugin example and its custom widget (based on the Analog Clock example), by introducing the concept of signals and slots.
QML is designed to be easily extensible to and from C. The classes in the Qt Declarative module allow QML components to be loaded and manipulated from C, and through Qt’s meta-object system, QML and C objects can easily communicate through Qt signals and slots.In addition, QML plugins can be written to create reusable QML components for distribution. Due to the plugin system in Qt does not supports signal/slots system, it’s hard for the main program to detect plugin’s sending buffer command. So in the current implementation, the main program send the handler of current connection to the plugin, and the plugin can do write operations with the handler. Qt 5 How to use the signal slot mechanism in the plugin, Programmer Sought. Qt 5 How to use the signal slot mechanism in the plugin. Tags: Qt Plugin Signal slot. For a large system, how to ensure scalability and maintainability is very important. Qt provides us with a plug-in system that can better solve the scalability problem. Qt/C - Lesson 024. Signals and Slot in Qt5. 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. QGIS 3 Plugins - Qt Designer for Plugins. Qt Designer is an easy-to-use program to build UI’s for Qt frameworks. Luckily, QGIS ships the program with its core on all operating systems and should be available as an executable on your computer.
The World Time Clock Plugin example consists of two classes:
*WorldTimeClock is a custom clock widget with hour and minute hands that is automatically updated every few seconds.
*WorldTimeClockPlugin exposes the WorldTimeClock class to Qt Designer.
First we will take a look at the WorldTimeClock class which extends the Custom Widget Plugin example’s AnalogClock class by providing a signal and a slot. Then we will take a quick look at the WorldTimeClockPlugin class, but this class is in most parts identical to the Custom Widget Plugin example’s implementation.
Finally we take a look at the plugin’s project file. The project file for custom widget plugins needs some additional information to ensure that they will work within Qt Designer. This is also covered in the Custom Widget Plugin example, but due to its importance (custom widget plugins rely on components supplied with Qt Designer which must be specified in the project file that we use) we will repeat it here.WorldTimeClock Class
The WorldTimeClock class inherits QWidget, and is a custom clock widget with hour and minute hands that is automatically updated every few seconds. What makes this example different from the Custom Widget Plugin example, is the introduction of the signal and slot in the custom widget class:
Slots download pc. Note the use of the QDESIGNER_WIDGET_EXPORT macro. This is needed to ensure that Qt Designer can create instances of the widget on some platforms, but it is a good idea to use it on all platforms.
We declare the setTimeZone() slot with an associated timeZoneOffset variable, and we declare an updated() signal which takes the current time as argument and is emitted whenever the widget is repainted.
In Qt Designer’s workspace we can then, for example, connect the WorldTimeClock widget’s updated() signal to a QTimeEdit’s setTime() slot using Qt Designer’s mode for editing signal and slots.
We can also connect a QSpinBox’s valueChanged() signal to the WorldTimeClock’s setTimeZone() slot.WorldTimeClockPlugin ClassQt Signals And Slots Tutorial
The WorldTimeClockPlugin class exposes the WorldTimeClock class to Qt Designer. Its definition is equivalent to the Custom Widget Plugin example’s plugin class which is explained in detail. The only part of the class definition that is specific to this particular custom widget is the class name:
The plugin class provides Qt Designer with basic information about our plugin, such as its class name and its include file. Furthermore it knows how to create instances of the WorldTimeClockPlugin widget. WorldTimeClockPlugin also defines the initialize() function which is called after the plugin is loaded into Qt Designer. The function’s QDesignerFormEditorInterface parameter provides the plugin with a gateway to all of Qt Designer’s API’s.
The WorldTimeClockPlugin class inherits from both QObject and QDesignerCustomWidgetInterface. It is important to remember, when using multiple inheritance, to ensure that all the interfaces (i.e. the classes that doesn’t inherit Q_OBJECT) are made known to the meta object system using the Q_INTERFACES() macro. This enables Qt Designer to use qobject_cast() to query for supported interfaces using nothing but a QObject pointer.
The implementation of the WorldTimeClockPlugin is also equivalent to the plugin interface implementation in the Custom Widget Plugin example (only the class name and the implementation of QDesignerCustomWidgetInterface::domXml() differ). The main thing to remember is to use the Q_EXPORT_PLUGIN2() macro to export the WorldTimeClockPlugin class for use with Qt Designer:
Without this macro, there is no way for Qt Designer to use the widget.The Project File: worldtimeclockplugin.pro
The project file for custom widget plugins needs some additional information to ensure that they will work as expected within Qt Designer:
The TEMPLATE variable’s value make qmake create the custom widget as a library. The CONFIG variable contains two values, designer and plugin:
*designer: Since custom widgets plugins rely on components supplied with Qt Designer, this value ensures that our plugin links against Qt Designer’s library (libQtDesigner.so).
