關(guān)于GCD的最最基本的知識，往往很多初學(xué)者都被忽略浑槽。理解上造成了偏差蒋失，而不注重去實(shí)踐， 所有我們需要認(rèn)真的總結(jié)一下括荡。

dispatch_asyn和dispatch_sync添加任務(wù)到dispatch隊(duì)列時高镐，是否創(chuàng)建線程呢，那么創(chuàng)建線程是創(chuàng)建一個呢還是多個呢畸冲，如果你自己能直接分的清楚就不必看我下文的實(shí)踐代碼了嫉髓， 下文的代碼做了詳細(xì)的說明观腊， 不僅僅是代碼，也包含了總結(jié)算行， 如果你想實(shí)踐的話梧油， 也可以跟著實(shí)踐一遍哦，這樣最好哦

以下主要總結(jié)了隊(duì)列的類型和隊(duì)列的堵塞

- (void)viewDidLoad {
    [super viewDidLoad];
    
    
    //********
    [self blockIntroduced];
    //********通過以上測試我們得出如下幾個結(jié)論
    
    /*
     1.dispatch_sync添加任務(wù)到隊(duì)列州邢，不會創(chuàng)建新的線程都是在當(dāng)前線程中處理的碰镜。無論添加到串行隊(duì)列里或者并行隊(duì)列里，都是串行效果肛鹏，因?yàn)檫@個方法是等任務(wù)執(zhí)行完成以后才會返回飞涂。
     2.dispatch_async添加任務(wù)到
       2-1：mainQueue不創(chuàng)建線程，在主線程中串行執(zhí)行
       2-2：globalQueue 和 并行隊(duì)列：根據(jù)任務(wù)系統(tǒng)決定開辟線程個數(shù)
       2-3：串行對列：創(chuàng)建一個線程：串行執(zhí)行呀枢。
     
    */
    
    
    [self blockedMainThread0];
    [self blockedMainThread1];
    [self blockedMainThread2];
    [self blockedMainThread3];
//    [self blockedMainThread];
    
}

-(void)blockIntroduced
{
    //隊(duì)列一般就是系統(tǒng)的主隊(duì)列和全局隊(duì)列還有自己手動創(chuàng)建的串行隊(duì)列和并行隊(duì)列
    dispatch_queue_t chuanxingduilie = dispatch_queue_create("chuanxingduilie", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t bingxingduilie = dispatch_queue_create("bingxingduilie", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
    
    //方法有兩個  dispatch_sync 和 dispatch_async  dispatch_async方法是立刻返回的胚股，也就是說把block內(nèi)容加到相應(yīng)隊(duì)列里后會立馬返回，而dispatch_sync要等到加到隊(duì)列里執(zhí)行完之后才會返回裙秋。
    //這時候總共有2*4 = 8種組合琅拌，接下來我們來說一下這八種組合
    
    
    //一：
    for(int i=0;i<3;i++){
        dispatch_sync(chuanxingduilie, ^{
            NSLog(@"dispatch_sync-chuanxingduilie-%d\n mainThread:%@",i,[NSThread currentThread]);
        });
    }
    /*
     2016-02-05 21:07:12.622 GcdTest[7229:164997] dispatch_sync-chuanxingduilie-0
     mainThread:<NSThread: 0x7fad93407dd0>{number = 1, name = main}
     2016-02-05 21:07:12.623 GcdTest[7229:164997] dispatch_sync-chuanxingduilie-1
     mainThread:<NSThread: 0x7fad93407dd0>{number = 1, name = main}
     2016-02-05 21:07:12.623 GcdTest[7229:164997] dispatch_sync-chuanxingduilie-2
     mainThread:<NSThread: 0x7fad93407dd0>{number = 1, name = main}

     */
    //第一種方式可見，串行隊(duì)列是在主線程里完成的摘刑，因?yàn)槭谴嘘?duì)列进宝，所以打印%d是有順序的。
    
    //二：
    for(int i=0;i<3;i++){
        dispatch_sync(bingxingduilie, ^{
            
            NSLog(@"dispatch_sync-bingxingduilie-%d\n mainThread:%@",i,[NSThread currentThread]);
    
