Qt: Difference between revisions

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* A complete library that provides SQL database access, XML parsing, thread management, network support, and a unified cross-platform application programming interface (API) for file handling.
* A complete library that provides SQL database access, XML parsing, thread management, network support, and a unified cross-platform application programming interface (API) for file handling.


The Qt framework is emerging as a major development platform and is the basis of the [[KDE]] software community, among other important open source and proprietary applications such as [[VLC]], [[VirtualBox]], [[Mathematica]] and many others.
The Qt framework as basis of the [[KDE]] software, as well as other important open source and proprietary applications such as [[VLC]], [[VirtualBox]], [[Mathematica]] and many others.


== Installation ==
== Installation ==

Revision as of 15:32, 27 January 2019

Related articles

Qt is a cross-platform application and widget toolkit that uses standard C++ but makes extensive use of a special code generator (called the Meta Object Compiler, or moc) together with several macros to enrich the language. Some of its more important features include:

  • Running on the major desktop platforms and some of the mobile platforms.
  • Extensive internationalization support.
  • A complete library that provides SQL database access, XML parsing, thread management, network support, and a unified cross-platform application programming interface (API) for file handling.

The Qt framework as basis of the KDE software, as well as other important open source and proprietary applications such as VLC, VirtualBox, Mathematica and many others.

Installation

Two versions of Qt are currently available in the official repositories. They can be installed with the following packages:

  • Qt 5.x is available in the qt5-base package, with documentation in the qt5-doc package.
  • Qt 4.x is available in the qt4 package, with documentation on AUR in the qt4-docAUR package.
  • Qt 3.x is available from the AUR in the qt3AUR package, with documentation on AUR in the qt3-docAUR package.

Default Qt toolkit

Qt packages do not provide the usual bins (e.g. qmake) in /usr/bin anymore. Instead -qt5, -qt4 and -qt3 symlinks are provided (e.g. qmake-qt5, qmake-qt4, qmake-qt3). This may cause compilation failures in Qt3/4 applications.

By installing qtchooserAUR you can restore the usual bins (e.g. qmake) in /usr/bin and setup the Qt toolkit to use. By default Qt5 is used.

Warning: qtchooserAUR is now in conflict with qt5-base. You can install it in /usr/local if you really need it, but it's not officially supported. See FS#51308.

Using environment variables

To define the default Qt toolkit, you can create QT_SELECT environment variable. For example, to use Qt4, set QT_SELECT=4.

Using configuration files

You can set the default Qt toolkit by creating a symlink ~/.config/qtchooser/default.conf to one of .conf files in /etc/xdg/qtchooser/ directory. For example, to set Qt4 symlink /etc/xdg/qtchooser/4.conf to ~/.config/qtchooser/default.conf: 

$ ln -s /etc/xdg/qtchooser/4.conf ~/.config/qtchooser/default.conf

Appearance

Qt5

Qt5 decides the style to use based on what desktop environment is used:

  • In KDE Plasma, it uses the actually selected Qt style. It can be configured using KDE System Settings (systemsettings5), the settings can be found in Appearance > Application Style > Widget Style.
  • In Cinnamon, GNOME, MATE, LXDE, Xfce, it uses GTK+ (QGtkStyle).
  • In other desktop environments, it uses Fusion.

To force a specific style, you can set the QT_STYLE_OVERRIDE environment variable. Specifically, set it to gtk2 if you want to use the GTK+ theme (Note: you will need to install the Qt style plugins mention below to get the GTK+ style). Qt5 applications also support the -style flag, which you can use to launch a Qt5 application with a specific style.

The following styles are included in Qt5: Fusion, Windows. Others can be installed from the official repositories:

  • Breeze — Artwork, styles and assets for the Breeze visual style for the Plasma Desktop.
https://cgit.kde.org/breeze.git || breeze
  • Oxygen — KDE Oxygen style.
https://cgit.kde.org/oxygen.git || oxygen
  • QtCurve — A configurable set of widget styles for KDE and Gtk.
https://cgit.kde.org/qtcurve.git || qtcurve-qt5
  • Adwaita-Qt — A style to bend Qt applications to look like they belong into GNOME Shell.
https://github.com/MartinBriza/adwaita-qt || adwaita-qt5AUR
  • Qt style plugins — Additional style plugins for Qt5, including GTK+, Cleanlooks, Motif, Plastique.
http://code.qt.io/cgit/qt/qtstyleplugins.git || qt5-styleplugins

Qt4

Qt4 application will try to mimic the behavior of the desktop environment they are running on, unless they run into some problems or hard-coded settings.

