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

Eike Ziller Eike.Ziller at qt.io
Mon Sep 11 10:57:25 CEST 2017

> On Sep 11, 2017, at 10:31, Elvis Stansvik <elvstone at gmail.com> wrote:
> 2017-09-11 9:24 GMT+02:00 Elvis Stansvik <elvstone at gmail.com>:
>> 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?
> I could think of one other "detail" question (then I'm finished, promise!):
> I see that MainWindow is the one responsible for creating the ICore
> instance (passing in itself to its constructor), and has been made a
> friend of ICore to be able to call its private constructor.
> What was the reason for that arrangement? Could not ICore itself be
> responsible for creating the MainWindow?

Again historically.
When I tried to change this, I started moving around so much code (because of dependencies on initialization order etc etc), that I decided it’s not worth the effort.

Br, Eike

> Elvis
>>> 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
>>> Elvis
>>>> Many thanks in advance,
>>>> Elvis
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Eike Ziller
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