GCD的詳解

開始GCD之前我們先來了解下任務(wù)和隊(duì)列的概念

任務(wù)

任務(wù)在程序里面可以理解為你寫的一段代碼, 執(zhí)行任務(wù)就是運(yùn)行代碼,關(guān)鍵在于如何執(zhí)行任務(wù)
這里就有同步執(zhí)行和異步執(zhí)行的概念了

  • 同步執(zhí)行（sync）:
    1钱反、同步添加任務(wù)到指定的隊(duì)列中瞎领，在添加的任務(wù)執(zhí)行結(jié)束之前，會(huì)一直等待，直到隊(duì)列里面的任務(wù)完成之后再繼續(xù)執(zhí)行
    2找蜜、只能在當(dāng)前線程中執(zhí)行任務(wù)，不具備開啟新線程的能力
  • 異步執(zhí)行（async）:
    1、異步添加任務(wù)到指定的隊(duì)列中，它不會(huì)做任何等待特石，可以繼續(xù)執(zhí)行任務(wù)
    2、可以在新的線程中執(zhí)行任務(wù)鳖链，具備開啟新線程的能力

隊(duì)列（Dispatch Queue）

隊(duì)列.png

串行隊(duì)列 （Serial Dispatch Queue）

串行隊(duì)列.png
  • 每次只有一個(gè)任務(wù)被執(zhí)行姆蘸。讓任務(wù)一個(gè)接著一個(gè)地執(zhí)行。（只開啟一個(gè)線程芙委，一個(gè)任務(wù)執(zhí)行完畢后逞敷，再執(zhí)行下一個(gè)任務(wù)）

并行隊(duì)列（Concurrent Dispatch Queue）

并行隊(duì)列.png
  • 可以讓多個(gè)任務(wù)并發(fā)（同時(shí)）執(zhí)行。（可以開啟多個(gè)線程灌侣，并且同時(shí)執(zhí)行任務(wù)）

GCD 的創(chuàng)建和使用

// 串行隊(duì)列
dispatch_queue_t queue = dispatch_queue_create("com.example.gcd.serial", DISPATCH_QUEUE_SERIAL);
// 并行隊(duì)列
dispatch_queue_t queue = dispatch_queue_create("com.example.gcd.concurrent", DISPATCH_QUEUE_CONCURRENT);
//全局隊(duì)列
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//主隊(duì)列
dispatch_queue_t queue = dispatch_get_main_queue();
//同步執(zhí)行
dispatch_sync(queue, ^{

});
//異步執(zhí)行
dispatch_async(queue, ^{
        
});

上面說了4種隊(duì)列,2種執(zhí)行方式,那么他們的組合肯定有多種,來寫點(diǎn)代碼運(yùn)行看看效果吧
1推捐、 同步執(zhí)行串行隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t serialQueue = dispatch_queue_create("com.example.gcd.serial", DISPATCH_QUEUE_SERIAL);
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_sync(serialQueue, ^{
            NSLog(@"%ld, 當(dāng)前線程 : %@", i, [NSThread currentThread]);
        });
    }
    
NSLog(@"--- end ---");

日志打印

2019-05-14 16:03:01.382002+0800 GCDDemo[5717:153795] --- begin ---
2019-05-14 16:03:01.382222+0800 GCDDemo[5717:153795] 0, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382349+0800 GCDDemo[5717:153795] 1, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382464+0800 GCDDemo[5717:153795] 2, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382563+0800 GCDDemo[5717:153795] 3, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382659+0800 GCDDemo[5717:153795] 4, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382785+0800 GCDDemo[5717:153795] 5, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382879+0800 GCDDemo[5717:153795] 6, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.382971+0800 GCDDemo[5717:153795] 7, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.383061+0800 GCDDemo[5717:153795] 8, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.395531+0800 GCDDemo[5717:153795] 9, 當(dāng)前線程 : <NSThread: 0x6000023313c0>{number = 1, name = main}
2019-05-14 16:03:01.395652+0800 GCDDemo[5717:153795] --- end ---