        });
    }
    /*
     2016-02-05 21:09:26.422 GcdTest[7241:165846] dispatch_sync-bingxingduilie-0
     mainThread:<NSThread: 0x7face9d01d20>{number = 1, name = main}
     2016-02-05 21:09:26.422 GcdTest[7241:165846] dispatch_sync-bingxingduilie-1
     mainThread:<NSThread: 0x7face9d01d20>{number = 1, name = main}
     2016-02-05 21:09:26.422 GcdTest[7241:165846] dispatch_sync-bingxingduilie-2
     mainThread:<NSThread: 0x7face9d01d20>{number = 1, name = main}

    */
    //第二種方式可以發(fā)現(xiàn)同樣還是在主線程里執(zhí)行并行隊(duì)列枷恕，(雖然是并行隊(duì)列党晋，但這時候依然在同一個線程里執(zhí)行)
    
    //三：
    /*
    for(int i=0;i<3;i++){
        NSLog(@"dispatch_sync-mainQueue");
        dispatch_sync(mainQueue, ^{
            
            NSLog(@"dispatch_sync-mainQueue-%d\n mainThread:%@",i,[NSThread currentThread]);
            
        });
    }
    //打印結(jié)果：dispatch_sync-mainQueue，發(fā)現(xiàn)主線程被堵塞活尊。原因在堵塞中分析隶校。此種方式暫時先屏蔽掉
    */
    
    //四：
    for(int i=0;i<3;i++){
        dispatch_sync(globalQueue, ^{
            
            NSLog(@"dispatch_sync-globalQueue-%d\n mainThread:%@",i,[NSThread currentThread]);
            
        });
    }
    /*
     2016-02-05 21:22:41.107 GcdTest[7338:172870] dispatch_sync-globalQueue-0
     mainThread:<NSThread: 0x7feadad050b0>{number = 1, name = main}
     2016-02-05 21:22:41.107 GcdTest[7338:172870] dispatch_sync-globalQueue-1
     mainThread:<NSThread: 0x7feadad050b0>{number = 1, name = main}
     2016-02-05 21:22:41.107 GcdTest[7338:172870] dispatch_sync-globalQueue-2
     mainThread:<NSThread: 0x7feadad050b0>{number = 1, name = main}
     */
    //打印結(jié)果可以看出依然是主線程。
    
    //五：
    for (int i=0; i<3; i++) {
        dispatch_async(chuanxingduilie, ^{
            if (i==1) {
                sleep(2);
            }
            NSLog(@"dispatch_async-chuanxingduilie-%d\n Thread:%@",i,[NSThread currentThread]);
        });
    }
    /*
     2016-02-05 22:32:14.957 GcdTest[7499:191789] dispatch_async-chuanxingduilie-0
     Thread:<NSThread: 0x7fd7eaf10c70>{number = 3, name = (null)}
     2016-02-05 22:32:16.962 GcdTest[7499:191789] dispatch_async-chuanxingduilie-1
     Thread:<NSThread: 0x7fd7eaf10c70>{number = 3, name = (null)}
     2016-02-05 22:32:16.963 GcdTest[7499:191789] dispatch_async-chuanxingduilie-2
     Thread:<NSThread: 0x7fd7eaf10c70>{number = 3, name = (null)}
     */
    //可以看出創(chuàng)建了一個線程蛹锰， 在串行隊(duì)列里串行執(zhí)行的
    
    
    //六：
    for (int i=0; i<3; i++) {
        dispatch_async(bingxingduilie, ^{
            if (i==1) {
                sleep(3);
            }
            NSLog(@"dispatch_async-bingxingduilie-%d\n Thread:%@",i,[NSThread currentThread]);
        });
    }
    /*
     2016-02-05 22:10:07.083 GcdTest[7425:184003] dispatch_async-bingxingduilie-2
     Thread:<NSThread: 0x7fcda24bcd00>{number = 4, name = (null)}
     2016-02-05 22:10:07.083 GcdTest[7425:183982] dispatch_async-bingxingduilie-0
     Thread:<NSThread: 0x7fcda2502310>{number = 3, name = (null)}
     2016-02-05 22:10:10.083 GcdTest[7425:184002] dispatch_async-bingxingduilie-1
     Thread:<NSThread: 0x7fcda2436180>{number = 5, name = (null)}
     */
    //分析可以看出創(chuàng)建了多個線程深胳，任務(wù)執(zhí)行順序不一定（時間差不多的話）
    
