A category to abstract the details of NSPointerArray

NSPointerArray+AbstractionHelpers.h

#import <Foundation/Foundation.h>

// This helps deal with NSPointerArray that stores pointers to objects that inherit from NSObject.
// Not sure if this will work for other pointer types!!
@interface NSPointerArray (Helpers)

/**
 *  Adds pointer to the given object to the array.
 *
 *  @param object Object whose pointer needs to be added to the array. 
 *  If a pointer to this object already exists in the array, you get a duplicate.
 *  Call containsObject first if you don't want that to happen.
 */
- (void)addObject:(id)object;

/**
 *  Checks if pointer to the given object is present in the array.
 *
 *  @param object Object whose pointer's presence needs to be checked.
 *
 *  @return YES if pointer to the given object is already present in the array; NO otherwise.
 */
- (BOOL)containsObject:(id)object;

/**
 *  Removes a pointer that matches the pointer to the passed in object.
 *
 *  @param object An object that's currently in the array. No ill effects if the object is not in the array.
 */
- (void)removeObject:(id)object;

- (void)removeAllNulls;

@end

NSPointerArray+AbstractionHelpers.m

#import "NSPointerArray+Helpers.h"

@implementation NSPointerArray (Helpers)

- (void)addObject:(id)object
{
    [self addPointer:(__bridge void *)object];
}

- (BOOL)containsObject:(id)object
{
    // get passed in object's pointer
    void * objPtr = (__bridge void *)object;
    for (NSUInteger i = 0; i < [self count]; i++) {
        void * ptr = [self pointerAtIndex:i];
        
        if (ptr == objPtr) {
            return YES;
        }
    }
    
    return NO;
}

- (void)removeObject:(id)object
{
    // get pointer to the passed in object
    void * objPtr = (__bridge void *)object;
    int objIndex = -1;
    for (NSUInteger i = 0; i < [self count]; i++) {
        void * ptr = [self pointerAtIndex:i];
        
        if (ptr == objPtr) {
            // pointers equal, found our object!
            objIndex = i;
            break;
        }
    }
    
    // make sure index is non-nil and not outside bounds
    if (objIndex >= 0 && objIndex < [self count]) {
        [self removePointerAtIndex:objIndex];
    }
}

- (void)removeAllNulls
{
    NSMutableSet *indexesToRemove = [NSMutableSet new];
    for (NSUInteger i = 0; i < [self count]; i++) {
        if (![self pointerAtIndex:i]) { // is the pointer null? then remove it
            [indexesToRemove addObject:@(i)];
        }
    }
    
    for (NSNumber *indexToRemove in indexesToRemove) {
        [self removePointerAtIndex:[indexToRemove unsignedIntegerValue]];
    }
}

@end

SampleObject.m

// Some object that pushes out notifications

#import "NSPointerArray+AbstractionHelpers.h"
#import "SampleObject.h"

@interface SampleObject () 

@property (nonatomic, strong) NSPointerArray *delegates;

@end

@implementation SampleObject

- (id)init
{
    if (self = [super init]) {
        _delegates = [NSPointerArray pointerArrayWithOptions:NSPointerFunctionsWeakMemory];
    }
    return self;
}

- (void)registerDelegate:(id<YourProtocolHere>)delegate
{
    if (!delegate) return;
    
    [self addDelegate:delegate];
    
    [self startUpdates];
}

- (void)removeDelegate:(id<YourProtocolHere>)delegate
{
    [self.delegates removeObject:delegate];
    
    // if last delegate gets removed, stop listening for updates/nil out depedency objects, if applicable
    if ([self.delegates count] == 0) {
        [self stopUpdates];
    }
}

- (void)addDelegate:(id<YourProtocolHere>)delegate
{
    if (![self.delegates containsObject:delegate]) {
        [self.delegates addObject:delegate];
    }
}

// ... some SampleObject logic here ...

// Helper method that can be called after each round of notifications to see if any delegates left
// Delegates can all eventually become nil due to being weak references
- (BOOL)shouldUpdatesContinue
{
    [self.delegates removeAllNulls];
    
    if ([self.delegates count] == 0) {
        return NO;
    }
    
    return YES;
}

@end

Super useful category - thanks!
I was wondering about the - (void)removeAllNulls;, isn't it the same as the - (void)compact; which is a part of the NSPointerArray?