BDSBlockingQueue.h

#import <Foundation/Foundation.h>

@interface BDSBlockingQueue : NSObject

- (void)enqueue:(id)item;
- (id)dequeue;

@end

BDSBlockingQueue.m

#import "BDSBlockingQueue.h"

#define NO_ITEM 0
#define HAS_ITEM 1

@interface BDSBlockingQueue ()

@property (nonatomic, retain) NSMutableArray *items;
@property (nonatomic, retain) NSConditionLock *condLock;

@end

@implementation BDSBlockingQueue

- (id)init
{
    if (self = [super init]) {
        self.items = [[NSMutableArray alloc] init];
        self.condLock = [[NSConditionLock alloc] initWithCondition:NO_ITEM];
    }
    return self;
}

- (void)enqueue:(id)item
{
    [self.condLock lock];
    [self.items addObject:item];
    [self.condLock unlockWithCondition:HAS_ITEM];
}

- (id)dequeue
{
    id item = nil;
    [self.condLock lockWhenCondition:HAS_ITEM];
    item = [self.items objectAtIndex:0];
    [self.items removeObjectAtIndex:0];
    NSUInteger count = [self.items count];
    [self.condLock unlockWithCondition:(count > 0) ? HAS_ITEM : NO_ITEM];
    return item;
}

@end