    
    //七：
    for (int i=0; i<3; i++) {
        dispatch_async(mainQueue, ^{
            if (i==1) {
                sleep(3);
            }
            NSLog(@"dispatch_async-mainQueue-%d\n Thread:%@",i,[NSThread currentThread]);
        });
    }
    /*
     Thread:<NSThread: 0x7fbb28704510>{number = 5, name = (null)}
     2016-02-05 22:15:03.709 GcdTest[7460:186485] dispatch_async-mainQueue-0
     Thread:<NSThread: 0x7fbb28703fb0>{number = 1, name = main}
     2016-02-05 22:15:06.710 GcdTest[7460:186485] dispatch_async-mainQueue-1
     Thread:<NSThread: 0x7fbb28703fb0>{number = 1, name = main}
     2016-02-05 22:15:06.710 GcdTest[7460:186485] dispatch_async-mainQueue-2
     Thread:<NSThread: 0x7fbb28703fb0>{number = 1, name = main}
     */
    
    //可以看出這種情況還是在當(dāng)前線程環(huán)境中執(zhí)行，并不創(chuàng)建線程铜犬，因?yàn)槭窃谥麝?duì)列里舞终，順序執(zhí)行
    
    //八：
    for (int i=0; i<3; i++) {
        dispatch_async(globalQueue, ^{
        
            NSLog(@"dispatch_async-globalQueue-%d\n Thread:%@",i,[NSThread currentThread]);
        });
    }
    /*
     2016-02-05 22:17:58.306 GcdTest[7482:188067] dispatch_async-globalQueue-1
     Thread:<NSThread: 0x7fa281e24580>{number = 4, name = (null)}
     2016-02-05 22:17:58.306 GcdTest[7482:188099] dispatch_async-globalQueue-2
     Thread:<NSThread: 0x7fa281e321c0>{number = 5, name = (null)}
     2016-02-05 22:17:58.307 GcdTest[7482:188069] dispatch_async-globalQueue-0
     Thread:<NSThread: 0x7fa281e262b0>{number = 3, name = (null)}
     */
   //可以看出創(chuàng)建了多個線程，執(zhí)行順序并不一定

}

-(void)blockedMainThread
{
  //打印結(jié)果是1  分析：mainQueue里存在任務(wù)1癣猾，同步線程任務(wù)敛劝，這兩個任務(wù)，當(dāng)執(zhí)行dispatch_sync時纷宇，把打印任務(wù)2加入主隊(duì)列夸盟，想要打印2必須等之前所有的任務(wù)都執(zhí)行完成，這時候因?yàn)橹麝?duì)列里有同步線程任務(wù)像捶，這時候相當(dāng)于自己在等自己執(zhí)行完成上陕，進(jìn)入死循環(huán)桩砰。
    NSLog(@"1"); // 任務(wù)1
    dispatch_sync(dispatch_get_main_queue(), ^{
        NSLog(@"2"); // 任務(wù)2
    });
    NSLog(@"3"); // 任務(wù)3

}

-(void)blockedMainThread0
{
    //打印結(jié)果123  分析：mainQueue里存在任務(wù)1，同步線程任務(wù)释簿，當(dāng)執(zhí)行dispatch_sync時亚隅，拿到全局隊(duì)列，
    NSLog(@"1"); // 任務(wù)1
    dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        NSLog(@"2"); // 任務(wù)2
    });
    NSLog(@"3"); // 任務(wù)3
}

