iOS多线程之GCD-同步、异步、并发、串行、线程组、栅栏函数、信号量等全网最全的总结-阿里云开发者社区

1. GCD简介

GCD全称：Grand Central Dispatch，译为大型的中枢调度器、纯C语言实现，提供了非常多强大的功能；

优势：旨在替代NSThread等线程技术，充分利用设备的多核。

2. GCD队列

队列（Dispatch Queue）

这里的队列指执行任务的等待队列，即用来存放任务的队列。队列是一种特殊的线性表，采用 FIFO（先进先出）的原则，即新任务总是被插入到队列的末尾，而读取任务的时候总是从队列的头部开始读取。每读取一个任务，则从队列中释放一个任务。

GCD的队列可以分为2大类型：

并发队列(Concurrent Dispatch Queue)

1）可以让多个任务并发(同时)执行(自动开启多个线程同时执行任务)

2）并发功能只有在异步(dispatch_async)函数下才有效

串行队列(Serial Dispatch Queue)

让任务一个接着一个地执行(一个任务执行完毕后，再执行下一个任务)

3. 同步、异步、并发、串行

同步和异步主要影响: 能不能开启新的线程

同步: 在当前线程中执行任务，不具备开启新线程的能力

异步: 在新的线程中执行任务，具备开启新线程的能力

并发和串行主要影响: 任务的执行方式

并发: 多个任务并发(同时)执行

串行: 一个任务执行完毕后，再执行下一个任务

4. 创建队列

可以使用 dispatch_queue_create 方法来创建队列。该方法需要传入两个参数：

第一个参数表示队列的唯一标识符

第二个参数 DISPATCH_QUEUE_SERIAL或NULL: 表示串行队列 DISPATCH_QUEUE_CONCURRENT: 表示并发队列

系统默认提供了主队列，主队列也属于一个串行队列，可通过dispatch_get_main_queue获取主队列，放在主队列中的任务都会放在主线程中执行，系统还提供了一个并发队列，dispatch_get_global_queue() 方法获得全局并发队列。

- (void)gcdCreateQueue {
    dispatch_queue_t queue1 = dispatch_queue_create("com.glt.test.queue1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t queue2 = dispatch_queue_create("com.glt.test.queue2", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t queue3 = dispatch_queue_create("com.glt.test.queue3", NULL);
    dispatch_queue_t queue4 = dispatch_get_main_queue();
    dispatch_queue_t queue5 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    NSLog(@"%@", queue1);
    NSLog(@"%@", queue2);
    NSLog(@"%@", queue3);
    NSLog(@"%@", queue4);
    NSLog(@"%@", queue5);
}
- (void)gcdCreateQueue {
    dispatch_queue_t queue1 = dispatch_queue_create("com.glt.test.queue1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t queue2 = dispatch_queue_create("com.glt.test.queue2", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t queue3 = dispatch_queue_create("com.glt.test.queue3", NULL);
    dispatch_queue_t queue4 = dispatch_get_main_queue();
    dispatch_queue_t queue5 = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    NSLog(@"%@", queue1);
    NSLog(@"%@", queue2);
    NSLog(@"%@", queue3);
    NSLog(@"%@", queue4);
    NSLog(@"%@", queue5);
}

运行结果：

2021-04-10 10:51:46.206755+0800 GCD[1482:45167] <OS_dispatch_queue_concurrent: com.glt.test.queue1>
2021-04-10 10:51:46.206850+0800 GCD[1482:45167] <OS_dispatch_queue_serial: com.glt.test.queue2>
2021-04-10 10:51:46.206930+0800 GCD[1482:45167] <OS_dispatch_queue_serial: com.glt.test.queue3>
2021-04-10 10:51:46.207000+0800 GCD[1482:45167] <OS_dispatch_queue_main: com.apple.main-thread>
2021-04-10 10:51:46.207061+0800 GCD[1482:45167] <OS_dispatch_queue_global: com.apple.root.default-qos>

