常見用法：

dispatch_queue_t queue = dispatch_get_global_qeueue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
  dispatch_group_t group = dispatch_group_create();
  for(id obj in array)
      dispatch_group_async(group, queue, ^{
          [self doSomethingIntensiveWith:obj];
      });
  //等group里的task都執(zhí)行完后執(zhí)行notify方法里的內(nèi)容,相當(dāng)于把wait方法及之后要執(zhí)行的代碼合到一起了
  dispatch_group_notify(group, queue, ^{
      [self doSomethingWith:array];
  });

信號(hào)量：

/// Returns the cached model class meta
+ (instancetype)metaWithClass:(Class)cls {
    if (!cls) return nil;
    static CFMutableDictionaryRef cache;
    static dispatch_once_t onceToken;
    static dispatch_semaphore_t lock;
    dispatch_once(&onceToken, ^{
        cache = CFDictionaryCreateMutable(CFAllocatorGetDefault(), 0, &kCFTypeDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);
        lock = dispatch_semaphore_create(1);
    });
    dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
    _YYModelMeta *meta = CFDictionaryGetValue(cache, (__bridge const void *)(cls));
    dispatch_semaphore_signal(lock);
    if (!meta || meta->_classInfo.needUpdate) {
        meta = [[_YYModelMeta alloc] initWithClass:cls];
        if (meta) {
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER);
            CFDictionarySetValue(cache, (__bridge const void *)(cls), (__bridge const void *)(meta));
            dispatch_semaphore_signal(lock);
        }
    }
    return meta;
}

關(guān)于信號(hào)量，一般可以用停車來比喻。
　　停車場(chǎng)剩余4個(gè)車位，那么即使同時(shí)來了四輛車也能停的下。如果此時(shí)來了五輛車，那么就有一輛需要等待。信號(hào)量的值就相當(dāng)于剩余車位的數(shù)目，dispatch_semaphore_wait函數(shù)就相當(dāng)于來了一輛車，dispatch_semaphore_signal就相當(dāng)于走了一輛車。停車位的剩余數(shù)目在初始化的時(shí)候就已經(jīng)指明了（dispatch_semaphore_create（long value）），調(diào)用一次dispatch_semaphore_signal，剩余的車位就增加一個(gè)；調(diào)用一次dispatch_semaphore_wait剩余車位就減少一個(gè)；當(dāng)剩余車位為0時(shí)，再來車（即調(diào)用dispatch_semaphore_wait）就只能等待。有可能同時(shí)有幾輛車等待一個(gè)停車位。有些車主沒有耐心，給自己設(shè)定了一段等待時(shí)間，這段時(shí)間內(nèi)等不到停車位就走了，如果等到了就開進(jìn)去停車。而有些車主就像把車停在這，所以就一直等下去。