Qt Create Slots
2021年4月30日Register here: http://gg.gg/ufkxr
*Qt Create Slots Software
*Qt Creator Slots
*Qt Create Slots Online
*Qt Creator Slots And Signals
*An introduction to creating PySide/PyQt signals and slots, using QObject. How signals and slots are useful, and what they can do when developing in PySide/PyQt.
*Create a TCP server in Qt is also very easy, indeed, the class QTcpServer already provide all we need to do the server. First, we need to listen to any ip, a random port and do something when a client is connected. Like that: server.listen(QHostAddress::Any, 4242); connect(&server, SIGNAL(newConnection), this, SLOT(onNewConnection)).
If i create a class from a base class with virtual slots, the slots never get called with the new connect-flavour. If i use the old connect-syntax, the slot gets called. What could be the problem? @ class BaseClass: public QObject public slots: virt. To establish a signal and slot connection between two widgets in a dialog, you first need to switch to Qt Designer’s Edit Signals/Slots mode. To do that, you can press the F4 key, select the EditEdit Signals/Slots option in the main menu, or click on the Edit Signals/Slots button on the toolbar. In order to use signals and slots, you need to have the QOBJECT macro in your class as well as identifying which functions should be the signals and the slots. Have a look at the documentation for a more in-depth explanation. After this, you need to set up the project file so that MOC can generate the necessary code.
This tutorial is an introduction to Qt programming and will teach you how to create an application with Qt and C++. Full project and source code are provided.Qt
Qt is a cross-platform framework and a collection of tools for creating applications which can run on desktop, embedded and mobile operating systems.
If you need to create an application with a GUI in C++ you should definitely consider Qt for it.A simple example application
This tutorial will guide you through the creation of a basic window with a menu bar, a menu and a status bar. The final result while look like this:
Nothing too fancy, but these are the basic elements of most desktop applications and you can use this code as starting template for your projects.
When working with Qt, especially when developing cross-platform software, it’s recommended to use Qt Creator as IDE. The main reason is that Qt Creator is the official Qt IDE and it offers full integration with the framework and all the other development tools. Using other IDEs is possible, but I don’t recommend it and for this tutorial I will use Qt Creator.Creating a new Qt project
The first step is to create a new project in Qt Creator from one of the available templates:
For a graphical application you want to select the Qt Widgets Application template.
The second step is selecting a name for your project and a location where to save it. For my project I decided to use “BasicApplication” for the name, but obviously you can use anything you want.
The third step is selecting the kit(s) for building your application.
Qt Creator kits are groups of settings used for building and running projects for different platforms.
I am developing on Linux so I am going for the kit Desktop Qt 5.7.0 GCC 64bit, but you might see different kits if developing on/for different platforms.
The fourth step is defining the details of the initial main window class that will be generated by the template.
You can leave everything as it is, but make sure to uncheck the Generate form field. The form file is used by the integrated visual interface editor Qt Designer. We won’t need it because we will be coding the GUI from scratch.
In the final step of the project creation you will get a summary of what defined in the previous steps.
Here it’s also possible to set a version control system to use for the project, but feel free to not set any for now.
Clicking on the button Finish will create the project and generate 3 C++ files (main.cpp, MainWindow.h and MainWindow.cpp) and a project file used by Qt Creator and qmake (the Qt building tool) to build the project.The main
The main function initialises the application, then creates and opens the main window:
The last line of code launches the Qt application and waits for it to return an exit code before terminating the program.The main window class
The main window is defined in the header file MainWindow.h and it inherits from QMainWindow:
It’s important to include the Q_OBJECT macro in all the classes that are inheriting from QObject.
In this simple example the constructor is the only public function:
Finally some private slots are declared:
Slots are handlers used to do something when an event happens. They will be explained in more detail later.The main window implementation
All the MainWindow methods, including the slots, are implemented in MainWindow.cpp.
Most of the code will be placed in the constructor because it’s there where the elements of the window are created.Window settings
We can start with some basic settings:
The function setWindowTitle sets the title of the window, whereas setMinimumSize sets the minimum size allowed when resizing it.Adding a menu bar
Most applications have a menu bar with menus. Creating one in Qt is pretty simple:
Once the menu bar is created you need to set it for the main window calling the setMenuBar function.
After creating and setting the menu bar it’s time to create a first menu:
The “&” before “File” will enable the keyboard shortcut ALT+F to open the menu.
