同步與異步是對(duì)應(yīng)的，它們是線程之間的關(guān)系，兩個(gè)線程之間要么是同步的，要么是異步的。
阻塞與非阻塞是對(duì)同一個(gè)線程來說的，在某個(gè)時(shí)刻，線程要么處于阻塞，要么處于非阻塞。
阻塞是使用同步機(jī)制的結(jié)果，非阻塞則是使用異步機(jī)制的結(jié)果。（不是存在同步非阻塞和異步阻塞這兩種情況嗎？）

• 在主線程中使用performSelectOnMain...waitUntilDone:YES居然不會(huì)死鎖,[NSOperationQueue mainQueue] addOperationWithBlock也不會(huì)死鎖.
  經(jīng)過以下測(cè)驗(yàn)暫時(shí)可以得出的結(jié)論:
  1 死鎖的本質(zhì)是線程等待(阻塞)與queue的FIFO沖突造成的
  2 addOperationWithBlock不阻塞當(dāng)前線程
  3 performSelectorOnMainThread用的不是mainQueue,或者根本沒用queue

/*
     2017-08-21 18:14:00.749968+0800 多線程[12431:8733044] start
     2017-08-21 18:14:03.248097+0800 多線程[12431:8733044] end
     2017-08-21 18:14:07.281684+0800 多線程[12431:8733044] block
     */
    NSLog(@"%@",@"start");
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
       
        NSLog(@"block");
    }];
    NSLog(@"end");
    /*
     waitUntilDone:YES ---> start - haha - end
     waitUntilDone:NO  ---> start - end - haha
     */
    NSLog(@"%@",@"start");
    [self performSelectorOnMainThread:@selector(haha) withObject:nil waitUntilDone:YES];
    NSLog(@"end");

• golbal_queue是否有可能在主線程執(zhí)行任務(wù)?還是只會(huì)在子線程?

    async只會(huì)在子線程.sync會(huì)在當(dāng)前線程(除了指定main_Queue)
    只要是serial_queue,不管是否異步,都會(huì)按順序執(zhí)行,如下,3完了才會(huì)4
  

dispatch_async(serial_queue, ^{
        sleep(3);
        NSLog(@"task 3");       // Thread x  (x != 1)
    });
    // 4) add a task asynchronously too
    dispatch_async(serial_queue, ^{
        NSLog(@"task 4");       // Thread x  (x != 1)
    });

• NSOpterationqueue呢?

    只會(huì)在子線程運(yùn)行,且不會(huì)阻塞當(dāng)前線程.設(shè)置的最大線程數(shù)不包括主線程
  

*在主線程執(zhí)行同步串行隊(duì)列居然不會(huì)死鎖,而且是按順序執(zhí)行

NSLog(@"start");
dispatch_queue_t serial_queue = dispatch_queue_create("com.Evan.test", DISPATCH_QUEUE_SERIAL);
    
dispatch_sync(serial_queue, ^{
    sleep(2);
    NSLog(@"%@",[NSThread currentThread]);
        
});
NSLog(@"end");

2017-06-27 18:44:51.564 gcd測(cè)試[8087:1684347] start
2017-06-27 18:44:53.566 gcd測(cè)試[8087:1684347] <NSThread: 0x608000075c40>{number = 1, name = main}
2017-06-27 18:44:53.566 gcd測(cè)試[8087:1684347] end

以下不會(huì)死鎖,但是如果把serial_queue2換成main_queue,或者serial_queue就會(huì)死鎖

NSLog(@"start");
    dispatch_queue_t serial_queue = dispatch_queue_create("com.Evan.test", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t serial_queue2 = dispatch_queue_create("com.Evan.test2", DISPATCH_QUEUE_SERIAL);
    
    dispatch_sync(serial_queue, ^{
        sleep(2);
        NSLog(@"%@",[NSThread currentThread]);
        
        dispatch_sync(serial_queue2, ^{
            NSLog(@"1");
        });
        NSLog(@"2");
        
    });
    NSLog(@"end");

為什么呢?因?yàn)?
假如換成serial_queue,首先由于是sync,所以會(huì)阻塞當(dāng)前線程(即主線程)任務(wù),等待serial_queue新加的任務(wù)NSLog(@"1")執(zhí)行完.但是NSLog(@"2")(ps:包括sleep和NSLog(@"%@",[NSThread currentThread])) 任務(wù)比NSLog(@"1")先添加進(jìn)串行隊(duì)列,所以要先執(zhí)行完NSLog(@"2")才能執(zhí)行NSLog(@"1").這樣就造成了sync和串行隊(duì)列FIFO的沖突.
換成main_queue同理,是sync和main_queue的FIFO沖突