10.3. QSettings: Saving and Restoring Application State

10.3. QSettings: Saving and Restoring Application State
	Chapter 10. MainWindows and Actions

[ fromfile: qsettings.xml id: qsettings ]

The settings/preferences/options need to be persistent.
The mechanism for that is included with QSettings.

A QSettings must be initialized with an application name, organization name, and organization domain before it is used for the first time.
This prevents settings from one application from accidentally overwriting those from another.
If, however, the QApplication itself is set with this information, as shown in Example 10.7, the QSettings default constructor uses it.

Example 10.7. src/widgets/mainwindow/mainwindow-main.cpp

#include "mymainwindow.h"
#include <QApplication>

int main( int argc, char ** argv ) {
    QApplication app( argc, argv );
    app.setOrganizationName("objectlearning");
    app.setOrganizationDomain("objectlearning.net");
    app.setApplicationName("mainwindow-test");
    MyMainWindow mw;
    mw.show();
    return app.exec();
}

Now that the QApplication is initialized, you can create QSettings instances using the default constructor.

As you develop a new QMainWindow application, the first persistent settings you may want to save are probably the window size and position.

QSettings manages persistent maps of key/value pairs.
It is a QObject, and uses something similar to QObject's property interface, setValue() and value(), to set and get its values.
It can be used to store any data that needs to be remembered across multiple executions.

Each combination of names defines a unique persistent map that does not clash with settings from other-named Qt applications.

Some possibilities for its storage include the Win32 registry (in Windows) and your $HOME/.settings directory (in Linux).

QMainWindow::saveState() returns a QByteArray that contains information about the main window's toolbars and dockwidgets.
To do this it makes use of the objectName property for each of those subwidgets, thus making it important that each name be unique.
The QSettings object stores that QByteArray with the key string "state".

QMainWindow::restoreState() takes a QByteArray, presumably created by saveState(), and uses the information that it holds to put the toolbars and dockwidgets into a prior arrangement.
Read the settings after constructing the object, as shown in Example 10.8.

Example 10.8. src/widgets/mainwindow/mymainwindow.cpp

[ . . . . ]

void MyMainWindow::readSettings() {
    QSettings settings;
    QPoint pos = settings.value("pos", QPoint(200, 200)).toPoint();
    QSize size = settings.value("size", QSize(400, 400)).toSize();
    QByteArray state = settings.value("state", QByteArray())
                                                   .toByteArray();
    restoreState(state);
    resize(size);
    move(pos);
}

You should write settings after the user wants to quit but before the window is closed.
The appropriate place for that is in an event handler, so that you can handle the event before the widget itself does.
Example 10.9 shows an event handler for closeEvent() that saves this information to QSettings.

Example 10.9. src/widgets/mainwindow/mymainwindow.cpp

[ . . . . ]

void MyMainWindow::closeEvent(QCloseEvent* event) {
    if (maybeSave()) {
        writeSettings();
        event->accept();
    } else {
        event->ignore();
    }
}


void MyMainWindow::writeSettings() {
    /* Save postion/size of main window */
    QSettings settings;
    settings.setValue("pos", pos());
    settings.setValue("size", size());
    settings.setValue("state", saveState());
}


10.2. Regions and QDockWidgets		10.4. Clipboard and Data Transfer Operations