*plugin: We also need to ensure that qmake considers the custom widget a plugin library.
When Qt is configured to build in both debug and release modes, Qt Designer will be built in release mode. When this occurs, it is necessary to ensure that plugins are also built in release mode. For that reason you might have to add a release value to your CONFIG variable. Otherwise, if a plugin is built in a mode that is incompatible with Qt Designer, it won’t be loaded and installed.
The header and source files for the widget are declared in the usual way, and in addition we provide an implementation of the plugin interface so that Qt Designer can use the custom widget.
It is important to ensure that the plugin is installed in a location that is searched by Qt Designer. We do this by specifying a target path for the project and adding it to the list of items to install:
The custom widget is created as a library, and will be installed alongside the other Qt Designer plugins when the project is installed (using make install or an equivalent installation procedure). Later, we will ensure that it is recognized as a plugin by Qt Designer by using the Q_EXPORT_PLUGIN2() macro to export the relevant widget information.Qt Public Slots
Note that if you want the plugins to appear in a Visual Studio integration, the plugins must be built in release mode and their libraries must be copied into the plugin directory in the install path of the integration (for an example, see C:/program files/trolltech as/visual studio integration/plugins).
For more information about plugins, see the How to Create Qt Plugins document.
Files:
© 2016 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.
An overview of Qt’s signals and slots inter-object communication mechanism.
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 .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.
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 queuedconnections ; 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 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.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 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 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 connect() function calls, or with uic’s automatic connections feature.A Real Example¶
The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.
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.Qt Plugin Signal Slot
After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), 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.Signals And Slot
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In this example, we simply extend the Custom Widget Plugin example and its custom widget (based on the Analog Clock example), by introducing the concept of signals and slots.
QML is designed to be easily extensible to and from C. The classes in the Qt Declarative module allow QML components to be loaded and manipulated from C, and through Qt’s meta-object system, QML and C objects can easily communicate through Qt signals and slots.In addition, QML plugins can be written to create reusable QML components for distribution. Due to the plugin system in Qt does not supports signal/slots system, it’s hard for the main program to detect plugin’s sending buffer command. So in the current implementation, the main program send the handler of current connection to the plugin, and the plugin can do write operations with the handler. Qt 5 How to use the signal slot mechanism in the plugin, Programmer Sought. Qt 5 How to use the signal slot mechanism in the plugin. Tags: Qt Plugin Signal slot. For a large system, how to ensure scalability and maintainability is very important. Qt provides us with a plug-in system that can better solve the scalability problem. Qt/C - Lesson 024. Signals and Slot in Qt5. 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. QGIS 3 Plugins - Qt Designer for Plugins. Qt Designer is an easy-to-use program to build UI’s for Qt frameworks. Luckily, QGIS ships the program with its core on all operating systems and should be available as an executable on your computer.
The World Time Clock Plugin example consists of two classes:
*WorldTimeClock is a custom clock widget with hour and minute hands that is automatically updated every few seconds.
*WorldTimeClockPlugin exposes the WorldTimeClock class to Qt Designer.
First we will take a look at the WorldTimeClock class which extends the Custom Widget Plugin example’s AnalogClock class by providing a signal and a slot. Then we will take a quick look at the WorldTimeClockPlugin class, but this class is in most parts identical to the Custom Widget Plugin example’s implementation.
Finally we take a look at the plugin’s project file. The project file for custom widget plugins needs some additional information to ensure that they will work within Qt Designer. This is also covered in the Custom Widget Plugin example, but due to its importance (custom widget plugins rely on components supplied with Qt Designer which must be specified in the project file that we use) we will repeat it here.WorldTimeClock Class
The WorldTimeClock class inherits QWidget, and is a custom clock widget with hour and minute hands that is automatically updated every few seconds. What makes this example different from the Custom Widget Plugin example, is the introduction of the signal and slot in the custom widget class:
Slots download pc. Note the use of the QDESIGNER_WIDGET_EXPORT macro. This is needed to ensure that Qt Designer can create instances of the widget on some platforms, but it is a good idea to use it on all platforms.
