- Qt 5.5
- QML Coding Conventions
- Qt 5.5.1 Reference Documentation

# 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; y: 20; height: 150                               // try to group related properties together
        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) }