Apple Photos: observing MLMediaGroup changes using MediaLibrary on macOS - objective-c

Question

I need to display Album structure of Apple Photos in my application on macOS. The only way to do it that I could find is MediaLibrary framework. I can traverse the tree structure using MLMediaGroup class without problems. But watching changes in it makes me puzzled. I would expect that I need to subscribe to KVO-notificatios for each individual media group and watch the changes in attributes.
But in reality when I create a new album/folder in Photos I get a notification that the rootMediaGroup has changed, and the addresses of this group as well as all child groups also change. This seems strange because now I have to rebuild the whole album-hierarchy. This is like watching directory changes and getting opaque notifications like "something in the filesystem has changed" without further details each time a user modifies a directory somewhere deep inside $HOME.
I'm using objective-c (in fact I need to integrate it with c++), although swift examples may also give me a hint. Below I provide a minimal example to illustrate the question.
PhotosObserver.h
#import <Foundation/Foundation.h>
#import <MediaLibrary/MediaLibrary.h>
#interface PhotosObserver: NSObject
{
#private
MLMediaLibrary *ml;
MLMediaSource *src;
MLMediaGroup *root;
}
#property(nonatomic, retain, nullable, readonly) MLMediaLibrary *ml;
#property(nonatomic, retain, nullable, readonly) MLMediaSource *src;
#property(nonatomic, retain, nullable, readonly) MLMediaGroup *root;
#end
PhotosObserver.m
#include "PhotosObserver.h"
static void* CTX_MEDIASOURCE = &CTX_MEDIASOURCE;
static void* CTX_MEDIAGROUP = &CTX_MEDIAGROUP;
static NSString* KPATH_SOURCES = #"mediaSources";
static NSString* KPATH_GROUPS = #"rootMediaGroup";
#implementation PhotosObserver
#synthesize ml;
#synthesize src;
#synthesize root;
-(id)init {
if(self = [super init]) {
self->ml = [[MLMediaLibrary alloc] init];
[self->ml addObserver: self forKeyPath:KPATH_SOURCES options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:CTX_MEDIASOURCE];
[self->ml mediaSources];
}
return self;
}
-(void)dealloc {
#try {
[self.ml removeObserver:self forKeyPath:KPATH_SOURCES context:CTX_MEDIASOURCE];
[self.src removeObserver:self forKeyPath:KPATH_GROUPS context:CTX_MEDIAGROUP];
}
#catch(NSException *) {}
}
-(void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
if(CTX_MEDIASOURCE == context)
{
NSLog(#"Loaded mediaSources");
self->src = [self->ml.mediaSources objectForKey:/*#"com.apple.Photos"*/MLMediaSourcePhotosIdentifier];
assert(self->src);
[self.src addObserver: self forKeyPath:KPATH_GROUPS options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld context:CTX_MEDIAGROUP];
[self.src rootMediaGroup];
}
else if(CTX_MEDIAGROUP == context)
{
MLMediaGroup *old = self->root;
if(!old)
NSLog(#"Loaded rootMediaGroup");
self->root = self.src.rootMediaGroup;
if(old && old != self.root)
{
NSLog(#"Oops, root group has changed: %# -> %#", old, self.root);
}
}
}
#end
When I run the program and create a folder/album, or rename an album in Photos, I get a notification that the root group is changed. For some reason I don't get notifications in 100% of cases but anyway quite often.
Q: Why does rootMediaGroup get changed at all, since the user cannot change it in Photos UI?
Q: How to watch for the changes without the need to rescan the whole MLMediaGroup hierarchy after each small user modification?