We declare the setTimeZone() slot with an associated timeZoneOffset variable, and we declare an updated() signal which takes the current time as argument and is emitted whenever the widget is repainted.
In Qt Designer’s workspace we can then, for example, connect the WorldTimeClock widget’s updated() signal to a QTimeEdit’s setTime() slot using Qt Designer’s mode for editing signal and slots.
We can also connect a QSpinBox’s valueChanged() signal to the WorldTimeClock’s setTimeZone() slot.WorldTimeClockPlugin ClassQt Signals And Slots Tutorial
The WorldTimeClockPlugin class exposes the WorldTimeClock class to Qt Designer. Its definition is equivalent to the Custom Widget Plugin example’s plugin class which is explained in detail. The only part of the class definition that is specific to this particular custom widget is the class name:
The plugin class provides Qt Designer with basic information about our plugin, such as its class name and its include file. Furthermore it knows how to create instances of the WorldTimeClockPlugin widget. WorldTimeClockPlugin also defines the initialize() function which is called after the plugin is loaded into Qt Designer. The function’s QDesignerFormEditorInterface parameter provides the plugin with a gateway to all of Qt Designer’s API’s.
The WorldTimeClockPlugin class inherits from both QObject and QDesignerCustomWidgetInterface. It is important to remember, when using multiple inheritance, to ensure that all the interfaces (i.e. the classes that doesn’t inherit Q_OBJECT) are made known to the meta object system using the Q_INTERFACES() macro. This enables Qt Designer to use qobject_cast() to query for supported interfaces using nothing but a QObject pointer.
The implementation of the WorldTimeClockPlugin is also equivalent to the plugin interface implementation in the Custom Widget Plugin example (only the class name and the implementation of QDesignerCustomWidgetInterface::domXml() differ). The main thing to remember is to use the Q_EXPORT_PLUGIN2() macro to export the WorldTimeClockPlugin class for use with Qt Designer:
Without this macro, there is no way for Qt Designer to use the widget.The Project File: worldtimeclockplugin.pro
The project file for custom widget plugins needs some additional information to ensure that they will work as expected within Qt Designer:
The TEMPLATE variable’s value make qmake create the custom widget as a library. The CONFIG variable contains two values, designer and plugin:
*designer: Since custom widgets plugins rely on components supplied with Qt Designer, this value ensures that our plugin links against Qt Designer’s library (libQtDesigner.so).
*plugin: We also need to ensure that qmake considers the custom widget a plugin library.
When Qt is configured to build in both debug and release modes, Qt Designer will be built in release mode. When this occurs, it is necessary to ensure that plugins are also built in release mode. For that reason you might have to add a release value to your CONFIG variable. Otherwise, if a plugin is built in a mode that is incompatible with Qt Designer, it won’t be loaded and installed.
The header and source files for the widget are declared in the usual way, and in addition we provide an implementation of the plugin interface so that Qt Designer can use the custom widget.
It is important to ensure that the plugin is installed in a location that is searched by Qt Designer. We do this by specifying a target path for the project and adding it to the list of items to install:
The custom widget is created as a library, and will be installed alongside the other Qt Designer plugins when the project is installed (using make install or an equivalent installation procedure). Later, we will ensure that it is recognized as a plugin by Qt Designer by using the Q_EXPORT_PLUGIN2() macro to export the relevant widget information.Qt Public Slots
Note that if you want the plugins to appear in a Visual Studio integration, the plugins must be built in release mode and their libraries must be copied into the plugin directory in the install path of the integration (for an example, see C:/program files/trolltech as/visual studio integration/plugins).
For more information about plugins, see the How to Create Qt Plugins document.
Files:
© 2016 The Qt Company Ltd. Documentation contributions included herein are the copyrights of their respective owners. The documentation provided herein is licensed under the terms of the GNU Free Documentation License version 1.3 as published by the Free Software Foundation. Qt and respective logos are trademarks of The Qt Company Ltd. in Finland and/or other countries worldwide. All other trademarks are property of their respective owners.
An overview of Qt’s signals and slots inter-object communication mechanism.
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 .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.
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 queuedconnections ; 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 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.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 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 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 connect() function calls, or with uic’s automatic connections feature.A Real Example¶
The following is an example of the header of a simple widget class without member functions. The purpose is to show how you can utilize signals and slots in your own applications.
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.Qt Plugin Signal Slot
After the class constructor and public members, we declare the class signals. The LcdNumber class emits a signal, overflow(), 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.Signals And Slot
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