Open Radar

 
Summary:
A finder extension (FinderSync) primary method (-beginObservingDirectoryAtURL:)  is not called only on /Volumes if registered to monitor root "/" folder.

Steps to Reproduce:
1. Created a sample FinderSync ("Finder Sync") application.
2. It is sandboxed
3. It is signed and embedded in our .app with signing.
4. @interface FinderSync : FIFinderSync
5. in my init, in the implementation I have:
self.myFolderURL = [NSURL fileURLWithPath:@"/"];
[FIFinderSyncController defaultController].directoryURLs = [NSSet setWithObject:self.myFolderURL]; 
note: So basically I'm watching the root folder including /Volumes;
6. I implement the protocol 
- (void)beginObservingDirectoryAtURL:(NSURL *)url {
    // The user is now seeing the container's contents.
    // If they see it in more than one view at a time, we're only told once.
    NSLog(@"beginObservingDirectoryAtURL:%@", url.filePathURL);
    self.beginObserving = [url copy];
}

Expected Results:
- (void)beginObservingDirectoryAtURL:(NSURL *)url 
should be called upon clicking on a folder on a mounted drive under /Volumes

Actual Results:
It is not called. Neither - (void)endObservingDirectoryAtURL:(NSURL *)url
But only on /Volumes, it works on any other folder on my tested systems.
Also the selected files are properly loaded under /Volumes, but not the folders.

Version/Build:
macOS 10.10.5, 10.12.3, 10.12.6, 10.13 developer beta 9 (17A360a)