結(jié)果分析:依次執(zhí)行, 沒有開辟新線程
2、同步執(zhí)行并行隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.example.gcd.serial", DISPATCH_QUEUE_CONCURRENT);
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_sync(concurrentQueue, ^{
            NSLog(@"%ld, 當(dāng)前線程 : %@", i, [NSThread currentThread]);
        });
    }
    
NSLog(@"--- end ---");

2019-05-14 16:04:34.245949+0800 GCDDemo[5739:154761] --- begin ---
2019-05-14 16:04:34.246151+0800 GCDDemo[5739:154761] 0, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246264+0800 GCDDemo[5739:154761] 1, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246373+0800 GCDDemo[5739:154761] 2, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246484+0800 GCDDemo[5739:154761] 3, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246593+0800 GCDDemo[5739:154761] 4, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246705+0800 GCDDemo[5739:154761] 5, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246801+0800 GCDDemo[5739:154761] 6, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246891+0800 GCDDemo[5739:154761] 7, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.246982+0800 GCDDemo[5739:154761] 8, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.258913+0800 GCDDemo[5739:154761] 9, 當(dāng)前線程 : <NSThread: 0x60000055d3c0>{number = 1, name = main}
2019-05-14 16:04:34.259039+0800 GCDDemo[5739:154761] --- end ---

結(jié)果分析: 同上
3侧啼、同步執(zhí)行全局隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_sync(globalQueue, ^{
            NSLog(@"%ld, 當(dāng)前線程 : %@", i, [NSThread currentThread]);
        });
    }
    
NSLog(@"--- end ---");

2019-05-14 16:06:44.980750+0800 GCDDemo[5767:155906] --- begin ---
2019-05-14 16:06:44.981023+0800 GCDDemo[5767:155906] 0, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981168+0800 GCDDemo[5767:155906] 1, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981307+0800 GCDDemo[5767:155906] 2, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981410+0800 GCDDemo[5767:155906] 3, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981526+0800 GCDDemo[5767:155906] 4, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981640+0800 GCDDemo[5767:155906] 5, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981738+0800 GCDDemo[5767:155906] 6, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981833+0800 GCDDemo[5767:155906] 7, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:44.981976+0800 GCDDemo[5767:155906] 8, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:45.016921+0800 GCDDemo[5767:155906] 9, 當(dāng)前線程 : <NSThread: 0x6000016993c0>{number = 1, name = main}
2019-05-14 16:06:45.017021+0800 GCDDemo[5767:155906] --- end ---

結(jié)果分析:同上
4玖姑、同步執(zhí)行主隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    
    dispatch_sync(mainQueue, ^{
        NSLog(@"1");
    });
    
    dispatch_sync(mainQueue, ^{
        NSLog(@"2");
    });
    
    dispatch_sync(mainQueue, ^{
        NSLog(@"3");
    });
    
NSLog(@"--- end ---");

2019-05-14 15:19:00.807434+0800 GCDDemo[5154:130087] --- begin ---
(lldb)
崩潰截圖.png

結(jié)果崩潰了
這邊涉及到了線程死鎖的概念, 簡(jiǎn)單說一下:這段代碼本身(這里稱做任務(wù)F,下同)就先添加再主隊(duì)列里, NSLog(@"1"), NSLog(@"2"),NSLog(@"3")分別一次后添加到主隊(duì)列里面, NSLog(@"1")這個(gè)任務(wù)在等待任務(wù)F執(zhí)行完, 任務(wù)F要想執(zhí)行完那必須等到NSLog(@"1"), NSLog(@"2"), NSLog(@"3")著三個(gè)任務(wù)都執(zhí)行完了才能執(zhí)行完, 這樣就存在了任務(wù)F和NSLog(@"1")互相等待, 這就形成了主線程的死鎖.(建議自己詳細(xì)了解一下線程的死鎖)
5、異步執(zhí)行串行隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t serialQueue = dispatch_queue_create("com.example.gcd.serial", DISPATCH_QUEUE_SERIAL);
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_async(serialQueue, ^{
            NSLog(@"%ld, 當(dāng)前線程 : %@", i, [NSThread currentThread]);
        });
    }
    
