I'm a bit confused about where the application should send a bunch of files. Let me suppose that an application accepts a number of images with NSOpenPanel at a time from the user. The application applies graphic filters to them. And it's now ready to save processed files. Before they forced us to sandbox applications, we were allowed to export processed files to application folder in Application Support without NSSavePanel. If you wanted to save files elsewhere, then you had to use NSSavePanel. If the application is sandboxed, it cannot send files to NSApplicationSupportDirectory/{app name}(which points to the containers folder assigned to this application)? My first sandboxed application was rejected a few days ago merely because a text field showed a path to container's application support folder. So if you have a bunch of files to export, you have to prompt the user to ask where to save each file? AppSandboxDesignedGuide, which Apple, Inc. has issued, has nothing to say exactly about where to save files except that it says "Your app has unrestricted read/write access to the container and its subdirectories." I think this PDF guide is a printed version of this web site. I'm asking this question here because I have some doubts and reviewers were often wrong at least when I submitted applications to them two years ago.
Thank you for your advice.
If the files are only for the application itself to use you can save the files in "Application Support/", which under the sandbox is under your container, just as before - just use the APIs to construct the path to that folder (and create it, it doesn't exist automatically, just as before).
If you are outputting files for the user to access then you don't put them in the container - that folder is meant to be hidden from ordinary users, though yours is the first time I've heard that even showing the path got you a rejection, but Apple are pretty random.
Here are three choices of where to put your files:
First is to ask the user. This is what you would normally do anyway, you shouldn't just dump files somewhere.
Second is a situation that the sandbox makes harder - when where the file should is is implicit, e.g. a graphic conversion program might sensibly output the converted file with the same name but different extension in the same folder as the original. This was finally addressed by Apple around 10.8.3 or something with "Related Items" - Apple's docs for this are here. Essentially in the Document Types in the Info.plist you must list all the extensions you handle - both in and out - and add a NSIsRelatedItemType key with the value of YES to all those you might convert between. E.g. For TextEdit .rtf, .rtfd and .txt are flagged in this way so TextEdit can open as one format and save as another.
Third, if you wish to put all your files in one location, say in a "Converted Items" folder. Then you ask the user once to specify this folder and then save a security-scoped bookmark to that folder in your applications defaults or elsewhere in your app's container. On subsequent executions you can access this bookmark and regain access to the folder. For an introduction to this start with Apple's Security-Scoped Bookmarks and Persistent Resource Access. This is really no harder than pre-sandbox as any decent app would always ask for the location of the folder from the user, the difference is the need to save the security-scoped bookmark so the user doesn't need to give permission every time.
HTH.