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

Christian Gagneraud chgans at gmail.com
Sun Sep 10 12:06:32 CEST 2017

On 10 September 2017 at 21:31, Elvis Stansvik <elvstone at gmail.com> wrote:
> 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?

I'm not a Qtc dev, but like you i spent some time studying some of the
QtC internals, and especially the extension system and the
document/editor system.

I think the way QtC load and initialise plugins allows for a 2-way
communication without any ambiguity.

The plugin's initialise() is call in the bottom-to-top order, so when
your initialise() is called, you know that the plugins you depend upon
are initialised.

Then comes the extensionsInitialized() call, when this one is called,
you know for sure that all the plugins that depends on you have had
their initialise() called.
So it is a good place, for example, to query the object pool for all
objects that implements one of your interface and that have been
registered by your dependees.

My 2 cents.

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