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

Elvis Stansvik elvstone at gmail.com
Mon Sep 11 09:24:51 CEST 2017

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

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).


> Elvis
>> Many thanks in advance,
>> Elvis

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