[Qt-creator] Couple of questions about the design of Qt Creator

[Eike Ziller]

> On Sep 11, 2017, at 09:24, Elvis Stansvik <elvstone at gmail.com> wrote:
> 
> 2017-09-10 11:31 GMT+02:00 Elvis Stansvik <elvstone at gmail.com>:
>> 2017-09-10 11:03 GMT+02:00 Elvis Stansvik <elvstone at gmail.com>:
>>> Hi all,
>>> 
>>> In a quest to find inspiration for good Qt application architectures,
>>> I've been looking at the plugin based one you're using in Qt Creator.
>>> It strikes me as a really nice design.
>>> 
>>> I've been reading the available docs on it, and dug into the code a
>>> bit. This may be a bit much to ask, but I was wondering if any of you
>>> devs could answer a few questions that popped up? It would be much
>>> appreciated!
>>> 
>>> It's really just two questions, about two different topics:
>>> 
>>> 1. The Invoker / invoke<...> Thingie:
>>> 
>>> You have ExtensionSystem::Invoker and the associated invoke<..>
>>> helper, which are syntactic sugar for achieving "soft" extension
>>> points. It seems it's not used that much (?). I grepped for
>>> "Invoker|invoke<" in the code and could only find a few uses of it. I
>>> also grepped for "invokeMethod" to see if the approach was being used
>>> "manually" so to speak (without the sugar), and found a few more hits.
>>> 
>>> What was the motivation for adding this? I assume it's for cases where
>>> you want a looser coupling between plugins (no linking, no shared
>>> header), but can you give an example of when you really wanted that
>>> looser coupling and why?
>>> 
>>> 2. The Plugin System in General:
>>> 
>>> Is there anything about the plugin system in its current form, or how
>>> it is used, that you would do fundamentally different if you could do
>>> it all over again? Any areas that you find messy/awkward, that need a
>>> re-think/makeover? In short: What are the biggest warts in the code in
>>> your opinion?
>> 
>> As soon as I hit send, I realized I have a third question:
>> 
>> 3. Communication Between Plugins:
>> 
>> There seems to be two main mechanisms through which plugins
>> communicate: Either objects that implement shared interfaces are added
>> to the plugin manager object pool and picked up by downstream or
>> upstream plugins (in the top-down or bottom-up phase of plugin
>> initialization, respectively), or a singleton instance is acquired and
>> calls made on it.
>> 
>> Is the former approach used when dependants provide functionality to
>> their dependees (which are unknown), and the latter approach used when
>> dependees use their dependants (which are known)? Is that the deciding
>> factor?
> 
> And finally, a couple of more down-to-earth questions:
> 
> 1. ICore, the class is concrete, so why the I in the name? Was it
> abstract at one point?

Yes historically.

> How do you decide whether a class should get
> the interface 'I' in its name?

It’s a mess ;)
I suppose the trend goes to not prepend the ‘I’.

> The same with e.g. IContext, though
> that one at least has a few virtuals and is used as a base class (but
> no pure ones AFAICS, so still concrete).

Historically these classes where “pure” virtual (except for the QObject base).
We moved to a more “configurable” approach then to avoid the need to create subclasses for every little thing, while keeping the option open in many cases.

> 2. The relatively liberal use of singleton classes. We all know that
> is a debated subject, and I don't have an opinion either way. I'm just
> interested in if you have some (spoken or unspoken) policy regarding
> singletons in the project. Do you want to minimize the use of them, or
> is it OK for newer code, or is it judged on a case-by-case basis? Have
> you had any moments where you really wish you hadn't used singletons?
> (e.g. I know it can sometimes hurt testability).

We always had a liberal amount of singletons in Qt Creator, and we even moved most of them to be classes with mostly static methods a while ago. There are no plans to move away from that. If you have a central hub for “managing” something, feel free to use a singleton/static methods.

Br, Eike

-- 
Eike Ziller
Principal Software Engineer

The Qt Company GmbH
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eike.ziller at qt.io
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