GCD应用：没什么卵用的 Global Queue Serial

//
//  GlobalQueueSerial.h
//  TestProj
//
//  Created by Wesley on 2016/11/17.
//  Copyright © 2016年 Wesley. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface GlobalQueueSerial : NSObject

//+(instancetype)sharedInstance;
//+(dispatch_queue_t)queue;
+(void)addBlock:(void (^)(void))block;

@end

//
//  GlobalQueueSerial.m
//  TestProj
//
//  Created by Wesley on 2016/11/17.
//  Copyright © 2016年 Wesley. All rights reserved.
//

#import "GlobalQueueSerial.h"

@interface GlobalQueueSerial()
{
    dispatch_queue_t _aQueue;
    dispatch_group_t _queueGroup;
}
@end

@implementation GlobalQueueSerial

+(instancetype)sharedInstance
{
    static GlobalQueueSerial*sharedInstance = nil;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[GlobalQueueSerial alloc] init];
    });
    return sharedInstance;
}

+(dispatch_queue_t)queue
{
    return [[GlobalQueueSerial sharedInstance] queue];
}

+(dispatch_group_t)group
{
    return [[GlobalQueueSerial sharedInstance] group];
}

+(void)addBlock:(void (^)(void))block
{
    dispatch_queue_t aQueue = [GlobalQueueSerial queue];
    // create queue group
    dispatch_group_t queueGroup = [GlobalQueueSerial group];
    // task 1
    dispatch_group_wait(queueGroup, DISPATCH_TIME_FOREVER);
    dispatch_group_async(queueGroup, aQueue, block);
}

-(instancetype)init
{
    if (self = [super init]) {
        _aQueue = dispatch_queue_create("sticker", DISPATCH_QUEUE_SERIAL);
        _queueGroup = dispatch_group_create();
    }
    return self;
}

-(dispatch_queue_t)queue
{
    if (!_aQueue) {
        _aQueue = dispatch_queue_create("sticker", DISPATCH_QUEUE_SERIAL);
    }
    return _aQueue;
}

-(dispatch_group_t)group
{
    if (!_queueGroup) {
        _queueGroup = dispatch_group_create();
    }
    return _queueGroup;
}

-(void)dealloc
{
#if(!__has_feature(objc_arc))
    dispatch_release(queueGroup);
#endif
}

@end

Usage:

for (int i = 0; i < 10; ++i) {
    [GlobalQueueSerial addBlock:^{
        NSLog(@"task %d begin.", i);
        NSLog(@"task %d end.", i);
    }];
}