remember user selected folder and reopen it by NSOpenPanel under Sandbox - directory

Question

Apple review require my app to remember the folder that user selected last time. But I can not make it under Sandbox. the -setDirectoryURL seems not working under Sandbox.
What should I do? Thank you for your help!
// read string saveFolder from NSUserDefaults
NSOpenPanel * myPanel = [NSOpenPanel openPanel];
[myPanel setTitle:#"Select Output Folder"];
[myPanel setCanChooseDirectories:YES];
[myPanel setCanCreateDirectories:YES];
[myPanel setAllowsMultipleSelection:NO];
[myPanel setCanChooseFiles:NO];
NSLog(#"before openpanel:folder=%#",saveFolder); // <== return normal
[myPanel setDirectoryURL:[NSURL URLWithString:saveFolder.stringValue]];
NSLog(#"readback:folder=%#",[[myPanel URL] path]); // <== return nil here
if ([myPanel runModal] == NSOKButton)
{
//
saveFolder = [[myPanel URL] path]];
// then save the saveFolder string to NSUserDefaults
//
}

Answer 1

it seems we should add and use this entitlement:
com.apple.security.files.bookmarks.app-scope
SAVE THE URL:
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSError *error = nil;
NSData *bookmarkData = [[myPanel URL] bookmarkDataWithOptions:NSURLBookmarkCreationWithSecurityScope includingResourceValuesForKeys:nil relativeToURL:nil error:&error];
if (!error)
{
[defaults setObject:bookmarkData forKey:#"iData"];
[defaults synchronize];
}
READ BACK:
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
bookmarkFileURL = nil;
NSData *bookmarkData = [defaults objectForKey:#"iData"];
if (bookmarkData != nil)
{
NSError *error=nil;
bookmarkFileURL = [NSURL URLByResolvingBookmarkData:bookmarkData options:NSURLBookmarkResolutionWithSecurityScope relativeToURL:nil bookmarkDataIsStale:nil error:&error];
if (error != nil) bookmarkFileURL = nil;
}
if (bookmarkFileURL)
{
[bookmarkFileURL startAccessingSecurityScopedResource];
}
//
//
[bookmarkFileURL stopAccessingSecurityScopedResource];