[SailfishDevel] Problem in interaction qml and c++

[David Llewellyn-Jones]

On 23/04/2020 10:50, michael fuchs wrote:
> Am 23.04.2020 08:59 schrieb David Llewellyn-Jones:
>> On 23/04/2020 09:51, Michael Fuchs wrote:
>>> How can AlbumList access mImgDB and mNetAccess?
>>>
>>> One solution could be to register the controller itself in qml and
>>> then write
>>> sth like this in qml:
>>>
>>> ====
>>> AlbumList {
>>>   imgDB: controller.imgDB
>>>   netAccess: controller.netAccess
>>> }
>>> ====
>>>
>>> but it's redundant.
>>
>> This doesn't look like a bad approach to me (although passing the
>> controller itself might be cleaner).
>>
>> An alternative might be to make Controller a singleton, that way your
>> AlbumList class could access it from the C++ without needing to be
>> explicitly passed a handle to it.
>>
>> But whether you can do that depends on the how Controller is used.
> 
> Thanks a lot.
> I'm not familiar with singletons, but this sounds promising.
> I found this simple example:
> 
> https://gist.github.com/pazdera/1098119

Yeah, this looks like a decent example: clear and complete.

As you can see from this, you essentially end up with a global object
you can access from anywhere. The downside is that you only ever have
one of them, so if you need more than one Controller, or (as Julian
mentions) think it might be needed in future, then this probably isn't
the right approach.

> I hope this also works for Q_OBJECT s.
> Maybe I should rather make netAccess and imgDB itself singletons?

I don't recall any issues with Q_OBJECTS being singletons, however if
you plan to use it in your QML you may also need to do some work
registering it as a singleton there too.

There's an example here:

https://doc.qt.io/qt-5/qqmlengine.html#qmlRegisterSingletonType

But I wouldn't worry about this unless you're already registering your
Controller as a QML type and need to use it from QML. As Julien also
pointed out, mixing singletons in C++ and QML adds complexity that can
lead to errors.

David
-- 
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