[SO-a/74386650/5690248] `TDWHasObserver` category leverages private API information to tell whether an object is an existing observer of the instance

NSObject+TDWHasObserver.h

//
//  NSObject+TDWHasObserver.h
//
//  Created by Aleksandr Medvedev on 09.11.2022.
//

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

OBJC_EXTERN NSErrorDomain const TDWObserverLookupErrorDomain;

typedef NS_ENUM(int16_t, TDWObserverLookupError) {
    TDWObserverLookupErrorUnknown               = -1,
    TDWObserverLookupErrorInconsistentLayout    = 100,
};

__attribute__((objc_direct_members))
@interface NSObject (TDWHasObserver)

- (BOOL)tdw_hasObserver:(id)observer
             forKeyPath:(nullable NSString *)keyPath
                context:(nullable void *)context
                  error:(NSError *__autoreleasing _Nullable *_Nullable)error;

@end

NS_ASSUME_NONNULL_END

NSObject+TDWHasObserver.m

//
//  NSObject+TDWHasObserver.m
//
//  Created by Aleksandr Medvedev on 09.11.2022.
//

#define INIT_IVAR_MACRO(ivar, name, ivarList, errorFormat, errorPtr) if (!ivar) { \
    ivar = [self tdw_p_findIvarByName:name inList:ivarList];\
    if (!ivar) {\
        *errorPtr = [NSError errorWithDomain:TDWObserverLookupErrorDomain\
                                        code:TDWObserverLookupErrorInconsistentLayout\
                                    userInfo:@{\
            NSLocalizedDescriptionKey: [NSString stringWithFormat:errorFormat, name],\
            NSDebugDescriptionErrorKey: [self tdw_p_makeDebugInformation]\
        }];\
    }\
}

#import "NSObject+TDWHasObserver.h"
#import <objc/runtime.h>

NSErrorDomain const TDWObserverLookupErrorDomain = @"the.dreams.wind.error_domain.observer_lookup";

__attribute__((objc_direct_members))
@implementation NSObject (TDWHasObserver)

#pragma mark Actions

- (BOOL)tdw_hasObserver:(id)observer forKeyPath:(NSString *)keyPath context:(void *)context error:(NSError **)error {
    NSError *internalError;
    BOOL result = NO;

    NSArray *observances = [self tdw_p_obtainObservancesWithError:&internalError];
    if (observances && !internalError) {
        Ivar *observanceIvarList = NULL;
        Ivar *observerIvar = NULL;
        Ivar *keyPathIvar = NULL;
        Ivar *contextIvar = NULL;

        for (id observance in observances) {
            if (!observanceIvarList) {
                observanceIvarList = class_copyIvarList([observance class], NULL);
                if (!observanceIvarList) {
                    // TODO: report error
                    result = NO;
                    break;
                }
            }
            INIT_IVAR_MACRO(observerIvar,
                            @"_observer",
                            observanceIvarList,
                            @"Could not find %@ ivar position for the observance object",
                            &internalError)
            if (internalError) {
                result = NO;
                break;
            }

            if (object_getIvar(observance, *observerIvar) != observer) {
                continue;
            }

            if (!keyPath && !context) {
                // Search by observer only
                result = YES;
                break;
            } else {
                BOOL keyPathFound = [self tdw_p_handleSearchForKeyPath:keyPath
                                                            observance:observance
                                                               ivarPtr:keyPathIvar
                                                              ivarList:observanceIvarList
                                                                 error:&internalError];
                if (internalError) {
                    result = NO;
                    break;
                }
                BOOL contextFound = [self tdw_p_handleSearchForContext:context
                                                            observance:observance
                                                               ivarPtr:contextIvar
                                                              ivarList:observanceIvarList
                                                                 error:&internalError];
                if (internalError) {
                    result = NO;
                    break;
                }
                if (keyPathFound && contextFound) {
                    result = YES;
                    break;
                }
            }

