dispatch_sync
dispatch_async函数的”async”意味着非同步，就是将指定的Block非同步的追加到指定的Dispatch Queue中，disptch_async函数不做任何等待；相对应的”sync”意味着同步，也就是说将指定的Block同步追加到Dispatch Queue中，在追加Block结束前，dispatch_sync函数会一直等待
应用情景：执行Main Dispatch Queue时，使用另外的线程Global Dispatch Queue进行处理，处理结束后立即使用所得到的结果
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
dispatch_sync(queue,^{/*处理*/});
注意：死锁问题
dispatch_queue_t queue = dispatch_get_main_queue()
dispatch_sync(queue,^{NSLog(@"Hello ?")});
dispatch_queue_t queue = dispatch_queue_create("com,example.gcd.MySerialDispachQueue", NULL);
dispatch_async(queue,^{
dispatch_sync(queue, ^{
NSLog(@"hello");
})
});
dispatch_apply
该函数按指定的次数将指定的Block追加到指定的Dispatch Queue中，并等待全部处理执行结束
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_apply(10, queue, ^(size_t index) {
NSLog(@"%zu",index);
});
NSLog(@"done");
dispatch_suspend/dispatch_resume
当追加大量处理到Dispatch Queue时，在追加处理的过程中，有时希望不执行已追加的处理，这时只要挂起Dispatch Queue，当可以执行时再恢复
//挂起
dispatch_suspend(queue);
//恢复
dispatch_resume(queue);
Dispatch Semaphore
是持有计数的信号，该计数是多线程编程中的计数类型信号。计数值为0时等待，计数为1或大于1时，减去1而不等待
//create
dispatch_semaphore_t semaphore =dispatch_semaphore_create(1);
//持有
dispatch_retain(semaphore);
//释放
dispatch_release(semaphore);
//阻塞等待
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, 1ull*NSEC_PER_SEC);
long result = dispatch_semaphore_wait(semaphore, time);
if (result == 0) {
/*计数值大于0*/
}else{
/*等待time时间后，计数值仍然为0*/
}
在等待时间内，接收到single消息 dispatch_semaphore_wait立即返回
dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
NSMutableArray *array = [[NSMutableArray alloc]init];
for (int i = 0; i < 1000; i++) {
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
[array addObject:[NSNumber numberWithInt:i]];
dispatch_semaphore_signal(semaphore);
}
dispatch_release(semaphore);
dispatch_once
dispatch_once函数是保证在应用程序执行中只执行一次指定处理的API
static dispatch_once_t pred;
dispatch_once(&pred,^{
/*初始化*/
});
单例模式：
+ (AnObject *)sharedSnashotRecordsMgr
{
static AnObject *_sharedInstance = nil;
static dispatch_once_t predicate;
dispatch_once(&predicate, ^{
_sharedInstance = [[AnObject alloc] init];
});
return _sharedInstance;
}