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Re: Android input methods (was: Re: textconv.c)

From: Eli Zaretskii
Subject: Re: Android input methods (was: Re: textconv.c)
Date: Sun, 12 Feb 2023 17:57:00 +0200

> From: Po Lu <luangruo@yahoo.com>
> Cc: emacs-devel@gnu.org
> Date: Sun, 12 Feb 2023 23:27:00 +0800
> Eli Zaretskii <eliz@gnu.org> writes:
> > I don't see why this is important, since you can switch the buffer
> > temporarily.  We do this all over the place, since insdel.c always
> > works on the current buffer.
> The text is copied into a C ``char *'', not into another buffer.

A buffer in Emacs is just a linear array of bytes, so the above
supports copying into a buffer and from a buffer as well.

Once again, look at the callers of copy_text, and you will see it.

> On Android, input methods work the other way around.  They do the text
> insertion and deletion themselves, all whilst querying the text editor
> about the position of the caret (point) and the text around it for
> reference.  Emacs is only told to insert or delete text at a specific
> position in its buffer, and is obligated to inform the input method
> about changes around the caret.

Maybe I'm missing something, but where's the problem in this?  We have
all this implemented in insdel.c, and a command can tell Emacs to
insert, delete, or replace text.  IOW, so far you describe the
communications between Emacs and the input method as a stream of
insert/delete/replace commands, for which we already have all the
infrastructure, and the only thing that's missing is the low-level
communication protocol with the input method, which should issue those

> If Emacs makes a change to the buffer outside the area in which the
> input method expresses interest, then it is obligated to ``restart'' the
> input method.  This takes a significant amount of time to complete.

What does Emacs have to do to "restart" an input method? what does
this mean in practice?

And how much time is "a significant amount of time" in this case?

> Sometimes, the input method will also tell Emacs to mark a portion of
> the buffer as ``preconversion text'' (or a ``composing span''), which is
> an ephemeral region which may be replaced by the input method by some
> other text, or deleted altogether.

Again, I see no problems with this: markers will handle such a region.

> at that point, the input method asks for the contents of the buffer
> before point again, and repeats the whole process.

What does "asks for the contents of the buffer" mean in practice? what
does the input method tell Emacs, and what does it expect from Emacs
in response? if it expects us to send it the text it requested, then
how (by which medium and protocol) is that text sent by Emacs to the
input method?

> All of this is behavior I have observed CJK and English input methods
> perform.  An input method is not obligated to behave in any way like
> what I have described above, as long as it constrains its edits to some
> reasonable position (600 characters) around the caret; if it makes edits
> any further away from the caret than that, the behavior of the
> application is undefined.

"Undefined" meaning that input methods will not usually do this?  Or
what does it mean?

> But the input method will stop working properly until the next time
> it is reset if it doesn't see the replacement reflected in Emacs's
> own buffer contents.

How can the input method know that the replacement is reflected in the
Emacs buffer?

> Sometimes, an input method will also monitor changes to the caret
> position.  At this point, Emacs is obligated to report any changes to
> the on screen caret to the input method, so it knows where it should
> begin to make edits from.

Again no problem for us: "we have the technology", in the form of
buffer-modification hooks.

> An input method might also ask for a region of text to be ``extracted'',
> which means Emacs must report each change to the buffer that modifies
> said region to the input method


> In any case, the conclusion is that Emacs must present a completely
> correct view of the buffer contents of the selected window and the
> location of its point to the input method, correctly report edits made
> by the input method to the buffer contents and any edits made by Emacs
> after that, and dilligently report changes to extracted text and/or
> reset the input method on ``major changes'' such as the selected buffer
> or window changing, or edits happening outside extracted text.

And the problem with doing what the input method expects is...?

> Now, it is sometimes possible to disable the input method and to simply
> work with an on screen keyboard (which is what the Android port
> currently does), but that precludes entering any non-ASCII text, and is
> a luxury which is only affored by several input methods.

You mean, if I select, say, a Cyrillic keyboard and start typing, what
Emacs will see is the above complex insert/delete/replace commands
instead of a series of single Cyrillic characters?  And this only
happens with non-ASCII characters, but not with ASCII?

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