In my editor (I use vim), edit the file so that only one of the changes appear
Save the file (but don't quit the editor)
Commit the changed file to svn
Hit "undo" in the editor enough times for the second set of changes reappear
Save the file again
Commit the second set of changes.
This is a simplistic approach that assumes one set of changes is reasonably easy to undo. For more complex situations, I would give up and commit both changes without worrying about it.
Now that I use git, this is something I hope I'll never have to do again!
I use either a local darcs repo, or just merge the changes in gradually. With merging (opendiff opens FileMerge, a merge program that comes with Xcode; replace with your favorite merge tool):
merge the related changes, save the merge, quit the merge program
svn ci -m 'first hunk' file
mv file.new file
svn ci -m 'second hunk' file
if more than one unrelated hunk in the file, rinse and repeat (but why would you wait so long before committing?!)
Also, if you know git, you can use git-svn to maintain a local git repo and sync your commits to an svn master server; works great in my limited experience.
Try using svn diff > out.patch then copy the out.patch file to out.patch.add and out.patch.modify
Only when you have a working patch file revert the original file using svn revert out.c.
Edit the patch files by hand so that they only contain the hunks for adding or modifying. Apply them to the original file using the patch command, test if the addition worked, then svn commit the addition.
Wash rinse repeat for the out.patch.modify patch.
If the changes are separate in the file as your initial question stated - added a new method, changed an existing method - this will work
This is a very tedious solution - although I'm not convinced you should have any reason to separate your commits.
You also could have checked out multiple working copies of the same source to apply your work against:
svn co http://location/repository methodAdd
svn co http://location/repository methodModify
Be sure to svn up and test to make sure all is well.
With git-svn you can make a local GIT repository of the remote SVN repository, work with it using the full GIT feature set (including partial commits) and then push it all back to the SVN repository.
Open all the files you want to split in editor-of-choice
Using a different tool set (on Win, use Spike's suggestion (the old version)) back out the second set
Commit
go back to your editor-of-choice and save all the files
It's a little riskier than Spike's full suggestion but can be easier to do. Also make sure you try it on something else first as some editors will refuse to save over a file that has changed out from under them unless you reload that file (losing all your changes)
Tortoise SVN 1.8 now supports this with it's "Restore after commit" feature. This allow you to make edits to a file, with all of the edits being undone after the commit
Per the documentation:
To commit only the parts of the file that relate to one specific issue:
in the commit dialog, right-click on file, choose "restore after commit"
edit the file in e.g. TortoiseMerge: undo the changes that you don't want to commit yet
This is possible using TortoiseSvn (Windows) since v1.8.
4.4.1. The Commit Dialog
If your working copy is up to date and there are no conflicts, you are ready to commit your changes. Select any
file and/or folders you want to commit, then TortoiseSVN → Commit....
<snip>
4.4.3. Commit only parts of files
Sometimes you want to only commit parts of the changes you made to a file. Such a situation usually
happens when you're working on something but then an urgent fix needs
to be committed, and that fix happens to be in the same file you're
working on.
right click on the file and use Context Menu → Restore after commit.
This will create a copy of the file as it is. Then you can edit the
file, e.g. in TortoiseMerge and undo all the changes you don't want to
commit. After saving those changes you can commit the file.
After the commit is done, the copy of the file is restored
automatically, and you have the file with all your modifications that
were not committed back.
I think an easier option than generating diff files, reverting, etc, would be to have two copies of the repository checked out, and use a visual diff tool like DeltaWalker to copy hunks from one to the other.
The first copy would be the one you actually work off of, and the second would just be for this purpose. Once you've made a ton of changes to the first, you can copy one section over to the second, commit it, copy another section, commit it, etc.
Try VisualSVN for Visual Studio. The latest 6.1 release introduces the QuickCommit feature. You can partially commit selected changes in a file using the new Commit this Block and Commit Selection context menu commands in the Visual Studio editor.