NSLog(@"--- end ---");

2019-05-14 16:08:38.492805+0800 GCDDemo[5793:157042] --- begin ---
2019-05-14 16:08:38.492977+0800 GCDDemo[5793:157042] --- end ---
2019-05-14 16:08:38.493042+0800 GCDDemo[5793:157082] 0, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493195+0800 GCDDemo[5793:157082] 1, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493331+0800 GCDDemo[5793:157082] 2, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493441+0800 GCDDemo[5793:157082] 3, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493559+0800 GCDDemo[5793:157082] 4, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493667+0800 GCDDemo[5793:157082] 5, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493780+0800 GCDDemo[5793:157082] 6, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.493911+0800 GCDDemo[5793:157082] 7, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.508214+0800 GCDDemo[5793:157082] 8, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}
2019-05-14 16:08:38.508361+0800 GCDDemo[5793:157082] 9, 當(dāng)前線程 : <NSThread: 0x600001c69480>{number = 3, name = (null)}

結(jié)果分析: 先走的開始和結(jié)束, 任務(wù)依次執(zhí)行, 開辟了新線程,只開辟了一條新線程
6慨菱、異步執(zhí)行并行隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t concurrentQueue = dispatch_queue_create("com.example.gcd.serial", DISPATCH_QUEUE_CONCURRENT);
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_async(concurrentQueue, ^{
            NSLog(@"%ld, 當(dāng)前線程 : %@", i, [NSThread currentThread]);
        });
    }
    
NSLog(@"--- end ---");

2019-05-14 16:14:01.097373+0800 GCDDemo[5837:159234] --- begin ---
2019-05-14 16:14:01.097566+0800 GCDDemo[5837:159234] --- end ---
2019-05-14 16:14:01.097710+0800 GCDDemo[5837:159276] 0, 當(dāng)前線程 : <NSThread: 0x600003aa4200>{number = 3, name = (null)}
2019-05-14 16:14:01.097712+0800 GCDDemo[5837:159278] 3, 當(dāng)前線程 : <NSThread: 0x600003a84a40>{number = 6, name = (null)}
2019-05-14 16:14:01.097710+0800 GCDDemo[5837:159275] 1, 當(dāng)前線程 : <NSThread: 0x600003ae3f80>{number = 5, name = (null)}
2019-05-14 16:14:01.097783+0800 GCDDemo[5837:159283] 4, 當(dāng)前線程 : <NSThread: 0x600003aab5c0>{number = 7, name = (null)}
2019-05-14 16:14:01.097783+0800 GCDDemo[5837:159277] 2, 當(dāng)前線程 : <NSThread: 0x600003abfb00>{number = 4, name = (null)}
2019-05-14 16:14:01.097793+0800 GCDDemo[5837:159284] 5, 當(dāng)前線程 : <NSThread: 0x600003aab580>{number = 8, name = (null)}
2019-05-14 16:14:01.097827+0800 GCDDemo[5837:159285] 6, 當(dāng)前線程 : <NSThread: 0x600003abfb40>{number = 9, name = (null)}
2019-05-14 16:14:01.097882+0800 GCDDemo[5837:159286] 7, 當(dāng)前線程 : <NSThread: 0x600003a84ac0>{number = 10, name = (null)}
2019-05-14 16:14:01.097895+0800 GCDDemo[5837:159275] 9, 當(dāng)前線程 : <NSThread: 0x600003ae3f80>{number = 5, name = (null)}
2019-05-14 16:14:01.097909+0800 GCDDemo[5837:159278] 8, 當(dāng)前線程 : <NSThread: 0x600003a84a40>{number = 6, name = (null)}

