[Qt-qml] Cannot store javascript object in property, why?

[michael.brasser at nokia.com]

On 17/12/2010, at 11:54 PM, ext Ville M. Vainio wrote:

> Currently, you cannot store javascript objects as properties in qml
> components, e.g. you can't do
> 
> propery JSRef myobj
> 
> function setCallback(f) { myobj = f };
> 
> On irc #qt-qml, this explanation was quoted:
> 
> QQQ
> In QML "property var" cannot store JavaScript native objects, as we
> cannot bind to them. This might change in a future release. For now,
> more explicit warnings are generated when you accidentally do this.
> QQQ

The core problem is that properties on a JS object don't give any notification when they are changed (in contrast to a QObject, which uses NOTIFY signals to inform when a property has changed). This means if you bind to such a property, e.g.

Text { text: myobj.textProp }

the binding will *not* be updated if you do something like

function myFunction() { myobj.textProp = "New Text" }

This is the reason we don't currently support it -- we didn't want to make it possible for these bindings to "silently fail" (and from memory providing a warning for such bindings was also non-trivial).

> I can't claim to understand the explanation, but I'm curious whether
> we will have this feature in the near future - as the lack of it seems
> to be a pretty fundamental shortcoming in QML, and workarounds (like
> storing the reference in a .js file, or using C++) seem quite hacky in
> comparison.


We certainly want to make JS/QML integration more seamless, but I'm not sure exactly how or when this will happen. It's not currently planned as part of the next release (QtQuick 1.1).

Regards,
Michael