1. Meet Qt 5¶

1.1.1. Qt5 Focus¶

Qt 5 is a complete refreshment of the very successful Qt 4 release. With Qt 4.8, the Qt 4 release is almost 7 years old. It’s time to make an amazing toolkit even more amazing. Qt 5 is focused on the the following:

• Outstanding Graphics: Qt Quick 2 is based on OpenGL (ES) using a scene graph implementation. The recomposed graphics stack allows a new level of graphic effects combined with an ease of use never seen before in this field.
• Developer Productivity: QML and JavaScript are the primary means for UI creation. The back-end will be driven by C++. The split between JavaScript and C++ allows a fast iteration for front-end developers concentrating on creating beautiful user interfaces and back-end C++ developers concentrating on stability, performance and extending the runtime.
• Cross-platform portability: With the consolidated Qt Platform Abstraction, it is now possible to port Qt to a wider range of platforms easier and faster. Qt 5 is structured around the concept of Qt Essentials and Add-ons, which allows OS developer to focus on the essentials modules and leads to a smaller runtime altogether.
• Open Development: Qt is now a truly open-governance project hosted at Qt-Project. The development is open and community driven.

1.2.1. Qt Quick¶

Qt Quick is the umbrella term for the user interface technology used in Qt5. Qt Quick itself is a collection of several technologies:

• QML - Markup language for user interfaces
• JavaScript - The dynamic scripting language
• Qt C++ - The highly portable enhanced c++ library

Similar to HTML, QML is a markup language. It is composed of tags called elements in QtQuick enclosed in curly brackets Item {}. It was designed from the ground up for the creation of user interfaces, speed and easier reading for developers. The user interface can be enhanced using JavaScript code. Qt Quick is easily extend able with your own native functionality using Qt C++. In short the declarative UI is called the front-end and the native parts are called the back-end. This allows you to separate the computing intensive and native operation of your application from the user interface part.

In a typical project the front-end is developed in QML/JavaScript and the back-end code, which interfaces with the system and does the heavy lifting is developed using Qt C++. This allows a natural split between the more design oriented developers and the functional developers. Typically the back-end is tested using Qt own unit testing framework and exported for the front-end developers to be used.

1.2.2. Digesting an User Interface¶

Let’s create a simple user interface using QtQuick, which showcases some aspects of the QML language. At the end we will have a paper windmill with rotating blades.

We start with an empty document called main.qml. All QML files will have the ending .qml. As a markup language (like HTML) a QML document needs to have one and only one root element, which in our case is the Image element with a width and height based on the background image geometry:

import QtQuick 2.3

Image {
    id: root
    source: "images/background.png"
}

As QML does not make any restriction which element type is the root element we use an Image element with the source property set to our background image as the root element.

The foreground elements pole and pin wheel of our user interface are placed as separate images.

The pole needs to be placed in the horizontal center of the background towards the bottom. And the pinwheel can be placed in the center of the background.

Normally your user interface will be composed of many different element types and not only image elements like in this example.

Image {
    id: root
    ...
    Image {
        id: pole
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.bottom: parent.bottom
        source: "images/pole.png"
    }

    Image {
        id: wheel
        anchors.centerIn: parent
        source: "images/pinwheel.png"
    }
    ...
}

To place the pin wheel at the central location we use a complex property called anchor. Anchoring allows you to specify geometric relations between parent and sibling objects. E.g. Place me in the center of another element ( anchors.centerIn: parent ). There are left, right, top, bottom, centerIn, fill, verticalCenter and horizontalCenter relations on both ends. Sure they need to match, it does not make sense to anchor my left side to the top side of an element.

So we set the pinwheel to be centered in the parent our background.

To make the showcase a little bit more interesting, we would like to make the scene interactive. The idea is to rotate the wheel when the user pressed the mouse somewhere in the scene.

We use the MouseArea element and make it as big as our root element.

Image {
    id: root
    ...
    MouseArea {
        anchors.fill: parent
        onClicked: wheel.rotation += 90
    }
    ...
}

The mouse area emit signals when a user clicks inside it covered area. You can hook onto this signal overriding the onClicked function. In this case the reference the wheel image and change its rotation by +90 degree.

Now the wheel will rotate, but it is still not fluent yet. The rotation property changes immediately. What we would like that the property changes by 90 degree over time. Now animations come into play. An animation defines how a property change is distributed over a duration. To enable this we use an animation type called property behavior. The Behaviour does specify an animation for a defined property for every change applied to that property. In short every time the property changes, the animation is run. This is only one of several ways of declaring an animation in QML.

Image {
    id: root
    Image {
        id: wheel
        Behavior on rotation {
            NumberAnimation {
                duration: 250
            }
        }
    }
}

Now whenever the property rotation of the wheel changes it will be animated using a NumberAnimation with a duration of 250 ms. So each 90 degree turn will take 250 ms.

Now the wheel looks already much better. I hope this has given you a short idea of how Qt Quick programming works.

1.3.1. Qt Modules¶

The Qt Essentials modules are mandatory for a Qt enabled platform. They offer the foundation to develop a modern Qt 5 Application using Qt Quick 2.

Core-Essential Modules

The minimal set of Qt 5 modules to start QML programming.

Qt Addon Modules

Besides the essential modules, Qt offers additional modules for software developers, which are not part of the release. Here is a short list of add-on modules available.

• Qt 3D - A set of APIs to make 3D graphics programming easy and declarative.
• Qt Bluetooth - C++ and QML APIs for platforms using Bluetooth wireless technology.
• Qt Contacts - C++ and QML APIs for accessing addressbooks / contact databases
• Qt Location - Provides location positioning, mapping, navigation and place search via QML and C++ interfaces. NMEA backend for positioning
• Qt Organizer - C++ and QML APIs for accessing organizer events (todos, events, etc.)
• Qt Publish and Subscribe
• Qt Sensors - Access to sensors via QML and C++ interfaces.
• Qt Service Framework - Enables applications to read, navigate and subscribe to change notifications.
• Qt System Info - Discover system related information and capabilities.
• Qt Versit - Support for vCard and iCalendar formats
• Qt Wayland - Linux only. Includes Qt Compositor API (server), and Wayland platform plugin (clients)
• Qt Feedback - Tactile and audio feedback to user actions.
• Qt JSON DB - A no-SQL object store for Qt.

1.3.2. Supported Platforms¶

Qt supports a variety of platforms. All major desktop and embedded platforms are supported. Through the Qt Application Abstraction, nowadays it’s easier to port Qt over to your own platform if required.

Testing Qt 5 on a platform is time consuming. A sub-set of platforms was selected by the qt-project to build the reference platforms set. These platforms are thoroughly tested through the system testing to ensure the best quality. Mind you though: no code is error free.