- Qt Emit Keyword
- Qt Signal Slot Emit
- Qt Emit Signal From Thread
- Qt Signal Slot Example
- Qt Signal Emit Example
(A Python signal is one that is emitted in Python code; a Qt signal is one emitted from an underlying C object.) We can use this syntax both to emit Python and Qt signals, and to connect to them. The PunchingBag inherits from QObject so it can emit signals; it has a signal called punched, which carries no data; and it has a punch method which does nothing but emit the punched signal. To make our PunchingBag useful, we need to connect its punched signal to a slot that does something. Feb 08, 2008 If the signal is a Qt signal, the type names must be the C type names, such as int and QString. C type names can be rather complex, with each type name possibly including one or more of const,., and &. When we write them as signal (or slot) signatures we can drop any consts and &s, but must keep any.s. May 28, 2014 The main application creates dialogs A and B and then connects the signal from A to the signal slot in B. Emitting the signal: A emits the signal by calling emit. Qt Signals and Slots, Connecting and Disconnecting. Show more Show less. Labels API; CPP.
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␡- Subclassing QDialog
How Qt Signals and Slots Work - Part 3 - Queued and Inter Thread Connections. Activate to prepare a Qt::QueuedConnection slot call. The code showed here has been slightly simplified and commented: static void queuedactivate(QObject.sender. Or is about to emit the signal that will wait for the parent thread, which won't go back to its. Example SLOT/SIGNAL between two object QT. Ask Question Asked 2 years, 8 months ago. Here is a simple example of how to emit signals and slots. Qt Signal Slot No Matching Function For Call To. Signal slot custom struct issue. Hot Network Questions.
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This chapter will teach you how to create dialog boxes using Qt.
This chapter is from the book C++ GUI Programming with Qt4, 2nd Edition
This chapter is from the book
This chapter is from the book
2. Creating Dialogs
- Subclassing QDialog
- Signals and Slots in Depth
- Rapid Dialog Design
- Shape-Changing Dialogs
- Dynamic Dialogs
- Built-in Widget and Dialog Classes
This chapter will teach you how to create dialog boxes using Qt. Dialog boxes present users with options and choices, and allow them to set the options to their preferred values and to make their choices. They are called dialog boxes, or simply 'dialogs', because they provide a means by which users and applications can 'talk to' each other.
Most GUI applications consist of a main window with a menu bar and toolbar, along with dozens of dialogs that complement the main window. It is also possible to create dialog applications that respond directly to the user's choices by performing the appropriate actions (e.g., a calculator application).
We will create our first dialog purely by writing code to show how it is done. Then we will see how to build dialogs using Qt Designer, Qt's visual design tool. Using Qt Designer is a lot faster than hand-coding and makes it easy to test different designs and to change designs later.
Subclassing QDialog
Our first example is a Find dialog written entirely in C++. It is shown in Figure 2.1. We will implement the dialog as a class in its own right. By doing so, we make it an independent, self-contained component, with its own signals and slots.
Figure 2.1 The Find dialog
The source code is spread across two files: finddialog.h and finddialog.cpp. We will start with finddialog.h.
Lines 1 and 2 (and 27) protect the header file against multiple inclusions.
Line 3 includes the definition of QDialog, the base class for dialogs in Qt. QDialog is derived from QWidget.
Lines 4 to 7 are forward declarations of the Qt classes that we will use to implement the dialog. A forward declaration tells the C++ compiler that a class exists, without giving all the detail that a class definition (usually located in a header file of its own) provides. We will say more about this shortly.
Next, we define FindDialog as a subclass of QDialog:
The Q_OBJECT macro at the beginning of the class definition is necessary for all classes that define signals or slots.
The FindDialog constructor is typical of Qt widget classes. The parent parameter specifies the parent widget. The default is a null pointer, meaning that the dialog has no parent.
The signals section declares two signals that the dialog emits when the user clicks the Find button. If the Search backward option is enabled, the dialog emits findPrevious(); otherwise, it emits findNext().
