[Development] QML Singletons

Mohamed Fawzi Fawzi.Mohamed at digia.com
Tue Dec 11 19:59:39 CET 2012

I very much prefer the static keyword approach, as it does not depend on how the file is imported.

Note that one could make even nested static declarations work if one sees them as
static x in Type1 Type2=> add an instance of x identified with Type1Id.Type2Id.x (or the AST node of x, or the offset in the file) if not already in the module, and return that instance.
Not sure it will be very useful, but technically it should be doable.

Instantiation of the whole module would instantiate just the the non static parts, and resubstitute the static parts (as they are already allocated).
Indeed it would probably be better to disallow the assignment of properties on static objects via instantiation.

But maybe allowing a single root static is clearer, especially wrt. how to access its properties

From: development-bounces+fawzi.mohamed=digia.com at qt-project.org [development-bounces+fawzi.mohamed=digia.com at qt-project.org] on behalf of Alan Alpert [416365416c at gmail.com]
Sent: Tuesday, December 11, 2012 5:22 PM
To: Hartmann Thomas
Cc: <development at qt-project.org>; Knoll Lars
Subject: Re: [Development] QML Singletons

On Tue, Dec 11, 2012 at 1:52 AM, Thomas Hartmann
<Thomas.Hartmann at digia.com> wrote:
> Am 11/12/2012 10:23, schrieb Knoll Lars:
> Hi,
>>>> it's a bit weird that you need a qmldir for that. Wouldn't it be better if we could mark this as singleton in the implementation (ie. inside single.qml)? Maybe use a new keyword for that? static is already a reserved keyword in Ecmascript 5.1, so we could maybe write single.qml as:
>>>> static QtObject {
>>>>     property int myproperty;
>>>>     ...
>>>> }

It's more of a file level distinction, but that could work. So when a
QML file has the static keyword on its root object, it is loaded as a
singleton type? And the static keyword is invalid when not on a root

Keep in mind that this should be valid, even though it's a tree with
four objects:
static QtObject {
    property int myProperty
    property MyItem subItem: MySubItem { MySubSubItem{}, MySubSubItem{}}

>>> But how would the engine know about single.qml being static? AFAIK all
>>> .qml parsing is done on demand. The engine would have to know that
>>> single.qml is a singleton from another source (Or scan all .qml files
>>> upfront for the static keyword).
>> Sure, but it won't matter until you reference Single somewhere. The singleton should not get loaded before that anyway. And once you have a reference to it, the QML engine will try to load the file and then figure out it's static. That would require some work in the engine, but I think it might be doable.
> Ah. I thought the singletons would still be referenced by one global id.
> If the singleton is accessed by an object definition then the question
> is what exactly is allowed.
> All properties would probably have to be read only for example, or we
> would depend on the order of instantiation.
> MySingleton {
>     id: myLocalAccessId
> }
> MyItem {
>     localProperty: myLocalAcessId.property
> }
> Not sure if just using an id would not be simpler:
> MyItem {
>     localProperty: mySingleton.property
> }

Actually the way to *use* singletons in QML is already defined,
because you can declare a C++ type as a singleton (

So using the singleton will look like this

MyItem {
    localProperty: MySingleton.property

Attempting to create an instance of the type, even like MySingleton {
id: mySingleton }, would result in an error.

Alan Alpert
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