5. 创建任务

dispatch_sync创建同步任务，dispatch_async创建异步线程任务

- (void)gcdCreateTask {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test.queue1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"同步任务队列");
    });
    dispatch_async(queue, ^{
        NSLog(@"异步任务队列");
    });
}
- (void)gcdCreateTask {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test.queue1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"同步任务队列");
    });
    dispatch_async(queue, ^{
        NSLog(@"异步任务队列");
    });
}

6. 队列组合

不同的队列组合产生的效果和影响不同，具体总结如下：

接下来通过样例来分别看一下组合效果（文末附源码工程链接）：

1）同步+串行

不会开启新的线程、把任务添加到当前线程中、任务一个接一个的执行

- (void)syncSerial {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_SERIAL);
    dispatch_sync(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}- (void)syncSerial {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_SERIAL);
    dispatch_sync(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 10:57:25.460191+0800 GCD[1540:51156] 任务1
2021-04-10 10:57:25.460303+0800 GCD[1540:51156] 当前线程: <NSThread: 0x60000294c180>{number = 1, name = main}
2021-04-10 10:57:27.460477+0800 GCD[1540:51156] 任务2
2021-04-10 10:57:27.460732+0800 GCD[1540:51156] 当前线程: <NSThread: 0x60000294c180>{number = 1, name = main}
2021-04-10 10:57:27.460895+0800 GCD[1540:51156] 任务3
2021-04-10 10:57:27.461054+0800 GCD[1540:51156] 当前线程: <NSThread: 0x60000294c180>{number = 1, name = main}
2021-04-10 10:57:25.460191+0800 GCD[1540:51156] 任务1
2021-04-10 10:57:25.460303+0800 GCD[1540:51156] 当前线程: <NSThread: 0x60000294c180>{number = 1, name = main}
2021-04-10 10:57:27.460477+0800 GCD[1540:51156] 任务2
2021-04-10 10:57:27.460732+0800 GCD[1540:51156] 当前线程: <NSThread: 0x60000294c180>{number = 1, name = main}
2021-04-10 10:57:27.460895+0800 GCD[1540:51156] 任务3
2021-04-10 10:57:27.461054+0800 GCD[1540:51156] 当前线程: <NSThread: 0x60000294c180>{number = 1, name = main}

2）同步+并发

不会开启新的线程、把任务添加到当前线程中、任务一个接一个的执行、并发队列在异步任务中才有效

- (void)syncConcurrent {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}
- (void)syncConcurrent {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_sync(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 11:00:59.308934+0800 GCD[1570:54003] 任务1
2021-04-10 11:00:59.309035+0800 GCD[1570:54003] 当前线程: <NSThread: 0x6000031b81c0>{number = 1, name = main}
2021-04-10 11:01:01.309702+0800 GCD[1570:54003] 任务2
2021-04-10 11:01:01.309948+0800 GCD[1570:54003] 当前线程: <NSThread: 0x6000031b81c0>{number = 1, name = main}
2021-04-10 11:01:01.310101+0800 GCD[1570:54003] 任务3
2021-04-10 11:01:01.310262+0800 GCD[1570:54003] 当前线程: <NSThread: 0x6000031b81c0>{number = 1, name = main}

3）异步+串行

会开启新的线程、把任务添加到当前线程中、任务一个接一个的执行

- (void)asyncSerial {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}
- (void)asyncSerial {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_SERIAL);
    dispatch_async(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 11:04:22.612780+0800 GCD[1612:57386] 任务1
2021-04-10 11:04:22.612881+0800 GCD[1612:57386] 当前线程: <NSThread: 0x6000009a2a40>{number = 5, name = (null)}
2021-04-10 11:04:24.615244+0800 GCD[1612:57386] 任务2
2021-04-10 11:04:24.615514+0800 GCD[1612:57386] 当前线程: <NSThread: 0x6000009a2a40>{number = 5, name = (null)}
2021-04-10 11:04:24.615686+0800 GCD[1612:57386] 任务3
2021-04-10 11:04:24.615854+0800 GCD[1612:57386] 当前线程: <NSThread: 0x6000009a2a40>{number = 5, name = (null)}

