Qt5 Slots Example

4/6/2022

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Qt5 Slot Example
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Qt Slot Example

The example shows a tooltip for the main QWidget. Window.setWindowTitle('ToolTip'); We set a tooltip for the QWidget widget with the setToolTip method. Figure: A tooltip Qt5 Cursors. A cursor is a small icon that indicates the position of the mouse pointer. In the next example will show various cursors that we can use in our programs. Platform, for example, as normal Windows applications when developing on a Windows platform. Qt Widgets has not been designed to benefit from a Graphical Processing Unit (GPU) since it is using a raster paint engine (QPainter) when drawing its graphical elements. While being better in many regards, the new connection syntax in Qt5 has one big weakness: Connecting overloaded signals and slots. In order to let the compiler resolve the overloads we need to use staticcasts to member function pointers, or (starting in Qt 5.7) qOverload and friends.

Nearly all of the code for the Application example is in the MainWindow class, which inherits QMainWindow. QMainWindow provides the framework for windows that have menus, toolbars, dock windows, and a status bar. The application provides File, Edit, and Help entries in the menu bar, with the following popup menus:

The status bar at the bottom of the main window shows a description of the menu item or toolbar button under the cursor.

To keep the example simple, recently opened files aren't shown in the File menu, even though this feature is desired in 90% of applications. Furthermore, this example can only load one file at a time. The SDI and MDI examples show how to lift these restrictions and how to implement recently opened files handling.

MainWindow Class Definition

Here's the class definition:

The public API is restricted to the constructor. In the protected section, we reimplement QWidget::closeEvent() to detect when the user attempts to close the window, and warn the user about unsaved changes. In the private slots section, we declare slots that correspond to menu entries, as well as a mysterious documentWasModified() slot. Finally, in the private section of the class, we have various members that will be explained in due time.

MainWindow Class Implementation

We start by including <QtWidgets>, a header file that contains the definition of all classes in the Qt Core, Qt GUI and Qt Widgets modules. This saves us from the trouble of having to include every class individually. We also include mainwindow.h.

You might wonder why we don't include <QtWidgets> in mainwindow.h and be done with it. The reason is that including such a large header from another header file can rapidly degrade performances. Here, it wouldn't do any harm, but it's still generally a good idea to include only the header files that are strictly necessary from another header file.

In the constructor, we start by creating a QPlainTextEdit widget as a child of the main window (the this object). Then we call QMainWindow::setCentralWidget() to tell that this is going to be the widget that occupies the central area of the main window, between the toolbars and the status bar.

Then we call createActions() and createStatusBar(), two private functions that set up the user interface. After that, we call readSettings() to restore the user's preferences.

We establish a signal-slot connection between the QPlainTextEdit's document object and our documentWasModified() slot. Whenever the user modifies the text in the QPlainTextEdit, we want to update the title bar to show that the file was modified.

At the end, we set the window title using the private setCurrentFile() function. We'll come back to this later.

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When the user attempts to close the window, we call the private function maybeSave() to give the user the possibility to save pending changes. The function returns true if the user wants the application to close; otherwise, it returns false. In the first case, we save the user's preferences to disk and accept the close event; in the second case, we ignore the close event, meaning that the application will stay up and running as if nothing happened.

The newFile() slot is invoked when the user selects File New from the menu. We call maybeSave() to save any pending changes and if the user accepts to go on, we clear the QPlainTextEdit and call the private function setCurrentFile() to update the window title and clear the windowModified flag.

The open() slot is invoked when the user clicks File Open. We pop up a QFileDialog asking the user to choose a file. If the user chooses a file (i.e., fileName is not an empty string), we call the private function loadFile() to actually load the file.

The save() slot is invoked when the user clicks File Save. If the user hasn't provided a name for the file yet, we call saveAs(); otherwise, we call the private function saveFile() to actually save the file.

In saveAs(), we start by popping up a QFileDialog asking the user to provide a name. If the user clicks Cancel, the returned file name is empty, and we do nothing.

The application's About box is done using one statement, using the QMessageBox::about() static function and relying on its support for an HTML subset.

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The tr() call around the literal string marks the string for translation. It is a good habit to call tr() on all user-visible strings, in case you later decide to translate your application to other languages. The Internationalization with Qt overview covers tr() in more detail.

The documentWasModified() slot is invoked each time the text in the QPlainTextEdit changes because of user edits. We call QWidget::setWindowModified() to make the title bar show that the file was modified. How this is done varies on each platform.

