QML Coding Conventions

This document contains the QML coding conventions that we follow in our documentation and examples and recommend that others follow.

QML Object Declarations

Throughout our documentation and examples, QML object attributes are always structured in the following order:

• id
• property declarations
• signal declarations
• JavaScript functions
• object properties
• child objects
• states
• transitions

For better readability, we separate these different parts with an empty line.

For example, a hypothetical photo QML object would look like this:

Rectangle {
    id: photo                                               // id on the first line makes it easy to find an object

    property bool thumbnail: false                          // property declarations
    property alias image: photoImage.source

    signal clicked                                          // signal declarations

    function doSomething(x)                                 // javascript functions
    {
        return x + photoImage.width
    }

    color: "gray"                                           // object properties
    x: 20                                                   // try to group related properties together
    y: 20
    height: 150
    width: {                                                // large bindings
        if (photoImage.width > 200) {
            photoImage.width;
        } else {
            200;
        }
    }

    Rectangle {                                             // child objects
        id: border
        anchors.centerIn: parent; color: "white"

        Image {
            id: photoImage
            anchors.centerIn: parent
        }
    }

    states: State {                                         // states
        name: "selected"
        PropertyChanges { target: border; color: "red" }
    }

    transitions: Transition {                               // transitions
        from: ""
        to: "selected"
        ColorAnimation { target: border; duration: 200 }
    }
}

Grouped Properties

If using multiple properties from a group of properties, consider using group notation instead of dot notation if it improves readability.

For example, this:

Rectangle {
    anchors.left: parent.left; anchors.top: parent.top; anchors.right: parent.right; anchors.leftMargin: 20
}

Text {
    text: "hello"
    font.bold: true; font.italic: true; font.pixelSize: 20; font.capitalization: Font.AllUppercase
}

could be written like this:

Rectangle {
    anchors { left: parent.left; top: parent.top; right: parent.right; leftMargin: 20 }
}

Text {
    text: "hello"
    font { bold: true; italic: true; pixelSize: 20; capitalization: Font.AllUppercase }
}

Lists

If a list contains only one element, we generally omit the square brackets.

For example, it is very common for a component to only have one state.

In this case, instead of:

states: [
    State {
        name: "open"
        PropertyChanges { target: container; width: 200 }
    }
]

we will write this:

states: State {
    name: "open"
    PropertyChanges { target: container; width: 200 }
}

JavaScript Code

If the script is a single expression, we recommend writing it inline:

Rectangle { color: "blue"; width: parent.width / 3 }

If the script is only a couple of lines long, we generally use a block:

Rectangle {
    color: "blue"
    width: {
        var w = parent.width / 3
        console.debug(w)
        return w
    }
}

If the script is more than a couple of lines long or can be used by different objects, we recommend creating a function and calling it like this:

function calculateWidth(object)
{
    var w = object.width / 3
    // ...
    // more javascript code
    // ...
    console.debug(w)
    return w
}

Rectangle { color: "blue"; width: calculateWidth(parent) }

For long scripts, we will put the functions in their own JavaScript file and import it like this:

import "myscript.js" as Script

Rectangle { color: "blue"; width: Script.calculateWidth(parent) }

If the code is longer than one line and hence within a block, we use semicolons to indicate the end of each statement:

MouseArea {
    anchors.fill: parent
    onClicked: {
        var scenePos = mapToItem(null, mouseX, mouseY);
        console.log("MouseArea was clicked at scene pos " + scenePos);
    }
}

Related Information

• Best Practices for QML and Qt Quick