4）异步+并发

会开启新的线程、把任务添加到新的线程中、并发的执行任务

- (void)asyncConcurrent {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 11:09:06.322720+0800 GCD[1664:61535] 任务1 - <NSThread: 0x6000002d1700>{number = 6, name = (null)}
2021-04-10 11:09:06.322726+0800 GCD[1664:61536] 任务3 - <NSThread: 0x6000002c5e80>{number = 7, name = (null)}
2021-04-10 11:09:08.326379+0800 GCD[1664:61541] 任务2 - <NSThread: 0x60000029a840>{number = 4, name = (null)}2021-04-10 11:09:06.322720+0800 GCD[1664:61535] 任务1 - <NSThread: 0x6000002d1700>{number = 6, name = (null)}
2021-04-10 11:09:06.322726+0800 GCD[1664:61536] 任务3 - <NSThread: 0x6000002c5e80>{number = 7, name = (null)}
2021-04-10 11:09:08.326379+0800 GCD[1664:61541] 任务2 - <NSThread: 0x60000029a840>{number = 4, name = (null)}

5）同步+主队列

不会开启新的线程、把任务添加到当前线程中、产生死锁(2和3相互等待产生死锁)

- (void)syncMain {
//    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_SERIAL);//不会产生死锁
    dispatch_queue_t queue = dispatch_get_main_queue();//产生死锁
    NSLog(@"1");
    dispatch_sync(queue, ^{
        NSLog(@"2");
    });
    NSLog(@"3");
}
- (void)syncMain {
//    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_SERIAL);//不会产生死锁
    dispatch_queue_t queue = dispatch_get_main_queue();//产生死锁
    NSLog(@"1");
    dispatch_sync(queue, ^{
        NSLog(@"2");
    });
    NSLog(@"3");
}

运行结果：

2021-04-10 11:11:30.578822+0800 GCD[1694:64014] 1

6）异步+主队列

不会开启新的线程、把任务添加到当前线程中、任务一个接一个的执行

- (void)asyncMain {
    dispatch_queue_t queue = dispatch_get_main_queue();
    NSLog(@"1");
    dispatch_async(queue, ^{
        NSLog(@"2");
    });
    NSLog(@"3");
}

运行结果：

2021-04-10 11:24:34.478703+0800 GCD[1755:73277] 1
2021-04-10 11:24:34.478825+0800 GCD[1755:73277] 3
2021-04-10 11:24:34.481464+0800 GCD[1755:73277] 2
2021-04-10 11:24:34.478703+0800 GCD[1755:73277] 1
2021-04-10 11:24:34.478825+0800 GCD[1755:73277] 3
2021-04-10 11:24:34.481464+0800 GCD[1755:73277] 2

7）同步+全局队列

会开启新的线程、把任务添加到当前线程中、没有开启新的线程、任务一个接一个的执行

- (void)syncGlobal {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_sync(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}
- (void)syncGlobal {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_sync(queue, ^{
        NSLog(@"任务1");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        sleep(2);
        NSLog(@"任务2");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"任务3");
        NSLog(@"当前线程: %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 11:26:45.632522+0800 GCD[1778:75212] 任务1
2021-04-10 11:26:45.632626+0800 GCD[1778:75212] 当前线程: <NSThread: 0x600003910900>{number = 1, name = main}
2021-04-10 11:26:47.633010+0800 GCD[1778:75212] 任务2
2021-04-10 11:26:47.633255+0800 GCD[1778:75212] 当前线程: <NSThread: 0x600003910900>{number = 1, name = main}
2021-04-10 11:26:47.633437+0800 GCD[1778:75212] 任务3
2021-04-10 11:26:47.633594+0800 GCD[1778:75212] 当前线程: <NSThread: 0x600003910900>{number = 1, name = main}

