[webkit-dev] SharedWorkers alternate design
mjs at apple.com
Tue May 26 10:36:08 PDT 2009
On May 26, 2009, at 10:21 AM, Darin Adler wrote:
> On May 26, 2009, at 10:16 AM, Drew Wilson wrote:
>> OK, I've got two strong votes for the interface + static factory
>> approach. Any objections from the rest of the WebKit team? If I
>> don't hear any counter proposals, I'll do that.
> I think it's unpleasant to pay run-time cost for a compile-time
> choice. Sure, sometimes the extra cost is no big deal, but sometimes
> it can be a big deal and I see no reason to choose idioms that use
> virtual functions if there are equally good or better ones that don't.
> Are there really no better techniques than abstract base classes and
> virtual functions for this sort of compile-time switch? How about
> the technique used for ResourceRequest and ResourceResponse? Maybe
> Darin Fisher can explain that one.
I agree with Darin's comments here. We've tried hard to avoid using
runtime polymorphism for compile-time choices. Here it's probably not
performance-critical, but it can be avoided.
The ResourceRequestBase / ResourceRequest model (due to Darin Fisher)
seems pretty clean to me. I would like to see more of our classes with
port-specific implementation details move to this style. I think it
could work for SharedWorkerRepository.
The basic idea is this. Let's say you have a class FooBar.
- You define a FooBarBase class that has the cross-platform interface
and data members. But not all the methods are actually implemented in
the cross-platform code. All of its constructors are protected so the
class cannot be instantiated directly.
- Each port subclasses FooBarBase to define FooBar, adding
constructors, platform-specific data members, and any needed platform-
specific private helpers or type conversions.
- Each port implements the methods of FooBarBase that are platform-
specific, freely downcasting to FooBar when needed since we have
guaranteed that every instance of FooBarBase is actually a FooBar.
- Cross-platform code using the class just uses FooBar. The Base class
is an implementation detail.
(Darin F., please correct me if I have not done justice to this
Note that this method has no runtime cost - there's no need to use
virtual methods or other forms of runtime indirection. And there's no
need to #ifdef any headers, everything is controlled purely by
including the right platform specific FooBar.h so it can be handled by
include paths. It's a little subtle at first but I think it results in
nice, understandable code.
I think we should document this technique as the preferred way to make
classes with port-specific implementation details and convert more of
WebCore/platform/ to this technique, as well as using it for new
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