This is a custom undo manager that can coalesce similar changes within the same group. This class is thread safe.
PSPDFKit creates instances of this class internally and makes them available on various objects. You should not create new instances of this class yourself.
NoteOnly use a perform/lock block if you’re not in any other lock controlled by PSPDFKit.
@interface PSPDFUndoController : NSObject
class PSPDFUndoController : NSObject