An example of how syncing tags could be made hopefully easier by separating a tag into two objects: - One that is recorded to the database, with a unique identifier. - And one that the are assigned to notes, which then points to a recorded tag.

TFFTagging

@protocol TFFTagging

@property (readonly, nonatomic) NSString *name;

@end

// Tags in sidebar UI use TFFRecordedTag
// Tags in notes UI use TFFAttachableTag
// Same interface for what makes a tag, which is its name.

@interface TFFRecordedTag : NSObject <TFFTagging>

@property (readwrite, nonatomic) NSString *name;
@property (readwrite, nonatomic) NSUUID *uniqueID;

@end


@interface TFFAttachableTag : NSObject <TFFTagging>

@property (nonatomic) TFFRecordedTag *recordedTag;

- (instancetype)initWithRecordedTag:(TFFTag *)recordedTag;

@end



@implementation TFFRecordedTag

- (instancetype)init
{
  self = [super init];
	if (self) {
		(self.uniqueID) = [NSUUID new];
	}
	return self;
}

@end


@implementation TFFAttachableTag

- (instancetype)initWithRecordedTag:(TFFTag *)recordedTag
{
	self = [super init];
	if (self) {
		(self.recordedTag) = recordedTag;
	}
	return self;
}

- (NSString *)name
{
	return (self.recordedTag.name);
}

@end


@interface TFFTagSystem

- (BOOL)tagName:(NSString *)tagNameA isEquivalentToTagName:(NSString *)tagNameB;
- (BOOL)tagNameIsPreferred:(NSString *)nameA toTagName:(NSString *)nameB; // e.g. Paris vs paris, which is preferred?
- (NSString *)normalizeTagName:(NSString *)inputTagName; // Used for keys in `self.recordedTags`;


@property (nonatomic) NSMutableDictionary *recordedTagsNormalizedNames; // Map of normalized tags name to TFFRecordedTag.


- (TFFRecordedTag *)recordedTagWithName:(NSString *)name creating:(BOOL)create;


- (NSSet *)acceptedTagsOfIntroducedRecordedTags:(NSSet *)introducedRecordedTags; // New tags introduced by another device.

@end


@implementation TFFTagSystem

- (BOOL)tagName:(NSString *)tagNameA isEquivalentToTagName:(NSString *)tagNameB
{
	return [tagNameA compare:tagNameB options:NSCaseInsensitiveSearch] == NSOrderedSame;
}

- (BOOL)tagNameIsPreferred:(NSString *)nameA toTagName:(NSString *)nameB
{
	// Capital letters are before lowercase letters in the unicode alphabet so I think this works.
	return [tagNameA compare:tagNameB options:NSCaseInsensitiveSearch] != NSOrderedDescending; // Go with ascending or same.
}

- (NSString *)normalizeTagName:(NSString *)inputTagName;
{
	// See also http://nshipster.com/cfstringtransform/ for some further ideas.
	// Ideally would use -lowercaseStringWithLocale: but each device might have a different locale from each other!
	
	return [inputTagName lowercaseString];
}


- (TFFRecordedTag *)recordedTagWithName:(NSString *)name creating:(BOOL)create
{
	NSMutableDictionary * recordedTagsNormalizedNames = (self.recordedTagsNormalizedNames);
	NSString *normalizedTagName = [self normalizeTagName:name];
	
	TFFRecordTag *recordedTag = recordedTagsNormalizedNames[normalizedTagName];
	if (!recordedTag && create) {
		recordedTag = [TFFRecordedTag new];
		(recordedTag.name) = name;
		recordedTagsNormalizedNames[normalizedTagName] = recordedTag;
	}
	
	return recordedTag;
}

- (NSSet *)acceptedTagsOfIntroducedRecordedTags:(NSSet *)introducedRecordedTags
{
	NSMutableSet *acceptedTags = [NSMutableSet set];
	
	for (TFFRecordedTag *newRecordedTag in introducedRecordedTags) {
		NSString *newTagName = (newRecordedTag.name);
		TFFRecordedTag *existingTag = [self recordedTagWithName:newTagName creating:NO];
		if (existingTag) {
			NSString *existingTagName = (existingTag.name);
			BOOL existingNameIsPreferred = [self tagNameIsPreferred:existingTagName andString:newTagName];
			if (existingNameIsPreferred)
				break; // Don't add it to acceptedTags.
		}
		
		[acceptedTags addObject:newRecordedTag];
	}
	
	return acceptedTags;
	
	// Then on all notes, do [note syncIntroducingNewRecordedTags:acceptedTags];
}

@end



@interface TFFNote : NSObject

@property NSMutableArray /* TFFAttachableTag */ *attachedTags; // or ideally NSMutableOrderedSet

- (void)addTag:(TFFAttachableTag *)attachableTag;

- (void)syncIntroducingNewRecordedTags:(NSSet /* TFFRecordedTag */ *)newRecordedTags;

@end


@implementation TFFNote

// - (void)addTag:(TFFAttachableTag *)attachableTag; unimplemented

- (void)syncIntroducingNewRecordedTags:(NSSet /* TFFRecordedTag */ *)newRecordedTags
{
	NSMutableArray *attachedTags = (self.attachedTags);
	
	// The number of tags is likely going to be pretty low, so having a double loop should be ok.
	// But this could be probably optimised by using a NSDictionary or similar somehow instead.
	for (TFFRecordedTag *newRecordedTag in newRecordedTags) {
		NSString *recordedTagName = (newRecordedTag.name);
		
		for (TFFAttachableTag *attachableTag in attachedTags) {
			if ([TFFTagSystem tagName:recordedTagName isEquivalentToTagName:(attachableTag.name)]) {
				// Reassign the recorded tag that our tag is connected to.
				(attachableTag.recordedTag) = newRecordedTag;
				break;
			}
		}
	}
}

@end