blockmaybefixed.m

#import <Foundation/Foundation.h>
 
@interface A : NSObject
- (void)doThingWithObject:(id)o completion:(void(^)(void))completionHandler;
@end
 
@implementation A
- (void)doThingWithObject:(id)o completion:(void(^)(void))completionHandler
{
  NSLog(@"object: %@", o);
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    if(completionHandler) completionHandler();
  });
}
@end
 
int main(int argc, char *argv[]) {
  @autoreleasepool {
    A *a = [A new];
    NSArray *someThings = @[@0, @1, @2];
	__block NSMutableArray *commands = [NSMutableArray new];
	void(^step)(void) = ^() {
		if (commands.count == 0) return;
		void(^command)(void) = [commands firstObject];
		[commands removeObjectAtIndex:0];
		command();
	};

	NSEnumerator *e = [someThings objectEnumerator];
	id cur = [e nextObject];
	while (cur) {
		[commands addObject:[^{
			[a doThingWithObject:cur completion:step];
		} copy]];
	
		cur = [e nextObject];
	}

	step();
    sleep(1);
  }
}