Writing Applications that Start Quickly

Delaying code execution.

Making an application start quickly is closely related to how much code is executed during the startup process. Some of the common operations that can affect startup time are:

• loading/parsing configuration files
• creating user interface components (dialogs, etc.)
• reading data files.

The simplest way to improve startup time is to defer some of these operations until after the user interface is visible. If possible, perform the operation on demand, for example, do not create a dialog until it is actually needed:

    MainWidget::MainWidget( QWidget *parent, Qt::WFlags f )
        : QMainWindow( parent, f ), settingsDialog(0)
    {
    }

    void MainWidget::showSettings()
    {
        // If settingsDialog has not yet been created, create it now.
        if ( !settingsDialog )
            settingsDialog = new SettingsDialog( this );
        settingsDialog->exec();
    }

If the operation is required immediately after the application is visible, a single shot timer may be used to start the processing after the main widget is visible:

    MainWidget::MainWidget( QWidget *parent, Qt::WFlags f )
        : QMainWindow( parent, f ), settingsDialog(0)
    {
        // After the event queue has been processed and the mainwindow is
        // visible, load the data.
        QTimer::singleShot(0, this, SLOT(loadData()));
    }

    void MainWidget::loadData()
    {
        // Open data file and populate data view.
    }