結(jié)果分析:順序就是亂的了, 開辟了新線程, 開辟了不止一條新線程
7、異步執(zhí)行全局隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_async(globalQueue, ^{
            NSLog(@"%ld", i);
        });
    }
    
NSLog(@"--- end ---");

2019-05-14 15:59:03.987523+0800 GCDDemo[5675:151762] --- begin ---
2019-05-14 15:59:03.987725+0800 GCDDemo[5675:151762] --- end ---
2019-05-14 15:59:03.987863+0800 GCDDemo[5675:151803] 1, 當(dāng)前線程 : <NSThread: 0x60000090c380>{number = 4, name = (null)}
2019-05-14 15:59:03.987862+0800 GCDDemo[5675:151806] 3, 當(dāng)前線程 : <NSThread: 0x60000097f700>{number = 6, name = (null)}
2019-05-14 15:59:03.987865+0800 GCDDemo[5675:151804] 2, 當(dāng)前線程 : <NSThread: 0x600000919b80>{number = 5, name = (null)}
2019-05-14 15:59:03.987887+0800 GCDDemo[5675:151805] 0, 當(dāng)前線程 : <NSThread: 0x600000914080>{number = 3, name = (null)}
2019-05-14 15:59:03.987895+0800 GCDDemo[5675:151823] 4, 當(dāng)前線程 : <NSThread: 0x60000094ed40>{number = 7, name = (null)}
2019-05-14 15:59:03.987928+0800 GCDDemo[5675:151824] 5, 當(dāng)前線程 : <NSThread: 0x60000090c3c0>{number = 8, name = (null)}
2019-05-14 15:59:03.988011+0800 GCDDemo[5675:151825] 6, 當(dāng)前線程 : <NSThread: 0x60000094eec0>{number = 9, name = (null)}
2019-05-14 15:59:03.988014+0800 GCDDemo[5675:151806] 8, 當(dāng)前線程 : <NSThread: 0x60000097f700>{number = 6, name = (null)}
2019-05-14 15:59:03.988037+0800 GCDDemo[5675:151805] 9, 當(dāng)前線程 : <NSThread: 0x600000914080>{number = 3, name = (null)}
2019-05-14 15:59:03.988040+0800 GCDDemo[5675:151826] 7, 當(dāng)前線程 : <NSThread: 0x60000097f900>{number = 10, name = (null)}

結(jié)果分析:同上
8戴甩、異步執(zhí)行主隊(duì)列

NSLog(@"--- begin ---");
    
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    
    for (NSInteger i = 0; i < 10; i++) {
        dispatch_async(mainQueue, ^{
            NSLog(@"%ld, 當(dāng)前線程 : %@", i, [NSThread currentThread]);
        });
    }
    
NSLog(@"--- end ---");

2019-05-14 15:56:57.099676+0800 GCDDemo[5648:150470] --- begin ---
2019-05-14 15:56:57.099889+0800 GCDDemo[5648:150470] --- end ---
2019-05-14 15:56:57.112562+0800 GCDDemo[5648:150470] 0, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.112749+0800 GCDDemo[5648:150470] 1, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.112850+0800 GCDDemo[5648:150470] 2, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.112945+0800 GCDDemo[5648:150470] 3, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.113067+0800 GCDDemo[5648:150470] 4, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.113166+0800 GCDDemo[5648:150470] 5, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.113297+0800 GCDDemo[5648:150470] 6, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.113417+0800 GCDDemo[5648:150470] 7, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.127392+0800 GCDDemo[5648:150470] 8, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}
2019-05-14 15:56:57.127536+0800 GCDDemo[5648:150470] 9, 當(dāng)前線程 : <NSThread: 0x600001ce13c0>{number = 1, name = main}

