[Development] Difficulties using Qt's new categorized logging. How to install filters?

[Mark]

On Tue, Oct 15, 2013 at 10:10 AM, Koehne Kai <Kai.Koehne at digia.com> wrote:
>
>> -----Original Message-----
>> From: Mark [mailto:markg85 at gmail.com]
>> Sent: Monday, October 14, 2013 5:40 PM
>> To: Koehne Kai
>> Cc: development at qt-project.org
>> Subject: Re: [Development] Difficulties using Qt's new categorized logging.
>> How to install filters?
>> [...]
>> Hi Kai,
>>
>> Thank you very much for your explanation! Would you also be able to
>> provide an example for installFilter? Since that is the one where i can't figure
>> out how it's supposed to work.
>
> Sure.
>
> So what you can do is define your own category filter:
>
> QLoggingCategory::CategoryFilter oldCategoryFilter;
>
> void myCategoryFilter(QLoggingCategory *category)
> {
>     if (qstrcmp(category->categoryName(), "qt.driver.usb") == 0) {
>         category->setEnabled(QtDebugMsg, true);
>     } else {
>         // let the default filter decide based on text rules
>         *oldCategoryFilter(category);
>     }
> }
>
> And install  it:
>
> oldCategoryFilter = QLoggingCategory::installFilter(myCategoryFilter);
>
>
> inside installFilter() myCategoryFilter will be called for every QLoggingCategory object already available. It will also be called whenever a new QLoggingCategory object is created, and must be reentrant (might be called from multiple threads concurrently).
>
>
> For this specific example you'd be arguably better of just defining a rule by setFilterRules. installFilter is mainly useful if you're e.g. defining your own logging framework, and say enable/disable categories based on some configuration coming from a file, or network ... . It can also be used to e.g. log all categories that are created during an application run :)
>
> I'm currently trying to improve the documentation based on your questions, please feel free to review:
>
> https://codereview.qt-project.org/#change,68114
>
> Regards
>
> Kai
>
> --
>    Kai Köhne, Senior Software Engineer - Digia, Qt
>    Digia Germany GmbH, Rudower Chaussee 13, D-12489 Berlin
>    Geschäftsführer: Mika Pälsi, Juha Varelius, Anja Wasenius
>    Sitz der Gesellschaft: Berlin. USt-IdNr: DE 286 306 868
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Hi Kai,

Now i finally get how it works! But why is it done this way? It just
doesn't smell like the Qt way of doing things. Here is an example of
how i would've expected something like this to work:


QLoggingCategory newCategory("some.new.category");
newCategory->setEnabled(QtDebugMsg, true);
QLoggingCategory::installFilter(newCategory);


Like that or something alike. So why isn't it done like that?
You have to agree with me that this looks a lot cleaner and much more
"the Qt way" then what is currently the situation?