Qml Styling

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QML provides several mechanisms for styling a UI. Below are three common approaches.

Approach 1: Custom Component

QML supports defining your own custom components. Below, we create a custom component TitleText that we can use whenever our UI requires a title. If we want to change the appearance of all the titles in our UI, we can then just edit TitleText.qml, and the changes will apply wherever it is used.

// TitleText.qml
Text {
    horizontalAlignment: Text.AlignHCenter
    font.pixelSize: 50
    color: "green"
}

// In use
TitleText {
    text: "Title 1"
}

Approach 2: Style Singleton

In this approach we define a Style QML singleton object that contains a collection of properties defining the style. As a QML singleton, a common instance can be access from anywhere which imports this directory. Note that QML singletons require a qmldir file with the singleton keyword preceding the Style type declaration; they cannot be picked up implicitly like other types. When using Qrc resources both the Style.qml and the qmldir file must be included in the .qrc resource file.

// Style.qml
pragma Singleton
import QtQuick 2.0

QtObject {
    property int textSize: 20
    property color textColor: "green"
}

// In use
import QtQuick 2.0
import "." // QTBUG-34418, singletons require explicit import to load qmldir file

Text {
    font.pixelSize: Style.textSize
    color: Style.textColor
    text: "Hello World"
}

Approach 3: Hybrid (Style Singleton + Custom Component)

In this approach, we have a Style singleton that is used by our custom component.

// Style.qml
pragma Singleton
import QtQuick 2.0

QtObject {
    property int titleAlignment: Text.AlignHCenter
    property int titleFontSize: 50
    property color titleColor: "green"
}

// TitleText.qml
import QtQuick 2.0
import "." // QTBUG-34418, singletons require explicit import to load qmldir file

Text {
    horizontalAlignment: Style.titleAlignment
    font.pixelSize: Style.titleFontSize
    color: Style.titleColor
}

// In use
TitleText {
    text: "Title 1"
}

Nesting QtObjects

If you need to nest QtObject to access more defined properties (i.e. border.width.normal) you can do the following:

// Style.qml
pragma Singleton

QtObject {
    property QtObject window: QtObject {
        property color background: "white"
    }
    property QtObject border: QtObject {
        property QtObject width: QtObject {
            property int normal: 1
            property int big: 3
        }
        property QtObject color: QtObject {
            property color normal: "gray"
            property color focus: "blue"
            property color disabled: "red"
        }
    }
}