結(jié)果分析:先執(zhí)行的開始和結(jié)束, 中間的任務(wù)是按順序執(zhí)行的,沒有開辟新線程

GCD之Group

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
     dispatch_group_t group = dispatch_group_create();
     dispatch_group_async(group, queue, ^{
         NSLog(@"%@",group);
         NSLog(@"blok0");
     });
     dispatch_group_async(group, queue, ^{
         NSLog(@"block1");
     });
     dispatch_group_async(group, queue, ^{
         NSLog(@"block2");
     });
     dispatch_group_notify(group, queue, ^{
         NSLog(@"done");
     });
//     dispatch_release(group); // 只有在MRC下才這么寫,ARC下會(huì)自動(dòng)管理

2019-05-14 16:24:25.308830+0800 GCDDemo[5922:163218] block1
2019-05-14 16:24:25.308830+0800 GCDDemo[5922:163217] block2
2019-05-14 16:24:25.308942+0800 GCDDemo[5922:163219] <OS_dispatch_group: 0x600003bc90e0>
2019-05-14 16:24:25.309028+0800 GCDDemo[5922:163219] blok0
2019-05-14 16:24:25.309138+0800 GCDDemo[5922:163219] done

結(jié)果分析: 最后才執(zhí)行的dispatch_group_notify中的任務(wù)(應(yīng)用場(chǎng)景:一個(gè)頁面有多個(gè)數(shù)據(jù)請(qǐng)求,等所有的請(qǐng)求都回調(diào)回來后才刷新界面)

GCD之Barrier

    dispatch_queue_t queue = dispatch_queue_create("com.example.gcd.ForBarrier", DISPATCH_QUEUE_CONCURRENT);
    for (NSInteger i = 0; i < 5; i++) {
        dispatch_async(queue, ^{
            NSLog(@"%ld", i);
        });
    }
    dispatch_barrier_async(queue, ^{
        NSLog(@"--- ---");
    });
    for (NSInteger i = 5; i < 10; i++) {
        dispatch_async(queue, ^{
            NSLog(@"%ld", i);
        });
    }

2019-05-14 16:34:06.856068+0800 GCDDemo[6028:168004] 3
2019-05-14 16:34:06.856057+0800 GCDDemo[6028:168005] 1
2019-05-14 16:34:06.856057+0800 GCDDemo[6028:168003] 0
2019-05-14 16:34:06.856057+0800 GCDDemo[6028:168006] 2
2019-05-14 16:34:06.856151+0800 GCDDemo[6028:168010] 4
2019-05-14 16:34:06.856279+0800 GCDDemo[6028:168010] --- ---
2019-05-14 16:34:06.856373+0800 GCDDemo[6028:168010] 5
2019-05-14 16:34:06.856381+0800 GCDDemo[6028:168006] 6
2019-05-14 16:34:06.856389+0800 GCDDemo[6028:168003] 7
2019-05-14 16:34:06.856403+0800 GCDDemo[6028:168004] 8
2019-05-14 16:34:06.856422+0800 GCDDemo[6028:168011] 9

結(jié)果分析:barrier就是欄的意思, 就像一個(gè)擋板以它界限分割開了, 上面的任務(wù)異步執(zhí)行, 下面的任務(wù)異步執(zhí)行

GCD之Semaphore

// 使用信號(hào)量可以做到并行隊(duì)列串行執(zhí)行 符喝，而且當(dāng)信號(hào)量為1時(shí)是按照代碼順序執(zhí)行的
- (void)GCD_semaphore{
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    dispatch_queue_t globalQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_async(globalQueue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//當(dāng)信號(hào)量>=1時(shí)，才會(huì)執(zhí)行下面的代碼甜孤，等待信號(hào)量-1
        NSLog(@"first start");
        NSLog(@"first end");
        dispatch_semaphore_signal(semaphore);//信號(hào)量+1
    });
    dispatch_async(globalQueue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"second start");
        NSLog(@"second end");
        dispatch_semaphore_signal(semaphore);//信號(hào)量+1
    });
    dispatch_async(globalQueue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"third start");
        NSLog(@"third end");
        dispatch_semaphore_signal(semaphore);//信號(hào)量+1
    });
}