The signals keyword is actually a macro. The C++ preprocessor converts it into standard C++ before the compiler sees it. Qt::CaseSensitivity is an enum type that can take the values Qt::CaseSensitive and Qt::CaseInsensitive.
In the class's private section, we declare two slots. To implement the slots, we will need to access most of the dialog's child widgets, so we keep pointers to them as well. The slots keyword is, like signals, a macro that expands into a construct that the C++ compiler can digest.
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For the private variables, we used forward declarations of their classes. This was possible because they are all pointers and we don't access them in the header file, so the compiler doesn't need the full class definitions. We could have included the relevant header files (<QCheckBox>, <QLabel>, etc.), but using forward declarations when it is possible makes compiling somewhat faster.
We will now look at finddialog.cpp, which contains the implementation of the FindDialog class.
First, we include <QtGui>, a header file that contains the definition of Qt's GUI classes. Qt consists of several modules, each of which lives in its own library. The most important modules are QtCore, QtGui, QtNetwork, QtOpenGL, QtScript, QtSql, QtSvg, and QtXml. The <QtGui> header file contains the definition of all the classes that are part of the QtCore and QtGui modules. Including this header saves us the bother of including every class individually.
In finddialog.h, instead of including <QDialog> and using forward declarations for QCheckBox, QLabel, QLineEdit, and QPushButton, we could simply have included <QtGui>. However, it is generally bad style to include such a big header file from another header file, especially in larger applications.
On line 4, we pass on the parent parameter to the base class constructor. Then we create the child widgets. The tr() function calls around the string literals mark them for translation to other languages. The function is declared in QObject and every subclass that contains the Q_OBJECT macro. It's a good habit to surround user-visible strings with tr(), even if you don't have immediate plans for translating your applications to other languages. We cover translating Qt applications in Chapter 18.
In the string literals, we use ampersands ('&') to indicate shortcut keys. For example, line 11 creates a Find button, which the user can activate by pressing Alt+F on platforms that support shortcut keys. Ampersands can also be used to control focus: On line 6 we create a label with a shortcut key (Alt+W), and on line 8 we set the label's buddy to be the line editor. A buddy is a widget that accepts the focus when the label's shortcut key is pressed. So when the user presses Alt+W (the label's shortcut), the focus goes to the line editor (the label's buddy).
On line 12, we make the Find button the dialog's default button by calling setDefault(true). The default button is the button that is pressed when the user hits Enter. On line 13, we disable the Find button. When a widget is disabled, it is usually shown grayed out and will not respond to user interaction.
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The private slot enableFindButton(const QString &) is called whenever the text in the line editor changes. The private slot findClicked() is called when the user clicks the Find button. The dialog closes itself when the user clicks Close. The close() slot is inherited from QWidget, and its default behavior is to hide the widget from view (without deleting it). We will look at the code for the enableFindButton() and findClicked() slots later on.
Since QObject is one of FindDialog's ancestors, we can omit the QObject:: prefix in front of the connect() calls.
Next, we lay out the child widgets using layout managers. Layouts can contain both widgets and other layouts. By nesting QHBoxLayouts, QVBoxLayouts, and QGridLayouts in various combinations, it is possible to build very sophisticated dialogs.
For the Find dialog, we use two QHBoxLayouts and two QVBoxLayouts, as shown in Figure 2.2. The outer layout is the main layout; it is installed on the FindDialog on line 35 and is responsible for the dialog's entire area. The other three layouts are sub-layouts. The little 'spring' at the bottom right of Figure 2.2 is a spacer item (or 'stretch'). It uses up the empty space below the Find and Close buttons, ensuring that these buttons occupy the top of their layout.
One subtle aspect of the layout manager classes is that they are not widgets. Instead, they are derived from QLayout, which in turn is derived from QObject. In the figure, widgets are represented by solid outlines and layouts are represented by dashed outlines to highlight the difference between them. In a running application, layouts are invisible.