8）异步+全局队列

会开启新的线程、把任务添加到新的线程中、并发的执行任务

- (void)asyncGlobal {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
    });
}
- (void)asyncGlobal {
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_async(queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 11:30:37.836797+0800 GCD[1814:79285] 任务1 - <NSThread: 0x6000032fac40>{number = 4, name = (null)}
2021-04-10 11:30:37.836805+0800 GCD[1814:79287] 任务3 - <NSThread: 0x6000032a96c0>{number = 6, name = (null)}
2021-04-10 11:30:39.836879+0800 GCD[1814:79283] 任务2 - <NSThread: 0x6000032a9080>{number = 7, name = (null)}

9）串行+异步+同步

死锁

- (void)concurrentSyncAsync {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test.queue1", DISPATCH_QUEUE_SERIAL);
    NSLog(@"1");
    dispatch_async(queue, ^{
        NSLog(@"4");
        dispatch_sync(queue, ^{
            NSLog(@"2");
        });
        NSLog(@"3");
    });
    NSLog(@"5");
}

运行结果：

2021-04-10 11:33:34.234259+0800 GCD[1834:81537] 1
2021-04-10 11:33:34.234365+0800 GCD[1834:81537] 5
2021-04-10 11:33:34.234377+0800 GCD[1834:81712] 4
2021-04-10 11:33:34.234259+0800 GCD[1834:81537] 1
2021-04-10 11:33:34.234365+0800 GCD[1834:81537] 5
2021-04-10 11:33:34.234377+0800 GCD[1834:81712] 4

7. 线程间通信

子线程执行耗时操作（图片下载、图片合成、文件下载等），回到主线程刷新UI

- (void)gcdMessage {
    NSLog(@"1");
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        sleep(2);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"2-刷新UI");
        });
    });
    NSLog(@"3");
}
- (void)gcdMessage {
    NSLog(@"1");
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        sleep(2);
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"2-刷新UI");
        });
    });
    NSLog(@"3");
}

运行结果：

2021-04-10 11:39:23.991812+0800 GCD[1865:85789] 1
2021-04-10 11:39:23.991908+0800 GCD[1865:85789] 3
2021-04-10 11:39:25.992573+0800 GCD[1865:85789] 2-刷新UI2021-04-10 11:39:23.991812+0800 GCD[1865:85789] 1
2021-04-10 11:39:23.991908+0800 GCD[1865:85789] 3
2021-04-10 11:39:25.992573+0800 GCD[1865:85789] 2-刷新UI

8. 栅栏函数

栅栏函数：dispatch_barrier_async会等待前边追加到并发队列中的任务全部执行完毕之后，再将指定的任务追加到该异步队列中；并且栅栏函数只能使用自己创建的并发队列、全局队列没有效果（使用全局队列相当于普通的异步任务），如果是主队列则依次执行；

dispatch_barrier_sync与dispatch_barrier_sync区别是：dispatch_barrier_sync内部的函数是在主线程中执行，dispatch_barrier_sync执行的任务是在新开辟的子线程中执行；

- (void)barrierTask {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);//栅栏函数有效果
//    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);//栅栏函数无效果
//    dispatch_queue_t queue = dispatch_get_main_queue();//栅栏函数有效果、队列依次执行
    dispatch_async(queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2 - %@", [NSThread currentThread]);
    });
    dispatch_barrier_sync(queue, ^{
        sleep(4);
        NSLog(@"栅栏任务 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务4 - %@", [NSThread currentThread]);
    });
}
- (void)barrierTask {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);//栅栏函数有效果
//    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);//栅栏函数无效果
//    dispatch_queue_t queue = dispatch_get_main_queue();//栅栏函数有效果、队列依次执行
    dispatch_async(queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        sleep(2);
        NSLog(@"任务2 - %@", [NSThread currentThread]);
    });
    dispatch_barrier_sync(queue, ^{
        sleep(4);
        NSLog(@"栅栏任务 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"任务4 - %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 11:44:44.907322+0800 GCD[1923:90606] 任务1 - <NSThread: 0x600000f3cdc0>{number = 6, name = (null)}
2021-04-10 11:44:46.911944+0800 GCD[1923:90607] 任务2 - <NSThread: 0x600000f2e2c0>{number = 5, name = (null)}
2021-04-10 11:44:50.913348+0800 GCD[1923:90420] 栅栏任务 - <NSThread: 0x600000f60900>{number = 1, name = main}
2021-04-10 11:44:50.913659+0800 GCD[1923:90609] 任务4 - <NSThread: 0x600000f64fc0>{number = 4, name = (null)}
2021-04-10 11:44:50.913642+0800 GCD[1923:90607] 任务3 - <NSThread: 0x600000f2e2c0>{number = 5, name = (null)}