Now that we have a menu we can populate it with entries, which are QAction objects:
As you can see creating an entry and adding it to a menu is pretty straightforward.
What’s more interesting is connecting menu entries to functions so that something happens when they are clicked. This is achieved using the Qt function connect which in its standard format takes 4 parameters:
*sender – the element generating an event
*signal – the event generated by the sender
*receiver – the element receiving the event
*slot – the handler to execute when an event is received
In the case of the first menu entry we have the following scenario
*sender : action (the menu entry)
*signal : &QAction::triggered (a menu entry has been clicked)
*receiver : this (the event is handled by the MainWindow)
*slot : &MainWindow::OnFileNew (when the menu entry is clicked this function is called)
When building a menu it’s also possible to add a horizontal line to to separate logic blocks. It only takes one line of code:
The final entry of the File menu is going to be one to exit from the application:
This entry is connected to the function close defined in QMainWindow which, as expected, will close the window when the menu entry is clicked.Adding a status bar
A status bar is useful to show messages to the user. Creating one is pretty simple:
As for the menu bar, it’s important to remember to set the active status bar calling the setStatusBar function.Implementing the slots
In this simple example all the slot functions do is setting a message to show in the status bar like in the following code:
Obviously this is just an example and a real application will do something more useful with them.Adding content to the window
Once the basic elements of the window have been created it’s time to add some content.
This is done by calling the function setCentralWidget and passing it a QWidget which contains the graphic elements the application needs to show and handle.
I decided to not add any central widget in my code to keep this example as simple as possible, but obviously you’ll need to do that in your application.Source code
The full source code of this tutorial is available on GitHub and released under the Unlicense license.References
All the Qt widgets discussed in this tutorial are documented in detail in the Qt Widgets C++ Classes section of the Qt documentation.
To learn more about Qt Creator check out the online manual.Conclusion
This was a simple tutorial to teach you how to create an application with Qt and C++. If you have any question about the code explained here feel free to leave a comment.
If you found it helpful please feel free to share it on social media using the share buttons below.Subscribe
Don’t forget to subscribe to the blog newsletter to get notified of future posts.
You can also get updates following me on Google+, LinkedIn and Twitter.
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 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.A Small Example
A minimal C++ class declaration might read:Qt Create Slots Software
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.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.
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’
https://diarynote.indered.space
*Qt Create Slots Software
*Qt Creator Slots
*Qt Create Slots Online
*Qt Creator Slots And Signals
*An introduction to creating PySide/PyQt signals and slots, using QObject. How signals and slots are useful, and what they can do when developing in PySide/PyQt.
*Create a TCP server in Qt is also very easy, indeed, the class QTcpServer already provide all we need to do the server. First, we need to listen to any ip, a random port and do something when a client is connected. Like that: server.listen(QHostAddress::Any, 4242); connect(&server, SIGNAL(newConnection), this, SLOT(onNewConnection)).
If i create a class from a base class with virtual slots, the slots never get called with the new connect-flavour. If i use the old connect-syntax, the slot gets called. What could be the problem? @ class BaseClass: public QObject public slots: virt. To establish a signal and slot connection between two widgets in a dialog, you first need to switch to Qt Designer’s Edit Signals/Slots mode. To do that, you can press the F4 key, select the EditEdit Signals/Slots option in the main menu, or click on the Edit Signals/Slots button on the toolbar. In order to use signals and slots, you need to have the QOBJECT macro in your class as well as identifying which functions should be the signals and the slots. Have a look at the documentation for a more in-depth explanation. After this, you need to set up the project file so that MOC can generate the necessary code.
This tutorial is an introduction to Qt programming and will teach you how to create an application with Qt and C++. Full project and source code are provided.Qt
Qt is a cross-platform framework and a collection of tools for creating applications which can run on desktop, embedded and mobile operating systems.
If you need to create an application with a GUI in C++ you should definitely consider Qt for it.A simple example application
This tutorial will guide you through the creation of a basic window with a menu bar, a menu and a status bar. The final result while look like this:
Nothing too fancy, but these are the basic elements of most desktop applications and you can use this code as starting template for your projects.
When working with Qt, especially when developing cross-platform software, it’s recommended to use Qt Creator as IDE. The main reason is that Qt Creator is the official Qt IDE and it offers full integration with the framework and all the other development tools. Using other IDEs is possible, but I don’t recommend it and for this tutorial I will use Qt Creator.Creating a new Qt project
The first step is to create a new project in Qt Creator from one of the available templates:
For a graphical application you want to select the Qt Widgets Application template.