-(void)blockedMainThread1
{
    /*
    //執(zhí)行結(jié)果：
    2016-02-05 22:42:30.661 GcdTest[7511:194820] 1
    2016-02-05 22:42:30.662 GcdTest[7511:194820] 5
    2016-02-05 22:42:30.662 GcdTest[7511:194929] 2 <NSThread: 0x7f9ca8709990>{number = 3, name = (null)}
     */
    dispatch_queue_t queue = dispatch_queue_create("com.demo.serialQueue", DISPATCH_QUEUE_SERIAL);
    NSLog(@"1"); // 任務(wù)1
    dispatch_async(queue, ^{
        NSLog(@"2 %@",[NSThread currentThread]); // 任務(wù)2
        
        dispatch_sync(queue, ^{
            NSLog(@"3"); // 任務(wù)3
        });
        NSLog(@"4"); // 任務(wù)4
    });
    NSLog(@"5"); // 任務(wù)5
}

-(void)blockedMainThread2
{
    //1,5,2,3,4 可以看出在全局隊(duì)列里拿到住隊(duì)列同步執(zhí)行是沒有問題的庶溶。
    NSLog(@"****************");
    NSLog(@"1"); // 任務(wù)1
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"2"); // 任務(wù)2
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"3 %@",[NSThread currentThread]); // 任務(wù)3
            //2016-02-05 22:51:00.044 GcdTest[7548:198417] 3 <NSThread: 0x7feb3b701be0>{number = 1, name = main}
        });
        NSLog(@"4"); // 任務(wù)4
    });
    NSLog(@"5"); // 任務(wù)5

}

-(void)blockedMainThread3
{
    //執(zhí)行結(jié)果：11煮纵，44
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"11"); // 任務(wù)1
        dispatch_sync(dispatch_get_main_queue(), ^{
            NSLog(@"22"); // 任務(wù)2
        });
        NSLog(@"33"); // 任務(wù)3
    });
    NSLog(@"44"); // 任務(wù)4
    while (1) {
    }
    NSLog(@"55"); // 任務(wù)5
}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

文章未完待續(xù)：待續(xù)內(nèi)容為dispatch里的大部分常用的方法，等我有時間了一定會更新上去偏螺， 如果大家喜歡文章行疏， 就關(guān)注哦~
有什么不明白的，可以直接評論回復(fù)套像，交流隘擎。

1.dispath_once

dispatch_once的作用就是只執(zhí)行一次，我們在寫單例的時候可以用到

+ (BWStatusBarOverlay *)shared {
    static dispatch_once_t pred = 0;
    __strong static id _sharedObject = nil;
    dispatch_once(&pred, ^{
        _sharedObject = [[self alloc] init];
    });
    return _sharedObject;
}

2.dispatch_after

//延遲0.5s以后執(zhí)行
   dispatch_time_t time = dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC*0.5);
    dispatch_after(time, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"doSomething");
    });

3.dispatch_group

    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, globalQueue, ^{
         sleep(1);
        NSLog(@"1");
    });
    dispatch_group_async(group, globalQueue, ^{
        sleep(2);
        NSLog(@"2");
    });
    dispatch_group_async(group, globalQueue, ^{
        sleep(3);
        NSLog(@"3");
    });
    dispatch_group_notify(group, globalQueue, ^{
        NSLog(@"Over!");
    });

4.dispatch_barrier_async

    dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-1");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-2");
    });
    dispatch_barrier_async(concurrentQueue, ^(){
        NSLog(@"dispatch-barrier-1+2");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-3");
    });
    dispatch_barrier_async(concurrentQueue, ^{
        NSLog(@"dispatch-barrier-1+2+3");
    });
    dispatch_async(concurrentQueue, ^(){
        NSLog(@"dispatch-4");
    });

最后不多說凉夯，需要看看蘋果官方文檔這里截圖看下關(guān)于GCD的東西

gcd-1.png

gcd-2.png

gcd-3.png

看官方的東西其實(shí)不會的東西太多太多，任重而道遠(yuǎn)采幌， 明天就回家過年嘍劲够， 祝愿每一個人新的一年都能對自己的目標(biāo)更近一步。