  • In KDE Plasma, it uses the actually selected Qt style. It can be configured using KDE System Settings (systemsettings5), the settings can be found in Appearance > Application Style > Widget Style.
  • In Cinnamon, GNOME, Xfce, it uses GTK+ (QGtkStyle).
  • In other desktop environments, it uses Windows.

For those who want to change the look and feel of Qt4 applications, the Qt Configuration (qtconfig-qt4) GUI tool is provided by the qt4 package. It offers a simple interface to configure the appearance of Qt4 applications including style, colors, fonts and some further options.

Note: If you use GTK+ style, then color and font settings will be ignored, and inherited from GTK+ 2.

Qt keeps all its configuration information in /etc/xdg/Trolltech.conf (system-wide) or ~/.config/Trolltech.conf (user-specific). The file is rather difficult to navigate because it contains a lot of information not related to appearance, but for any changes you can just add to the end of the file and overwrite any previous values (make sure to add your modification under the [Qt] header).

For example, to change the theme to QtCurve, add: 

~/.config/Trolltech.conf
...
[Qt]
style=QtCurve

The following styles are included in Qt4: CDE, Cleanlooks, GTK+, Motif, Plastique, Windows. Others can be installed from the official repositories or the AUR (most are written for KDE Plasma Desktop):

  • Breeze — Artwork, styles and assets for the Breeze visual style for the Plasma Desktop.
https://cgit.kde.org/breeze.git || breeze-kde4[broken link: Template:Aur-mirror]
https://cgit.kde.org/oxygen.git || oxygen-kde4[broken link: Template:Aur-mirror]
  • QtCurve — A configurable set of widget styles for KDE and Gtk.
https://cgit.kde.org/qtcurve.git || qtcurve-qt4
  • Adwaita-Qt — A style to bend Qt applications to look like they belong into GNOME Shell.
https://github.com/MartinBriza/adwaita-qt || adwaita-qt4AUR

Qt Style Sheets

An interesting way of customizing the look and feel of a Qt application is using Style Sheets, which are just simple CSS files. Using Style Sheets, one can modify the appearance of every widget in the application.

To run an application with a different style just execute: 

$ qt_application -stylesheet style.qss

For more information on Qt Style Sheets see the official documentation or other tutorials. As an example Style Sheet see this Dolphin modification.

GTK+ and Qt

If you have GTK+ and Qt applications, their looks might not exactly blend in very well. If you wish to make your GTK+ styles match your Qt styles please read Uniform look for Qt and GTK applications.

Configuration of Qt5 apps under environments other than KDE Plasma

Unlike Qt4, Qt5 does not ship a qtconfig utility to configure fonts, icons or styles. Instead, it will try to use the settings from the running desktop environment. In KDE Plasma or GNOME this works well, but in other less popular desktop environments or window managers it can lead to missing icons in Qt5 applications. One way to solve this is to fake the running desktop environment by setting XDG_CURRENT_DESKTOP=KDE or GNOME, and then using the corresponding configuration application to set the desired icon set.

Another solution is provided by the qt5ct package, which provides a Qt5 QPA independent of the desktop environment and a configuration utility. After installing the package, run qt5ct to set an icon theme, and set the environment variable QT_QPA_PLATFORMTHEME="qt5ct" so that the settings are picked up by Qt applications. Alternatively, use --platformtheme qt5ct as argument to the Qt5 application.

If the below errors are received, and some icons still do not appear in some of the apps, install oxygen and oxygen-icons: 

Icon theme "oxygen" not found.
Icon theme "oxygen" not found.
Error: standard icon theme "oxygen" not found!

Development

Supported platforms

Qt supports most platforms that are available today, even some of the more obscure ones, with more ports appearing every once in a while. For a more complete list see the Qt Wikipedia article.

