Intent of this code is to guide protected concurrent access to collection of states.

MVStateStorage.h

#import <Foundation/Foundation.h>

typedef id (^MVGetterBlock)(void);
typedef void (^MVSetterBlock)(id);

typedef id (^MVStateOpBlock)(id);
typedef NSOperation * (^MVOpSetupBlock)(MVGetterBlock, MVSetterBlock);

@interface MVStateStorage : NSObject {
	__block NSMutableDictionary *data;
	__block NSMutableDictionary *queues;
}

- (void)scheduleBlock:(MVStateOpBlock)block forKey:(id)key;
- (void)setupOperationForKey:(id)key withBlock:(MVOpSetupBlock)block;
- (void)cancelOperationsForKey:(id)key;
- (NSDictionary *)dictionaryRepresentation;
- (NSArray *)allKeys;

@end

MVStateStorage.m

#import "MVStateStorage.h"

@implementation MVStateStorage

- (id)init {
	self = [super init];
	if (self) {
		data   = [[NSMutableDictionary alloc] init];
		queues = [[NSMutableDictionary alloc] init];
	}
	return self;
}

- (void)dealloc {
	[queues enumerateKeysAndObjectsUsingBlock:^(id key, NSOperationQueue *obj, BOOL *s) {
		[obj cancelAllOperations];
		[obj waitUntilAllOperationsAreFinished];
		[obj setSuspended:YES];
	}];
	[queues release];
	[data release];
	[super dealloc];
}

- (NSOperationQueue *)queueForKey:(id)key {
	NSOperationQueue *q = [queues objectForKey:key];
	if (!q) {
		@synchronized(queues) {
			q = [[NSOperationQueue alloc] init];
			[q setMaxConcurrentOperationCount:1];
			[queues setObject:q forKey:key];
			[q release];
		}
	}
	return q;
}

- (void)cancelOperationsForKey:(id)key {
	[[self queueForKey:key] cancelAllOperations];
}

- (void)scheduleBlock:(MVStateOpBlock)block forKey:(id)key {
	NSOperationQueue *q = [self queueForKey:key];
	
	[q addOperationWithBlock:^{
		id val = nil;
		id res = nil;
		@synchronized(data) {
			val = [data objectForKey:key];
		}
		
		NSAutoreleasePool *p = [NSAutoreleasePool new];
		res = block(val);
		if (val != res) {
			if (!res) {
				[[NSOperationQueue currentQueue] cancelAllOperations];
				@synchronized(data) {
					[data removeObjectForKey:key];
				}
				return;
			}
			else {
				@synchronized(data) {
					[data setObject:res forKey:key];
				}
			}
		}
		[p release];
	}];
}

#define PREVENT \
if (preventAccess) {\
[NSException raise:NSInternalInconsistencyException format:@"Setter function called in operation *setup* block."];\
}

- (void)setupOperationForKey:(id)key withBlock:(MVOpSetupBlock)block {
	NSOperationQueue *q = [self queueForKey:key];
	
	__block BOOL preventAccess = YES;
	__block id cachedVal = nil;
	MVGetterBlock getter = ^{
		PREVENT
		if (cachedVal) return cachedVal;
		@synchronized(data) {
			return [data objectForKey:key];
		}
	};
	MVSetterBlock setter = ^(id newVal) {
		PREVENT
		@synchronized(data) {
			cachedVal = newVal;
			[data setObject:newVal forKey:key];
		}
	};
	
	NSAutoreleasePool *p = [NSAutoreleasePool new];
	NSOperation *op = block(getter, setter);
	preventAccess = NO;
	cachedVal = nil;
	[q addOperation:op];
	[p release];
}

#undef PREVENT

- (NSDictionary *)dictionaryRepresentation {
	@synchronized(data) {
		return [[[NSDictionary alloc] initWithDictionary:data copyItems:YES] autorelease];
	}
}

- (NSArray *)allKeys {
	@synchronized(data) {
		return [[[data allKeys] copy] autorelease];
	}
}

@end