GCD（2）

dispatch_sync

dispatch_async函數的」async」意味着非同步，就是將指定的Block非同步的追加到指定的Dispatch Queue中，disptch_async函數不作任何等待；相對應的」sync」意味着同步，也就是說將指定的Block同步追加到Dispatch Queue中，在追加Block結束前，dispatch_sync函數會一直等待

應用情景：執行Main Dispatch Queue時，使用另外的線程Global Dispatch Queue進行處理，處理結束後當即使用所獲得的結果

dispatch_queue_t  queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)

dispatch_sync(queue,^{/*處理*/});

注意：死鎖問題

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

該函數按指定的次數將指定的Block追加到指定的Dispatch Queue中，並等待所有處理執行結束

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

當追加大量處理到Dispatch Queue時，在追加處理的過程當中，有時但願不執行已追加的處理，這時只要掛起Dispatch Queue，當能夠執行時再恢復

//掛起

dispatch_suspend(queue);

 

//恢復

dispatch_resume(queue);

 

Dispatch Semaphore

是持有計數的信號，該計數是多線程編程中的計數類型信號。計數值爲0時等待，計數爲1或大於1時，減去1而不等待

//create

dispatch_semaphore_t semaphore =dispatch_semaphore_create(1);

 

//持有

dispatch_retain(semaphore);

 

//釋放

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) {

        /*計數值大於0*/

    }else{

        /*等待time時間後，計數值仍然爲0*/

}

在等待時間內，接收到single消息 dispatch_semaphore_wait當即返回

 

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函數是保證在應用程序執行中只執行一次指定處理的API

static dispatch_once_t pred;

dispatch_once(&pred,^{

/*初始化*/

});

單例模式：

+ (AnObject *)sharedSnashotRecordsMgr

{

    static AnObject *_sharedInstance = nil;

    static dispatch_once_t predicate;

    dispatch_once(&predicate, ^{

        _sharedInstance = [[AnObject alloc] init];

    });

    return _sharedInstance;

}