The second step is selecting a name for your project and a location where to save it. For my project I decided to use “BasicApplication” for the name, but obviously you can use anything you want.
The third step is selecting the kit(s) for building your application.
Qt Creator kits are groups of settings used for building and running projects for different platforms.
I am developing on Linux so I am going for the kit Desktop Qt 5.7.0 GCC 64bit, but you might see different kits if developing on/for different platforms.
The fourth step is defining the details of the initial main window class that will be generated by the template.
You can leave everything as it is, but make sure to uncheck the Generate form field. The form file is used by the integrated visual interface editor Qt Designer. We won’t need it because we will be coding the GUI from scratch.
In the final step of the project creation you will get a summary of what defined in the previous steps.
Here it’s also possible to set a version control system to use for the project, but feel free to not set any for now.
Clicking on the button Finish will create the project and generate 3 C++ files (main.cpp, MainWindow.h and MainWindow.cpp) and a project file used by Qt Creator and qmake (the Qt building tool) to build the project.The main
The main function initialises the application, then creates and opens the main window:
The last line of code launches the Qt application and waits for it to return an exit code before terminating the program.The main window class
The main window is defined in the header file MainWindow.h and it inherits from QMainWindow:
It’s important to include the Q_OBJECT macro in all the classes that are inheriting from QObject.
In this simple example the constructor is the only public function:
Finally some private slots are declared:
Slots are handlers used to do something when an event happens. They will be explained in more detail later.The main window implementation
All the MainWindow methods, including the slots, are implemented in MainWindow.cpp.
Most of the code will be placed in the constructor because it’s there where the elements of the window are created.Window settings
We can start with some basic settings:
The function setWindowTitle sets the title of the window, whereas setMinimumSize sets the minimum size allowed when resizing it.Adding a menu bar
Most applications have a menu bar with menus. Creating one in Qt is pretty simple:
Once the menu bar is created you need to set it for the main window calling the setMenuBar function.
After creating and setting the menu bar it’s time to create a first menu:
The “&” before “File” will enable the keyboard shortcut ALT+F to open the menu.
Now that we have a menu we can populate it with entries, which are QAction objects:
As you can see creating an entry and adding it to a menu is pretty straightforward.
What’s more interesting is connecting menu entries to functions so that something happens when they are clicked. This is achieved using the Qt function connect which in its standard format takes 4 parameters:
*sender – the element generating an event
*signal – the event generated by the sender
*receiver – the element receiving the event
*slot – the handler to execute when an event is received
In the case of the first menu entry we have the following scenario
*sender : action (the menu entry)
*signal : &QAction::triggered (a menu entry has been clicked)
*receiver : this (the event is handled by the MainWindow)
*slot : &MainWindow::OnFileNew (when the menu entry is clicked this function is called)
When building a menu it’s also possible to add a horizontal line to to separate logic blocks. It only takes one line of code:
The final entry of the File menu is going to be one to exit from the application:
This entry is connected to the function close defined in QMainWindow which, as expected, will close the window when the menu entry is clicked.Adding a status bar
A status bar is useful to show messages to the user. Creating one is pretty simple:
As for the menu bar, it’s important to remember to set the active status bar calling the setStatusBar function.Implementing the slots
In this simple example all the slot functions do is setting a message to show in the status bar like in the following code:
Obviously this is just an example and a real application will do something more useful with them.Adding content to the window
Once the basic elements of the window have been created it’s time to add some content.
This is done by calling the function setCentralWidget and passing it a QWidget which contains the graphic elements the application needs to show and handle.
I decided to not add any central widget in my code to keep this example as simple as possible, but obviously you’ll need to do that in your application.Source code
The full source code of this tutorial is available on GitHub and released under the Unlicense license.References
All the Qt widgets discussed in this tutorial are documented in detail in the Qt Widgets C++ Classes section of the Qt documentation.
To learn more about Qt Creator check out the online manual.Conclusion
This was a simple tutorial to teach you how to create an application with Qt and C++. If you have any question about the code explained here feel free to leave a comment.
If you found it helpful please feel free to share it on social media using the share buttons below.Subscribe
Don’t forget to subscribe to the blog newsletter to get notified of future posts.
You can also get updates following me on Google+, LinkedIn and Twitter.
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 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.A Small Example
A minimal C++ class declaration might read:Qt Create Slots Software
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.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.
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’
https://diarynote.indered.space
コメント