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

[Eike Ziller]

> On 12. Sep 2017, at 11:42, Elvis Stansvik <elvstone at gmail.com> wrote:
> 
> 2017-09-11 9:50 GMT+02:00 Eike Ziller <Eike.Ziller at qt.io>:
>> 
>>> 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.
> 
> Just a small additional question regarding singletons.
> 
> For example, in PluginManager, you have the following setup:
> 
> namespace ExtensionSystem {
> 
> static Internal::PluginManagerPrivate *d = 0;
> static PluginManager *m_instance = 0;
> 
> /*!
>    Gets the unique plugin manager instance.
> */
> PluginManager *PluginManager::instance()
> {
>    return m_instance;
> }
> 
> /*!
>    Creates a plugin manager. Should be done only once per application.
> */
> PluginManager::PluginManager()
> {
>    m_instance = this;
>    d = new PluginManagerPrivate(this);
> }
> 
> ...
> }
> 
> That is, instead of the normal singleton pattern (private constructor,
> lazy construction in instance()), the singleton aspect is by
> agreement.

- Control over the lifecycle. There is some code responsible for creating the singleton, which is also responsible to destruct it in a controlled and defined manner.
- Less checking. Since the lifecycle is defined, the (mostly static) methods do not need to ensure existence of the singleton. They directly access e.g. m_instance.

In many cases the constructor could be private with friend declaration for the code constructing the instance (but not in the PluginManager case, which is API in a separate library, and I’m no big fan of friend either).

> Also, the m_instance and d-pointer are not kept as members of the
> class, but globally in the enclosing namespace.

They are static in the .cpp, so only local.
There is just no need for them to be members of the public class.

Of course all of this is also a matter of taste and socialization ;)

Br, Eike

> 
> What is the reason for this approach?
> 
> Elvis
> 
>> 
>> Br, Eike
>> 
>> --
>> Eike Ziller
>> Principal Software Engineer
>> 
>> The Qt Company GmbH
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