9. 延时执行任务

- (void)gcdAfter {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"延时执行任务");
    });
}
- (void)gcdAfter {
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"延时执行任务");
    });
}

10. dispatch_once_t

仅执行1次，多线程环境下保证线程安全，注意：如果将onceToken设置为0，下次再调用依旧会执行，常用于销毁单例

- (void)gcdOnce {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"only one");
    });
}

11. dispatch_apply

按照指定的次数将指定的任务追加到指定的队列中，并等待全部队列执行结束,无论是在串行队列，还是并发队列中，dispatch_apply 都会等待全部任务执行完毕

- (void)gcdApply {
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"begain");
    dispatch_apply(6, queue, ^(size_t time) {
        NSLog(@"%zu - %@", time, [NSThread currentThread]);
    });
    NSLog(@"end");
}

运行结果：

2021-04-10 11:56:20.440608+0800 GCD[1959:97881] begain
2021-04-10 11:56:20.440731+0800 GCD[1959:97881] 0 - <NSThread: 0x600002b94a00>{number = 1, name = main}
2021-04-10 11:56:20.440806+0800 GCD[1959:98094] 1 - <NSThread: 0x600002bd5680>{number = 3, name = (null)}
2021-04-10 11:56:20.440815+0800 GCD[1959:97881] 3 - <NSThread: 0x600002b94a00>{number = 1, name = main}
2021-04-10 11:56:20.440816+0800 GCD[1959:98090] 2 - <NSThread: 0x600002b99980>{number = 6, name = (null)}
2021-04-10 11:56:20.440839+0800 GCD[1959:98093] 4 - <NSThread: 0x600002bd5980>{number = 5, name = (null)}
2021-04-10 11:56:20.440837+0800 GCD[1959:98089] 5 - <NSThread: 0x600002bcc880>{number = 4, name = (null)}
2021-04-10 11:56:20.440918+0800 GCD[1959:97881] end
2021-04-10 11:56:20.440608+0800 GCD[1959:97881] begain
2021-04-10 11:56:20.440731+0800 GCD[1959:97881] 0 - <NSThread: 0x600002b94a00>{number = 1, name = main}
2021-04-10 11:56:20.440806+0800 GCD[1959:98094] 1 - <NSThread: 0x600002bd5680>{number = 3, name = (null)}
2021-04-10 11:56:20.440815+0800 GCD[1959:97881] 3 - <NSThread: 0x600002b94a00>{number = 1, name = main}
2021-04-10 11:56:20.440816+0800 GCD[1959:98090] 2 - <NSThread: 0x600002b99980>{number = 6, name = (null)}
2021-04-10 11:56:20.440839+0800 GCD[1959:98093] 4 - <NSThread: 0x600002bd5980>{number = 5, name = (null)}
2021-04-10 11:56:20.440837+0800 GCD[1959:98089] 5 - <NSThread: 0x600002bcc880>{number = 4, name = (null)}
2021-04-10 11:56:20.440918+0800 GCD[1959:97881] end

12. 线程组

1）dispatch_group_async: 先把任务放到队列中，然后将队列放入队列组中

2）dispatch_group_notify: 监听dispatch_group_async中任务的完成状态，当所有的任务都执行完成后，最后执行dispatch_group_notify内部的任务

3）dispatch_group_wait: 阻塞当前任务，会先执行等待之前的任务完成后，再执行等待后的任务

4）dispatch_group_enter会将任务数+1，dispatch_group_leave会将任务数-1，当任务数为0的时候再执行dispatch_group_notify中的任务