        }
        free(observanceIvarList);
    }

    if (error && internalError) {
        *error = internalError;
    }
    
    return result;
}

#pragma mark Private

- (BOOL)tdw_p_handleSearchForKeyPath:(NSString *)keyPath observance:(id)observance ivarPtr:(Ivar *)ivarPtr
                            ivarList:(Ivar *)ivarList error:(NSError **)error {
    if (!keyPath) {
        // No keyPath, thus any value is acceptable
        return YES;
    }

    INIT_IVAR_MACRO(ivarPtr, @"_property", ivarList, @"Could not find %@ ivar position for the observance class", error)
    if (*error) {
        return NO;
    }

    id keyValueProperty = object_getIvar(observance, *ivarPtr);

    static Ivar keyPathIvar;
    static dispatch_once_t token;
    dispatch_once(&token, ^{
        Ivar *keyValuePropertyIvarList = class_copyIvarList([keyValueProperty superclass], NULL);
        keyPathIvar = *[self tdw_p_findIvarByName:@"_keyPath" inList:keyValuePropertyIvarList];
        free(keyValuePropertyIvarList);
    });

    if (!keyPathIvar) {
        *error = [NSError errorWithDomain:TDWObserverLookupErrorDomain
                                     code:TDWObserverLookupErrorInconsistentLayout
                                 userInfo:@{
            NSLocalizedDescriptionKey: @"Could not find keyPath ivar position",
            NSDebugDescriptionErrorKey: [self tdw_p_makeDebugInformation]
        }];
        return NO;
    }

    return [keyPath isEqual:object_getIvar(keyValueProperty, keyPathIvar)];
}

- (BOOL)tdw_p_handleSearchForContext:(void *)context observance:(id)observance ivarPtr:(Ivar *)ivarPtr
                            ivarList:(Ivar *)ivarList error:(NSError **)error {
    if (!context) {
        // No context, thus any value is acceptable
        return YES;
    }
    INIT_IVAR_MACRO(ivarPtr, @"_context", ivarList, @"Could not find %@ ivar position for the observance class", error)
    if (*error) {
        return NO;
    }
    return (__bridge void *)object_getIvar(observance, *ivarPtr) == context;
}

- (NSArray *)tdw_p_obtainObservancesWithError:(NSError **)error {
    id info = (id)self.observationInfo;
    if (!info) {
        return nil;
    }

    Ivar *ivarList = class_copyIvarList([info class], NULL);
    if (!ivarList) {
        *error = [NSError errorWithDomain:TDWObserverLookupErrorDomain
                                     code:TDWObserverLookupErrorUnknown
                                 userInfo:@{
            NSLocalizedDescriptionKey: @"Could not extract ivar list for the observation of the class",
            NSDebugDescriptionErrorKey: [self tdw_p_makeDebugInformation]
        }];
        return nil;
    }

    NSArray *observances;
    for (typeof(ivarList) ivarPtr = ivarList; *ivarPtr; ++ivarPtr) {
        id ivar = object_getIvar(info, *ivarPtr);
        if ([ivar isKindOfClass:[NSArray class]]) {
            // Assumes the observables are found
            observances = ivar;
            break;
        }
    }
    free(ivarList);
    return observances;
}

- (Ivar *)tdw_p_findIvarByName:(NSString *)substring inList:(Ivar *)ivarList {
    for (typeof(ivarList) ivarPtr = ivarList; *ivarPtr; ++ivarPtr) {
        // Naive search
        if (!strcmp(ivar_getName(*ivarPtr), substring.UTF8String)) {
            return ivarPtr;
        }
    }

    return NULL;
}

- (NSString *)tdw_p_makeDebugInformation {
    return [NSString stringWithFormat:@"Observable class requested: %@\nInstance: %@;\nObservation class: %@\nInstance: %@",
            [self class], self, [(id)self.observationInfo class], self.observationInfo];
}

@end