[Development] Another method of registering QML types

[BRM]

----- Original Message -----

> From: Alan Alpert <416365416c at gmail.com>
> On Thu, Nov 8, 2012 at 12:49 PM, BRM <bm_witness at yahoo.com> wrote:
>>>  From: Alan Alpert <416365416c at gmail.com>
>> 
>>>  To: development <development at qt-project.org>
>>>  Cc:
>>>  Sent: Thursday, November 8, 2012 12:57 PM
>>>  Subject: [Development] Another method of registering QML types
>>> 
>>>  Currently, there is no way to register QML files as types from C++.
>>>  This is the exact same functionality that qmldir provides, but I think
>>>  there are situations where you'll want to do this progamatically
>>>  instead of with a qmldir file. There is a very specific example I have
>>>  in mind: Platform Components.
>>> 
>>>  I'm suggesting another qmlRegisterTypes function, one that takes a 
> URL
>>>  instead of a C++ type:
>>> 
>>>  qmlRegisterType(const char* url, const char *uri, int versionMajor,
>>>  int versionMinor, const char *qmlName).
>>> 
>>>  https://codereview.qt-project.org/#change,39135 is a proof of concept
>>>  implementation.
>>> 
>>>  This would allow for a platform component import which looks like this:
>>>  if(platform=="desktoplinux")
>>>     
> qmlRegisterType("/usr/share/desktop/components/Button.qml", uri,
>>>  2, 0, "Button");
>>>  else if (platform=="meego")
>>>      qmlRegisterType("/usr/share/meego/components/Button.qml", 
> uri, 2,
>>>  0, "Button");
>>>  ...
>>> 
>>>  Except that the strings would also be generated procedurally, instead
>>>  of having a dozen lines of code per component type.
>>> 
>>>  The function isn't strictly for that usecase, there are other cases
>>>  where you might want dynamically directed types. The other case that
>>>  comes to mind is if you have an application with built-in types and
>>>  you want to add a file from qrc as a type to outside files (or
>>>  vice-versa). It would be nice to hear other usecases if anyone has
>>>  them, to ensure this API can meet those requirements.
>>> 
>>>  Any comments on this suggested addition to the QtDeclarative public 
> API?
>> 
>>  Sounds interesting. Here's another potential use case: server-side 
> interface definitions.
>> 
>>  I have a system now where QML (actually its predecessor QtScript) was of 
> interest to me (haven't gotten to using it yet) so that I could put 
> interfaces for the system in a (trusted) server component.
>>  The generic (multi-system, multi-customer) user interface would have 
> theoretically downloaded the interface descriptions (e.g. QML files) and then 
> loaded them.
>>  I never did it at the time as I hadn't figured out the storage side, 
> and got into doing some custom widgets for some of the functionality.
>> 
>>  >From this perspective it would be useful to be able to hand it a 
> QByteArray containing a QML file, or http/ftp/custom URLs for remote QML file 
> access.[1]
>> 
>>  One thought - it would probably be good to have a function that could 
> return other URLs that would also need to be loaded for successful use, perhaps 
> as an error condition, or qmlRegisterType() would need to be able to read the 
> embedded URLs and recurse to do the same.
>> 
>>  As I haven't gotten into using QML yet, I don't know how well QML 
> would meet that kind of use. It'd be a really cool one to support though.
>> 
> I think this is already supported, in that the URL transparency of QML
> means that if you have local assets with a remote file, it should
> automatically load the assets remotely to go with it. I might need a
> more specific example of what you mean if you're talking about
> something not already supported, but I know the following is
> supported:
> If you have a http://remote/Image.qml with code like Image { source:
> "graphic.png" } the png will be fetched transparently.

As I said, i have not had the chance to play with QML. So if what I said is already supported great!

Now, in my hypothetical case, I was planning on storing the dialog description probably as text within the server somehow. I never got to those details;
but the server I have does not present a normal URL style interface - that is, it's not an http server but a specific application server with a proprietary XML protocol;
so a client application would have to request the data and receive a big XML blob with the QML-based dialog and then somehow load it - thus it would be nice to be able to detect any imports not loaded so that it would be able to retrieve and load them.

Any how...that is just a hypothetical, but one that may be very useful to consider if people are not doing that already. If I had the time, I'd probably invest more into QML and do just I described; but I'd still have to figure out the server-side storage and transport issues for my server. (A web server is not an option; though if sftp is support, that would be.) Any how...enough dreaming...
 
>>  [1] Yes, I realize that it would enable some on-the-fly stuff that might 
> generate some security concerns. I would suggest that be documented so that 
> users know they have to load appropriately trusted materials if we did that. 
> They could just as easily write it to a temp file and load the temp file using 
> the regular API too.
> 
> That is the current alternative. You can do the exact same thing by
> writing out a temporary qmldir file to disk - it's just that I think
> that approach is horrible and unnecessary. (This is actually more of a
> security risk, because you could be overwriting the qmldir files for
> other imports, like "." ).

True - though I was thinking the application would just store it in say /tmp or /tmp/<user>.<application> and configure Qt to be able to use that dir as well for QML - e.g. something the application controlled, not Qt.

$0.02

Ben