Android

First of all, you need an Android SDK and NDK. Install SDK android-sdkAUR (some tools have been removed) or android-sdk-25.2.5AUR and NDK android-ndk-10eAUR from AUR or using Android Studio. It is highly recommended to install NDK version 10e because of some known issues. Next you are going to need Qt 5 for Android. You can install it from AUR as described below or build it yourself, you can find build instructions on Qt wiki page.

Or you can use a Qt Installer, for a full list see Official Qt installers.

Tools

The following are official Qt tools:

  • Qt Creator — A cross-platform IDE tailored for Qt that supports all of its features.
http://doc.qt.io/qtcreator/ || qtcreator
  • Qt Linguist — A set of tools that speed the translation and internationalization of Qt applications.
http://doc.qt.io/qt-5/qtlinguist-index.html || Qt 5: qt5-tools, Qt 4: qt4
  • Qt Assistant — A configurable and redistributable documentation reader for Qt qch files.
http://doc.qt.io/qt-5/qtassistant-index.html || Qt 5: qt5-tools, Qt 4: qt4
  • Qt Designer — A powerful cross-platform GUI layout and forms builder for Qt widgets.
http://doc.qt.io/qt-5/qtdesigner-manual.html || Qt 5: qt5-tools, Qt 4: qt4
  • Qt Quick Designer — A visual editor for QML files which supports WYSIWYG. It allows you to rapidly design and build Qt Quick applications and components from scratch.
http://doc.qt.io/qtcreator/creator-using-qt-quick-designer.html || qtcreator
  • qmlscene — A tool for loading QML documents that makes it easy to quickly develop and debug QML applications.
http://doc.qt.io/qt-5/qtquick-qmlscene.html || Qt 5: qt5-declarative, Qt 4 QML Viewer: qt4
  • qmake — A tool that helps simplify the build process for development project across different platforms, similar to cmake, but with fewer options and tailored for Qt applications.
http://doc.qt.io/qt-5/qmake-manual.html || Qt 5: qt5-base, Qt 4: qt4
  • uic — A tool that reads *.ui XML files and generates the corresponding C++ files.
http://doc.qt.io/qt-5/uic.html || Qt 5: qt5-base, Qt 4: qt4
  • rcc — A tool that is used to embed resources (such as pictures) into a Qt application during the build process. It works by generating a C++ source file containing data specified in a Qt resource (.qrc) file.
http://doc.qt.io/qt-5/rcc.html || Qt 5: qt5-base, Qt 4: qt4
  • moc — A tool that handles Qt's C++ extensions (the signals and slots mechanism, the run-time type information, and the dynamic property system, etc.).
http://doc.qt.io/qt-5/moc.html || Qt 5: qt5-base, Qt 4: qt4

Bindings

Qt has bindings for all of the more popular languages, for a full list see this list.

The following examples display a small 'Hello world!' message in a window.

C++

  • Package:
    • qt4 - Version 4.x of the Qt toolkit.
    • qt5-base - Version 5.x of the Qt toolkit.
  • Website: http://qt-project.org/
  • Build:
    • The Qt4 version: g++ $(pkg-config --cflags --libs QtGui) -o hello hello.cpp
    • The Qt5 version: g++ $(pkg-config --cflags --libs Qt5Widgets) -fPIC -o hello hello.cpp
  • Run with: ./hello
hello.cpp
#include <QApplication>
#include <QLabel>

int main(int argc, char **argv)
{
    QApplication app(argc, argv);
    QLabel hello("Hello world!");

    hello.show();
    return app.exec();
}

QML

hello.qml
import QtQuick 1.0

Rectangle {
    id: page
    width: 400; height: 100
    color: "lightgray"

    Text {
        id: helloText
        text: "Hello world!"
        anchors.horizontalCenter: page.horizontalCenter
        anchors.verticalCenter: page.verticalCenter
        font.pointSize: 24; font.bold: true
    }
}
Note: For version 5.x of the Qt toolkit, we need to import QtQuick 2.y.

