Background

Currently there are two ways to share global objects between QML components: share the global object as a context property from C++ or create the global object in the application root component where every other component has access to it. Latter approach creates strong coupling between the root component and sub-components and the sub-components can no longer be constructed in isolation because they depend on the properties of the root component. Also, sharing properties through a shared parent isn't really a solution for components libraries, which have no control of the context they have been created in. Context property approach is not a viable solution either for QML component libraries as introducing new context property produces a potential namespace conflicts when existing QML applications have already reserved the name of the context property for it's own use.

Syntax

One possible solution would be to provide a mechanism for sharing global elements between QML components using a special import syntax that would have to be defined under a mandatory namespace. It should be possible to create the shared object on demand, i.e. only when it gets imported from QML.

import libraryimportpath.globalobject versionnumber as Namespace
Item {
    Component.onCompleted: console.log(Namespace.property)
}

Use cases

Sharing default font between text elements.

import QtQuick.defaultFont 1.1 as DefaultFont
Text {
    font.family: DefaultFont.name
}

Sharing feedback object between button-like widgets.

import QtMobility.feedback.themeEffect 1.1 as ThemeEffect
Button {
    onPressed: ThemeEffect.play(ThemeEffect.Sensitive)
}

Sharing screen orientation information between application views.

import QtQuick.orientation 2.0 as Orientation
Window {
    rotation: Orientation.orientation == Orientation.Portrait ? 0 : 90
    Behavior on rotation { NumberAnimation { .. } } 
}

Sharing a model between different visual elements in the application, for example a clock model could be shared by the application status bar, alarm view and screen saver.

import MyApp.clockModel 1.0 as Clock
Statusbar {
    DigitalClock {
        minutes: Clock.minutes
        hours: Clock.hours
    }
}