The createActions() private function, which is called from the MainWindow constructor, creates QActions and populates the menus and two toolbars. The code is very repetitive, so we show only the actions corresponding to File New, File Open, and Help About Qt.

A QAction is an object that represents one user action, such as saving a file or invoking a dialog. An action can be put in a QMenu or a QToolBar, or both, or in any other widget that reimplements QWidget::actionEvent().

An action has a text that is shown in the menu, an icon, a shortcut key, a tooltip, a status tip (shown in the status bar), a 'What's This?' text, and more. It emits a triggered() signal whenever the user invokes the action (e.g., by clicking the associated menu item or toolbar button).

Instances of QAction can be created by passing a parent QObject or by using one of the convenience functions of QMenu, QMenuBar or QToolBar. We create the actions that are in a menu as well as in a toolbar parented on the window to prevent ownership issues. For actions that are only in the menu, we use the convenience function QMenu::addAction(), which allows us to pass text, icon and the target object and its slot member function.

Creating toolbars is very similar to creating menus. The same actions that we put in the menus can be reused in the toolbars. After creating the action, we add it to the toolbar using QToolBar::addAction().

The code above contains one more idiom that must be explained. For some of the actions, we specify an icon as a QIcon to the QAction constructor. We use QIcon::fromTheme() to obtain the correct standard icon from the underlying window system. If that fails due to the platform not supporting it, we pass a file name as fallback. Here, the file name starts with :. Such file names aren't ordinary file names, but rather path in the executable's stored resources. We'll come back to this when we review the application.qrc file that's part of the project.

The Edit Cut and Edit Copy actions must be available only when the QPlainTextEdit contains selected text. We disable them by default and connect the QPlainTextEdit::copyAvailable() signal to the QAction::setEnabled() slot, ensuring that the actions are disabled when the text editor has no selection.

Just before we create the Help menu, we call QMenuBar::addSeparator(). This has no effect for most widget styles (e.g., Windows and macOS styles), but for some styles this makes sure that Help is pushed to the right side of the menu bar.

QMainWindow::statusBar() returns a pointer to the main window's QStatusBar widget. Like with QMainWindow::menuBar(), the widget is automatically created the first time the function is called.

The readSettings() function is called from the constructor to load the user's preferences and other application settings. The QSettings class provides a high-level interface for storing settings permanently on disk. On Windows, it uses the (in)famous Windows registry; on macOS, it uses the native XML-based CFPreferences API; on Unix/X11, it uses text files.

The QSettings constructor takes arguments that identify your company and the name of the product. This ensures that the settings for different applications are kept separately.

We use QSettings::value() to extract the value of the geometry setting. The second argument to QSettings::value() is optional and specifies a default value for the setting if there exists none. This value is used the first time the application is run.

We use QWidget::saveGeometry() and Widget::restoreGeometry() to save the position. They use an opaque QByteArray to store screen number, geometry and window state.

The writeSettings() function is called from closeEvent(). Writing settings is similar to reading them, except simpler. The arguments to the QSettings constructor must be the same as in readSettings().

The maybeSave() function is called to save pending changes. If there are pending changes, it pops up a QMessageBox giving the user to save the document. The options are QMessageBox::Yes, QMessageBox::No, and QMessageBox::Cancel. The Yes button is made the default button (the button that is invoked when the user presses Return) using the QMessageBox::Default flag; the Cancel button is made the escape button (the button that is invoked when the user presses Esc) using the QMessageBox::Escape flag.

The maybeSave() function returns true in all cases, except when the user clicks Cancel or saving the file fails. The caller must check the return value and stop whatever it was doing if the return value is false.

In loadFile(), we use QFile and QTextStream to read in the data. The QFile object provides access to the bytes stored in a file.

We start by opening the file in read-only mode. The QFile::Text flag indicates that the file is a text file, not a binary file. On Unix and macOS, this makes no difference, but on Windows, it ensures that the 'rn' end-of-line sequence is converted to 'n' when reading.

If we successfully opened the file, we use a QTextStream object to read in the data. QTextStream automatically converts the 8-bit data into a Unicode QString and supports various encodings. If no encoding is specified, QTextStream assumes the file is written using the system's default 8-bit encoding (for example, Latin-1; see QTextCodec::codecForLocale() for details).

Since the call to QTextStream::readAll() might take some time, we set the cursor to be Qt::WaitCursor for the entire application while it goes on.