When the sublayouts are added to the parent layout (lines 25, 33, and 34), the sublayouts are automatically reparented. Then, when the main layout is installed on the dialog (line 35), it becomes a child of the dialog, and all the widgets in the layouts are reparented to become children of the dialog. The resulting parent–child hierarchy is depicted in Figure 2.3.
Figure 2.3 The Find dialog's parent–child relationships
Finally, we set the title to be shown in the dialog's title bar and we set the window to have a fixed height, since there aren't any widgets in the dialog that can meaningfully occupy any extra vertical space. The QWidget::sizeHint() function returns a widget's 'ideal' size.
This completes the review of FindDialog's constructor. Since we used new to create the dialog's widgets and layouts, it would seem that we need to write a destructor that calls delete on each widget and layout we created. But this isn't necessary, since Qt automatically deletes child objects when the parent is destroyed, and the child widgets and layouts are all descendants of the FindDialog.
Now we will look at the dialog's slots:
The findClicked() slot is called when the user clicks the Find button. It emits the findPrevious() or the findNext() signal, depending on the Search backward option. The emit keyword is specific to Qt; like other Qt extensions it is converted into standard C++ by the C++ preprocessor.
The enableFindButton() slot is called whenever the user changes the text in the line editor. It enables the button if there is some text in the editor, and disables it otherwise.
These two slots complete the dialog. We can now create a main.cpp file to test our FindDialog widget:
To compile the program, run qmake as usual. Since the FindDialog class definition contains the Q_OBJECT macro, the makefile generated by qmake will include special rules to run moc, Qt's meta-object compiler. (We cover Qt's meta-object system in the next section.)
For moc to work correctly, we must put the class definition in a header file, separate from the implementation file. The code generated by moc includes this header file and adds some C++ boilerplate code of its own.
Classes that use the Q_OBJECT macro must have moc run on them. This isn't a problem because qmake automatically adds the necessary rules to the makefile. But if you forget to regenerate your makefile using qmake and moc isn't run, the linker will complain that some functions are declared but not implemented. The messages can be fairly obscure. GCC produces error messages like this one:
Visual C++'s output starts like this:
If this ever happens to you, run qmake again to update the makefile, then rebuild the application.
Now run the program. If shortcut keys are shown on your platform, verify that the shortcut keys Alt+W, Alt+C, Alt+B, and Alt+F trigger the correct behavior. Press Tab to navigate through the widgets with the keyboard. The default tab order is the order in which the widgets were created. This can be changed using QWidget::setTabOrder().
Providing a sensible tab order and keyboard shortcuts ensures that users who don't want to (or cannot) use a mouse are able to make full use of the application. Full keyboard control is also appreciated by fast typists.
In Chapter 3, we will use the Find dialog inside a real application, and we will connect the findPrevious() and findNext() signals to some slots.
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Unlike a console mode application, which is executed in a sequential manner, a GUI based application is event driven. Functions or methods are executed in response to user’s actions like clicking on a button, selecting an item from a collection or a mouse click etc., called events.
Widgets used to build the GUI interface act as the source of such events. Each PyQt widget, which is derived from QObject class, is designed to emit ‘signal’ in response to one or more events. The signal on its own does not perform any action. Instead, it is ‘connected’ to a ‘slot’. The slot can be any callable Python function.
In PyQt, connection between a signal and a slot can be achieved in different ways. Following are most commonly used techniques −
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A more convenient way to call a slot_function, when a signal is emitted by a widget is as follows − Top slot machine apps.
Suppose if a function is to be called when a button is clicked. Here, the clicked signal is to be connected to a callable function. It can be achieved in any of the following two techniques −
or
Example
In the following example, two QPushButton objects (b1 and b2) are added in QDialog window. We want to call functions b1_clicked() and b2_clicked() on clicking b1 and b2 respectively.
Qt Emit Keyword
When b1 is clicked, the clicked() signal is connected to b1_clicked() function
Qt Signal Slot Emit
When b2 is clicked, the clicked() signal is connected to b2_clicked() function
Qt Emit Signal From Thread
Example
Qt Signal Slot Example
The above code produces the following output −