Basic syntax

Example:

 import QtQuick

 Rectangle {
     id: canvas
     width: 250
     height: 200
     color: "blue"

     Image {
         id: logo
         source: "pics/logo.png"
         anchors.centerIn: parent
         x: canvas.height / 5
     }
 }

Objects are specified by their type, followed by a pair of braces. Object types always begin with a capital letter. In the example above, there are two objects, a Rectangle; and its child, an Image. Between the braces, one can specify information about the object, such as its properties. Properties are specified as property: value. In the example above, we can see the Image has a property named source, which has been assigned the value pics/logo.png. The property and its value are separated by a colon.

The id property

Each object can be given a special unique property called an id. Assigning an id enables the object to be referred to by other objects and scripts. The first Rectangle element below has an id, myRect. The second Rectangle element defines its own width by referring to myRect.width, which means it will have the same width value as the first Rectangle element.

 Item {
     Rectangle {
         id: myRect
         width: 120
         height: 100
     }
     Rectangle {
         width: myRect.width
         height: 200
     }
 }

Note that an id must begin with a lower-case letter or an underscore, and cannot contain characters other than letters, digits and underscores.

Property bindings

A property binding specifies the value of a property in a declarative way. The property value is automatically updated if the other properties or data values change, following the reactive programming paradigm.

Property bindings are created implicitly in QML whenever a property is assigned a JavaScript expression. The following QML uses two property bindings to connect the size of the rectangle to that of otherItem.

 Rectangle {
     width: otherItem.width
     height: otherItem.height
 }

QML extends a standards-compliant JavaScript engine, so any valid JavaScript expression can be used as a property binding. Bindings can access object properties, make function calls, and even use built-in JavaScript objects like Date and Math.

Example:

 Rectangle {
     function calculateMyHeight() {
         return Math.max(otherItem.height, thirdItem.height);
     }
     anchors.centerIn: parent
     width: Math.min(otherItem.width, 10)
     height: calculateMyHeight()
     color: width > 10 ? "blue" : "red"
 }

States

States are a mechanism to combine changes to properties in a semantic unit. A button for example has a pressed and a non-pressed state, an address book application could have a read-only and an edit state for contacts. Every element has an "implicit" base state. Every other state is described by listing the properties and values of those elements which differ from the base state.

Example: In the default state, myRect is positioned at 0,0. In the "moved" state, it is positioned at 50,50. Clicking within the mouse area changes the state from the default state to the "moved" state, thus moving the rectangle.

 import QtQuick

 Item {
     id: myItem
     width: 200; height: 200

     Rectangle {
         id: myRect
         width: 100; height: 100
         color: "red"
     }
     states: [
         State {
             name: "moved"
             PropertyChanges {
                 target: myRect
                 x: 50
                 y: 50
             }
         }
     ]
     MouseArea {
         anchors.fill: parent
         onClicked: myItem.state = 'moved'
     }
 }

State changes can be animated using Transitions.

For example, adding this code to the above Item element animates the transition to the "moved" state:

 transitions: [
     Transition {
         from: "*"
         to: "moved"
         NumberAnimation { properties: "x,y"; duration: 500 }
     }
  ]

Animation

Animations in QML are done by animating properties of objects. Properties of type real, int, color, rect, point, size, and vector3d can all be animated.

QML supports three main forms of animation: basic property animation, transitions, and property behaviors.

The simplest form of animation is a PropertyAnimation, which can animate all of the property types listed above. A property animation can be specified as a value source using the Animation on property syntax. This is especially useful for repeating animations.

The following example creates a bouncing effect:

 Rectangle {
     id: rect
     width: 120; height: 200

     Image {
         id: img
         source: "pics/qt.png"
         x: 60 - img.width/2
         y: 0

         SequentialAnimation on y {
             loops: Animation.Infinite
             NumberAnimation { to: 200 - img.height; easing.type: Easing.OutBounce; duration: 2000 }
             PauseAnimation { duration: 1000 }
             NumberAnimation { to: 0; easing.type: Easing.OutQuad; duration: 1000 }
         }
     }
 }

Familiar concepts

QML provides direct access to the following concepts from Qt:

• QObject signals – can trigger callbacks in JavaScript
• QObject slots – available as functions to call in JavaScript
• QObject properties – available as variables in JavaScript, and for bindings
• QWindow – Window creates a QML scene in a window
• Q*Model – used directly in data binding (e.g. QAbstractItemModel)^[30][31][32]

Signal handlers

Signal handlers are JavaScript callbacks which allow imperative actions to be taken in response to an event. For instance, the MouseArea element has signal handlers to handle mouse press, release and click:

 MouseArea {
     onPressed: console.log("mouse button pressed")
 }

All signal handler names begin with "on".

How-tos

• Integrating QML and C++
• Important C++ Classes Provided By The Qt QML Module