I am right in assuming that:
2. Async methods have virtually no use outside the scope of event-driven
GTK+ applications (except perhaps as a way to implement additional Vala
features like Luca Bruno's generator, but at the cost of
runtime-performance).
3. There is always a way to write an equivalent program without async
methods (with just ordinary function calls), except for applications with a
graphical interface (GTK+ applications).
4. That a program avoiding async methods will always :
- be more readable (simpler).
- perform faster.
5. That async methods are not about runtime performance, but exclusively
about:
- callbacks for graphical interfaces.
- convenience (as an alternative to an ordinary function call, when
it seems convenient to do so).
6. That async methods are not at all meant to provide a way to run ancillary
tasks in the background, in order to gain runtime performance.
Serge.
On Fri, Jul 15, 2011 at 12:11 AM, Luca Bruno <[email protected]> wrote:
> On Thu, Jul 14, 2011 at 04:52:52PM -0500, Jim Peters wrote:
> > There seems to be special stuff associated with an async method which
> > I haven't found documented well anywhere. For async method 'method',
> > there are all these ways to use it:
> >
> > - Call 'method' from non-async code, starts it running until its first
> > 'yield', at which point it returns to the caller. (Correct?)
> >
> > method(args);
> >
> > - Get callback (from within method itself). This is the callback to
> > resume execution after the following 'yield'. (Correct?)
> >
> > SourceFunc callback = method.callback;
> >
> > - Use the resume callback from elsewhere. 'method' resumes and takes
> > control again, running until its next 'yield', at which point it
> > returns from this call. (Correct?)
> >
> > callback();
> >
> > - Give up control and return to the caller. This doesn't guarantee in
> > any way that the method will be resumed, i.e. callback() must be
> > called somewhere else. (Correct?)
> >
> > yield;
> >
> > - Give up control but arrange for a resume callback when idle. This
> > requires the main loop to be running. (Correct?)
> >
> > Idle.add(method.callback);
> > yield;
> >
> > - Call async method 'other_method' from 'method'. This automatically
> > sets up a callback for 'method' to resume itself and collect the
> > return value when 'other_method' completes. (Correct?)
> >
> > yield other_method(args);
>
> All correct.
>
> >
> > Question: At the C level, I guess this first calls forwards to
> > 'other_method' to start it before returning to the caller due to the
> > 'yield'. If the 'other_method' also yields, then there is no
> > problem, but if 'other_method' finishes without yielding (e.g. if it
> > can return the result right away without doing any asynchronous
> > work), then the 'method' callback would be called again, only a few
> > stack frames lower. In theory if 'method' called 'other_method'
> > repeatedly like this, the stack could overflow. Is this correct?
> > If so maybe that needs documenting.
>
> Variables in async methods are allocated on the heap. By the way I think I
> didn't understand well the problem you're raising.
>
> >
> > - Call with .begin(). Question: Is this just like the 'method()' call
> > but adding a callback request for when method() finishes? If so
> > will it also just run until the first 'yield' and then return?
> >
> > method.begin(args, callback);
>
> Yes.
>
> >
> > - Call with .end(). For use within the 'begin' callback, to get
> > method return value and clean up. Question: Is an .end() call
> > required for plain 'method()' style invocation?
> >
> > method.end(out_args);
>
> Can you rephrase?
>
> >
> > Also, questions:
> >
> > - Are there any other special features associated with async methods?
> > (Any other .<identifier> features, for example.)
> >
>
> No.
>
> > - When it is necessary to have a main loop running? Some of the
> > generated C code requests an idle callback. I guess this won't work
> > without a main loop running. But the Generator, for example, seems
> > to run through fine without a main loop.
>
> Mainloop is required only for async methods that never use yield.
>
> >
> > - The docs say "The .callback call is used to implicitly register a
> > _finish method for the async method". What does this mean? Also:
> > "end() is a syntax for the *_finish method". I'm confused.
>
> Try looking at the generated C code.
>
> --
> http://www.debian.org - The Universal Operating System
>
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