[cfe-dev] clang-format emacs integration: window-start dislocation when formatting-before-save

[Johann Klähn]

I have a prototype[1] that does adjustments via single edit operations
based on the '-output-replacements-xml' option (instead of replacing
the whole buffer; this would be similar to the difflib-approach, I
guess?). But when using this option the JSON-header containing the
'-cursor' information is not returned, so I have to rely on markers
inserted into the buffer to keep the cursor position. On the upside
this allows me to track the cursor per-window.

[1] https://github.com/kljohann/clang-format.el/tree/xml

On Sat, Jan 3, 2015 at 9:31 AM, Daniel Jasper <djasper at google.com> wrote:
> I have not heard of this problem before, but I am also not personally using
> emacs. Adding Johann Klähn, who has just proposed improvements to the emacs
> integration. Maybe he has experienced this problem, too.
>
> I think the "right" solution here might be what we do for the vim
> integration. Instead of replacing the entire buffer, we just compute a diff
> and only apply the diff chunks individually. This makes fiddling with the
> window starts completely unnecessary.
>
> On Fri, Jan 2, 2015 at 2:41 AM, Adam McKee <Adam.Matthew.McKee at gmail.com>
> wrote:
>>
>> As I wrote earlier, I've put this in my ~/.emacs to format my C++ buffers
>> before they're saved:
>>
>> (add-hook 'c++-mode-hook
>>     (lambda()
>>         (add-hook 'before-save-hook 'clang-format-buffer)))
>>
>> This worked fine when saving individual files with "C-x C-s".  However, I
>> encountered a problem when using "C-x s" (save-some-buffers) where the
>> window-start would be moved to the very first line, and the point would be
>> moved to the left margin on the middle line in the window.  In
>> clang-format.el, I added some debug messages to show what's happening with
>> the calls to goto-char() and set-window-start().  What I found is that
>> there's nothing wrong with those calls in the (save-some-buffers) case, and
>> they are the exact same as in the case where I save the buffer individually
>> with "C-x C-s".  Despite (clang-format-buffer) doing apparently the same
>> thing in both cases, with (save-some-buffers) _something_ is moving
>> window-start to the start of the file, which is not acceptable.
>>
>> I don't see a problem with clang-format.el to be fixed, and I don't know
>> how to proceed with this any more, so here I just wanted to give an account
>> of my experience with this, in the hope that it could be useful.
>>
>> What I've done to my clang-format.el for now is to remove the loop at the
>> end of the function that restores the window-starts of the windows showing
>> the formatted buffer.  The result of that is still is slightly disorienting
>> because the line containing the point is centered on the screen (if
>> possible), but it's far better than having the window-start moved to the
>> first line.
>>
>> Clang-formatting before save must be quite a common practice, so is anyone
>> doing so _not_ seeing the same window-start dislocation problem with
>> (save-some-buffers)?  If I can duplicate an emacs setup that doesn't have
>> this problem, that would help.
>>
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>