2019-05-14 16:39:09.310763+0800 GCDDemo[6083:170514] first start
2019-05-14 16:39:09.310924+0800 GCDDemo[6083:170514] first end
2019-05-14 16:39:09.311018+0800 GCDDemo[6083:170512] second start
2019-05-14 16:39:09.311106+0800 GCDDemo[6083:170512] second end
2019-05-14 16:39:09.311184+0800 GCDDemo[6083:170513] third start
2019-05-14 16:39:09.311247+0800 GCDDemo[6083:170513] third end

結(jié)果分析: 按順序依次執(zhí)行 (這就是異步里面的同步)
semaphore個(gè)人使用總結(jié):

 dispatch_semaphore_wait 函數(shù)的作用：
 1协饲、如果dsema信號(hào)量的值大于0畏腕，該函數(shù)所處線程就繼續(xù)執(zhí)行下面的語句，
 并且將信號(hào)量的值減1; 
 2茉稠、如果desema的值為0描馅，那么這個(gè)函數(shù)就阻塞當(dāng)前線程等待timeout（注意timeout的類型為dispatch_time_t，
 需要傳入對(duì)應(yīng)的類型參數(shù)）而线，如果等待的期間desema的值被dispatch_semaphore_signal函數(shù)加1了铭污，且該函數(shù)（即dispatch_semaphore_wait）所處線程獲得了信號(hào)量，那么就繼續(xù)向下執(zhí)行并將信號(hào)量減1膀篮。
 
 3嘹狞、如果等待期間沒有獲取到信號(hào)量或者信號(hào)量的值一直為0，那么等到timeout時(shí)誓竿，其所處線程自動(dòng)執(zhí)行其后語句磅网。

轉(zhuǎn)載請(qǐng)注明出處
原文鏈接: http://www.reibang.com/p/8321f35e30b9
參考資料: http://www.reibang.com/p/2d57c72016c6

補(bǔ)充:

GCD之定時(shí)器

    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    /*
     para1: 間隔幾秒
     para2: leeway (翻譯:最小的余地)精確度
     */
    dispatch_source_set_timer(timer, DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC, 0.1 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(timer, ^{
        //要執(zhí)行的任務(wù)
        NSLog(@"***");
    });
    
    //開始執(zhí)行定時(shí)器
    dispatch_resume(timer);

運(yùn)行完之后你會(huì)發(fā)現(xiàn)沒有日志輸出
原因是我們創(chuàng)建的這個(gè)_timer在這段代碼執(zhí)行完后就被銷毀了，可以看出GCD并沒有管理它的內(nèi)存筷屡，并沒有強(qiáng)持有它涧偷，所以我們需要自己想辦法讓它不被銷毀

@property (nonatomic, strong) dispatch_source_t timer;

- (void)GCD_timer {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
//    dispatch_source_t timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    //GCD創(chuàng)建的timer必須被持有
    self.timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
    /*
     para1: 間隔幾秒
     para2: leeway (翻譯:最小的余地)精確度
     */
    dispatch_source_set_timer(self.timer, DISPATCH_TIME_NOW, 1.0 * NSEC_PER_SEC, 0.1 * NSEC_PER_SEC);
    dispatch_source_set_event_handler(self.timer, ^{
        //要執(zhí)行的任務(wù)
        NSLog(@"***");
    });
    
    //開始執(zhí)行定時(shí)器
    dispatch_resume(self.timer);
}

2019-05-14 20:01:49.960536+0800 GCDDemo[8032:248654] ***
2019-05-14 20:01:50.995122+0800 GCDDemo[8032:248654] ***
2019-05-14 20:01:52.048355+0800 GCDDemo[8032:248653] ***
2019-05-14 20:01:52.960705+0800 GCDDemo[8032:248654] ***
2019-05-14 20:01:53.960540+0800 GCDDemo[8032:248654] ***
2019-05-14 20:01:55.051170+0800 GCDDemo[8032:248654] ***
2019-05-14 20:01:56.058094+0800 GCDDemo[8032:248666] ***

參考鏈接:http://www.reibang.com/p/69e2b9169365

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