QML, user interface language

Qt Markup Language is a declarative UI language. It is compatible with the Qt framework and interfaces with its API. It can be extended with C code holding calls to this API.

To use the runtime, Qt Declarative, must be added to the Qt framework.

Qml demo screenshoots
QML demos: gallery and browser

Why use QML?

Building an interface on the Qt environment is easier with QML as it is easier to describe a Java interface with JavaFX, which is similar (while other platforms make use of XML, such as XAML for example).
Just one instruction is needed to display "Hello World!" cons seven in Qt as you may see on the list of "Hello World!" in all languages.

The processing can be much slower however with than the use of direct calls to the Qt framework, so the language is best suited for small applications with a simple interface, and less for heavier applications. One may also use QML during the development phase and then optimize the final application by translating in Qt if it is too slow.
It is generally recommended to use QML for mobile and Qt only on the desktop, maybe because a loading time shorter with QML.

According to the editor, the runtime being installed on a server, a QML application can works on the Web with HTTP, making it an alternative to HTML. A HTML page may be generated also by QML to be viewed in a browser and be powered by JavaScript.

QML has a JSON rather than XML syntax

Choosing a C inspired syntax allows for faster file processing and a reduced size. It is the principle of JSON too.

Code samples

Drawing a rectangle enclosing the words "Hello World!".

Rectangle {
   width: 200
   height: 200
   color: mouse.pressed ? "blue" : "red"  
   Text {          
     text: "Hello World"          
     anchors.centerIn: parent      

Including JavaScript code ...

Rectangle {      
   function calculateHeight() 
     ... statements... 
     return x;      

   height: calculateHeight()      
   width: { 
      if (height > 100) 200; else 50 

More on the new Qt site. We can be found on qt-project.org a demonstration showing how to build a browser interface in QML, with all the necessary widgets.