When editing a page, it would help to have a "preview changes" or "show diff" button, which brings up a diff from the current page content to the proposed new page content. --[[JoshTriplett]]

Some discussion from the main [[/index/discussion]] page:

>It would be nice to be able to have a button to show "Differences" (or "Show Diff") when
>editing a page. Is that an option that can be enabled?
>
>> It's doable, it could even be done by a [[todo/plugin]], I think.
>> --[[Joey]]

---

I need help with this. Supposedly, such a plugin would provide a `formbuilder_setup`
hook which would add the button, and a cgi hook which would interecept
"Show Diff" clicks. This would show a diff between
`titlepage(possibly_foolish_untaint($form->field("page")))`
and the provided `$form->field("editcontent")`.

But:

  * How could the case of concurrent editing be handled ?
    Especially as (I think) only the RCS backend can know that
    this has happened.
  * May I hijack the `page_preview` template variable to show
    my diff output? (the patch below assumes this).
  * Using the formbuilder hook to add a button, and coerce `editpage()`
    into showing the edit form again rather that saving the page needs
    the following patch. (note that this is not the only template which has
    its submit buttons hardcoded; is this supposed to work around something?)

<pre>
Index: templates/editpage.tmpl
===================================================================
--- templates/editpage.tmpl     (révision 4130)
+++ templates/editpage.tmpl     (copie de travail)
@@ -57,9 +57,7 @@
 Optional comment about this change:<br />
 <TMPL_VAR FIELD-COMMENTS><br />
 </TMPL_IF>
-<input id="_submit" name="_submit" type="submit" value="Save Page" />
-<input id="_submit_2" name="_submit" type="submit" value="Preview" />
-<input id="_submit_3" name="_submit" type="submit" value="Cancel" />
+<TMPL_VAR FORM-SUBMIT>
 <TMPL_VAR HELPONFORMATTINGLINK>
 <TMPL_VAR FORM-END>
 
Index: IkiWiki/CGI.pm
===================================================================
--- IkiWiki/CGI.pm      (révision 4130)
+++ IkiWiki/CGI.pm      (copie de travail)
@@ -304,6 +304,7 @@
        eval q{use CGI::FormBuilder};
        error($@) if $@;
        my $form = CGI::FormBuilder->new(
+               title => "editpage",
                fields => \@fields,
                charset => "utf-8",
                method => 'POST',
@@ -321,7 +322,8 @@
        );
        
        run_hooks(formbuilder_setup => sub {
-               shift->(form => $form, cgi => $q, session => $session);
+               shift->(form => $form, cgi => $q, session => $session,
+                       buttons => \@buttons);
        });
        
        decode_form_utf8($form);
@@ -402,12 +404,12 @@
                        preprocess($page, $page,
                        filter($page, $page, $form->field('editcontent')), 0, 1))));
        }
-       else {
+       elsif ($form->submitted eq "Save Page") {
                $form->tmpl_param("page_preview", "");
        }
        $form->tmpl_param("page_conflict", "");
        
-       if (! $form->submitted || $form->submitted eq "Preview" || 
+       if ($form->submitted ne "Save Page" || 
            ! $form->validate) {
                if ($form->field("do") eq "create") {
                        my @page_locs;
</pre>

--[[JeremieKoenig]]