At the end, we call the private setCurrentFile() function, which we'll cover in a moment, and we display the string 'File loaded' in the status bar for 2 seconds (2000 milliseconds).

Saving a file is very similar to loading one. Here, the QFile::Text flag ensures that on Windows, 'n' is converted into 'rn' to conform to the Windows convension.

The setCurrentFile() function is called to reset the state of a few variables when a file is loaded or saved, or when the user starts editing a new file (in which case fileName is empty). We update the curFile variable, clear the QTextDocument::modified flag and the associated QWidget:windowModified flag, and update the window title to contain the new file name (or untitled.txt).

The strippedName() function call around curFile in the QWidget::setWindowTitle() call shortens the file name to exclude the path. Here's the function:

The main() Function

The main() function for this application is typical of applications that contain one main window:

The main function uses QCommandLineParser to check whether some file argument was passed to the application and loads it via MainWindow::loadFile().

The Resource File

As you will probably recall, for some of the actions, we specified icons with file names starting with : and mentioned that such file names aren't ordinary file names, but path in the executable's stored resources. These resources are compiled

The resources associated with an application are specified in a .qrc file, an XML-based file format that lists files on the disk. Here's the application.qrc file that's used by the Application example:

The .png files listed in the application.qrc file are files that are part of the Application example's source tree. Paths are relative to the directory where the application.qrc file is located (the mainwindows/application directory).

The resource file must be mentioned in the application.pro file so that qmake knows about it:

qmake will produce make rules to generate a file called qrc_application.cpp that is linked into the application. This file contains all the data for the images and other resources as static C++ arrays of compressed binary data. See The Qt Resource System for more information about resources.

Files:

Images:

Application Entry Point

The entry point for a QWindow based application is the QGuiApplication class. It manages the GUI application's control flow and main settings. We pass the command line arguments which can be used to pick up certain system wide options.

From there, we go on to create our window instance and then call the QWindow::show() function to tell the windowing system that this window should now be made visible on screen.

Once this is done, we enter the application's event loop so the application can run.

RasterWindow Declaration

We first start by including the <QtGui> header. This means we can use all classes in the Qt GUI module. Classes can also be included individually if that is preferred.

The RasterWindow class subclasses QWindow directly and provides a constructor which allows the window to be a sub-window of another QWindow. Parent-less QWindows show up in the windowing system as top-level windows.

The class declares a QBackingStore which is what we use to manage the window's back buffer for QPainter based graphics.

The raster window is also reused in a few other examples and adds a few helper functions, like renderLater().

RasterWindow Implementation

In the constructor we create the backingstore and pass it the window instance it is supposed to manage. We also set the initial window geometry.

Shortly after calling QWindow::show() on a created window, the virtual function QWindow::exposeEvent() will be called to notify us that the window's exposure in the windowing system has changed. The event contains the exposed sub-region, but since we will anyway draw the entire window every time, we do not make use of that.

The function QWindow::isExposed() will tell us if the window is showing or not. We need this as the exposeEvent is called also when the window becomes obscured in the windowing system. If the window is showing, we call renderNow() to draw the window immediately. We want to draw right away so we can present the system with some visual content.

The resize event is guaranteed to be called prior to the window being shown on screen and will also be called whenever the window is resized while on screen. We use this to resize the back buffer and call renderNow() if we are visible to immediately update the visual representation of the window on screen.

The renderNow function sets up what is needed for a QWindow to render its content using QPainter. As obscured windows have will not be visible, we abort if the window is not exposed in the windowing system. This can for instance happen when another window fully obscures this window.

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We start the drawing by calling QBackingStore::beginPaint() on the region we want to draw. Then we get the QPaintDevice of the back buffer and create a QPainter to render to that paint device.

To void leaving traces from the previous rendering and start with a clean buffer, we fill the entire buffer with the color white. Then we call the virtual render() function which does the actual drawing of this window.

After drawing is complete, we call endPaint() to signal that we are done rendering and present the contents in the back buffer using QBackingStore::flush().

The render function contains the drawing code for the window. In this minial example, we only draw the string 'QWindow' in the center.

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Rendering Asynchronously

We went through a few places where the window needed to repainted immediately. There are some cases where this is not desirable, but rather let the application return to the event loop and schedule the repaint for later. We achieve this by requesting an update, using QWindow::requestUpdate(), which will then be delivered when the system is ready to repaint.

We reimplement the virtual QObject::event() function to handle the update event. When the event comes in we call renderNow() to render the window right away.