Signals and Slots Across Threads. Qt supports these signal-slot connection types: Auto Connection (default) If the signal is emitted in the thread which the receiving object has affinity then the behavior is the same as the Direct Connection. Otherwise, the behavior is the same as the Queued Connection.'

Passing extra arguments to PyQt slots April 25, 2011 at 13:38 Tags Python, Qt. 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. 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. Qt is well known for its signals and slots mechanism. But how does it work? In this blog post, we will explore the internals of QObject and QMetaObject and discover how signals and slot work under the hood. In this blog article, I show portions of Qt5 code, sometimes edited for formatting and brevity. Nov 02, 2009 If the parameter types are incompatible, or if the signal or the slot doesn't exist, Qt will issue a warning at run-time if the application is built in debug mode. Similarly, Qt will give a warning if parameter names are included in the signal or slot signatures. So far, we have only used signals and slots.

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Subclassing QDialog

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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.

Qt Signal Slot Arguments

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.

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.

The

Qt Signals And Slots Passing Arguments