Python (PyQt)

hello-pyqt.py
import sys
from PyQt4 import QtGui

app = QtGui.QApplication(sys.argv)
label = QtGui.QLabel("Hello world!")

label.show()
sys.exit(app.exec_())

The Qt 5.x version is slighly different: 

hello-pyqt.py
import sys
from PyQt5 import QtWidgets

app = QtWidgets.QApplication(sys.argv)
label = QtWidgets.QLabel("Hello world!")

label.show()
sys.exit(app.exec_())

Python (PySide)

hello-pyside.py
import sys
from PySide.QtCore import *
from PySide.QtGui import *

app = QApplication(sys.argv)
label = QLabel("Hello world!")

label.show()
sys.exit(app.exec_())

C#

See QtSharp.

Ruby

hello.rb
require 'Qt4'

app = Qt::Application.new(ARGV)
hello = Qt::Label.new('Hello World!')

hello.show
app.exec

Perl

hello.pl
use QtGui4;

my $a = Qt::Application(\@ARGV);
my $hello = Qt::Label("Hello World!", undef);

$hello->show;
exit $a->exec;

Lua

hello.lua
label = qt.new_widget("QLabel")

label:setText("Hello World!")
label:show()
Note: QtLua is not designed to develop an application in pure Lua but rather to extend a Qt C++ application using Lua as scripting language.

Troubleshooting

Disable/Change Qt journal logging behaviour

When using KDE and/or any other Qt desktop environment debug info may be frequently be logged in the systemd journal.

Set QT_LOGGING_RULES as environment variable to change this behaviour, e.g. to completely disable logging: 

/etc/environment
QT_LOGGING_RULES='*=false'

To allow only debug logging, use QT_LOGGING_RULES="*.debug=false".

Icon theme is not applied

Since Qt 5.1 SVG support has moved into a module. Since qt5-base does not depend on qt5-svg it may happen that the qt5-base is installed but not qt5-svg. This results in deceptive icon theme behaviour. Since SVG is not supported the icons are silently skipped and the icon theme may seem to be unused. Installing qt5-svg explicitly solves the problem.

Theme not applied to root applications

As the user theme file ($XDG_CONFIG_HOME/Trolltech.conf), are not read by other accounts, the selected theme will not apply to X applications run as root. Possible solutions include:

  • Create symlinks, e.g 
    # ln -s /home/[username]/.config/Trolltech.conf /etc/xdg/Trolltech.conf
  • Configure system-wide theme file: /etc/xdg/Trolltech.conf
  • Adjust the theme as root

Qt4 style not respected

If pure Qt4 (non-KDE) applications do not stick with your selected Qt4 style, then you will probably need to tell Qt4 how to find KDE's styles (Oxygen, Phase etc.). You just need to set the environment variable QT_PLUGIN_PATH. E.g.: 

QT_PLUGIN_PATH=$HOME/.kde4/lib/kde4/plugins/:/usr/lib/kde4/plugins/

qtconfig-qt4 should then be able to find your kde styles and everything should look nice again!

Alternatively, you can symlink the Qt4 styles directory to the KDE4 styles one: 

# ln -s /usr/lib/{kde,qt}4/plugins/styles/theme_name

Applications using QML crash or do not work with Qt 5.8 - 5.10

Note: This is meant to be resolved with Qt 5.11 (see BBS thread 222486: sddm-helper crashes), published into the main repositories in late May 2018.

Between Qt 5.8 and 5.10, applications that rely on QML (such as SDDM or some KDE programs) may crash or fail to function correctly if they do not have execution privileges under /home or /var (e.g. if these are separate filesystems mounted as 'noexec'). This is a result of the qmlcache feature, which relies on being able to write files out to .cache directories and then execute them.

If you do not want to -- or cannot -- allow such execution privileges, a workaround is to set the following as appropriate in your environment variables: 

QML_DISABLE_DISK_CACHE=1

All Qt5-based applications fail to run after Qt5 update

If you get an error similar to 

Qt FATAL: Cannot mix incompatible Qt library (version 0x50900) with this library (version 0x50901)

then you are most likely using a Qt5 platform theme or style plugin which has not been recompiled against the latest version of Qt5. These usually use Qt private headers which means they depend on an exact version of Qt and not just a matching soname. Figure out which theme/style you are using by checking the QT_STYLE_OVERRIDE and QT_QPA_PLATFORMTHEME environment variables, and rebuild the AUR package that provides it.

See also

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