场景描述：我们要等前3个任务完成以后再执行其他的任务，并且这3个任务内部又有回到其他线程的操作

- (void)gcdGroup {
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
        dispatch_group_leave(group);
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"任务2 - %@", [NSThread currentThread]);
            dispatch_group_leave(group);
        });
    });
//    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
        dispatch_group_leave(group);
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"任务执行完毕 - %@", [NSThread currentThread]);
    });
}
- (void)gcdGroup {
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        NSLog(@"任务1 - %@", [NSThread currentThread]);
        dispatch_group_leave(group);
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
            NSLog(@"任务2 - %@", [NSThread currentThread]);
            dispatch_group_leave(group);
        });
    });
//    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        NSLog(@"任务3 - %@", [NSThread currentThread]);
        dispatch_group_leave(group);
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"任务执行完毕 - %@", [NSThread currentThread]);
    });
}

运行结果：

2021-04-10 12:00:52.647107+0800 GCD[1989:101516] 任务3 - <NSThread: 0x60000097e9c0>{number = 4, name = (null)}
2021-04-10 12:00:52.647116+0800 GCD[1989:101520] 任务1 - <NSThread: 0x600000940f40>{number = 6, name = (null)}
2021-04-10 12:00:55.647281+0800 GCD[1989:101346] 任务2 - <NSThread: 0x6000009501c0>{number = 1, name = main}
2021-04-10 12:00:55.647530+0800 GCD[1989:101516] 任务执行完毕 - <NSThread: 0x60000097e9c0>{number = 4, name = (null)}
2021-04-10 12:00:52.647107+0800 GCD[1989:101516] 任务3 - <NSThread: 0x60000097e9c0>{number = 4, name = (null)}
2021-04-10 12:00:52.647116+0800 GCD[1989:101520] 任务1 - <NSThread: 0x600000940f40>{number = 6, name = (null)}
2021-04-10 12:00:55.647281+0800 GCD[1989:101346] 任务2 - <NSThread: 0x6000009501c0>{number = 1, name = main}
2021-04-10 12:00:55.647530+0800 GCD[1989:101516] 任务执行完毕 - <NSThread: 0x60000097e9c0>{number = 4, name = (null)}

13. 信号量

dispatch_semaphore_create：创建一个Semaphore并设置信号的总量；

dispatch_semaphore_signal：发送信号，让信号+1；

dispatch_semaphore_wait：可以使总信号量-1，信号总量小于0时就会等待，阻塞所在线程；

dispatch_semaphore作用：

1）保持线程同步，将异步执行任务转换为同步执行任务

2）保证线程安全，为线程加锁

- (void)gcdSemaphoreSync {
    NSInteger ret = [self gcdRet];
    NSLog(@"异步获取函数返回值 - %ld", ret);
}
- (NSInteger)gcdRet {
    __block NSInteger count = 0;
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), queue, ^{//模拟异步网络请求
        count = 999;
        dispatch_semaphore_signal(semaphore);//+1后为0则继续向下执行
    });
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);//-1后小于0开始阻塞等待
    return count;
}
//保证线程安全，为线程加锁
- (void)gcdSemaphoreLock {
    if (!_semaphore) {
        _semaphore = dispatch_semaphore_create(1);
    }
    dispatch_queue_t queue = dispatch_queue_create("com.glt.test", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        dispatch_semaphore_wait(self->_semaphore, DISPATCH_TIME_FOREVER);//-1后等于0开始向下进行，之后再执行到这里此处会小于0，产生阻塞，会等待解锁后继续向下执行
        NSLog(@"线程锁-begain");
        sleep(3);
        NSLog(@"线程锁-end");
        dispatch_semaphore_signal(self->_semaphore);//+1后为0，解锁，则继续执行
    });
}

方法调用：

  [self gcdSemaphoreSync];
    [self gcdSemaphoreLock];
    [self gcdSemaphoreLock];
    [self gcdSemaphoreLock];  [self gcdSemaphoreSync];
    [self gcdSemaphoreLock];
    [self gcdSemaphoreLock];
    [self gcdSemaphoreLock];

运行结果：

2021-04-10 12:04:02.670569+0800 GCD[2009:103890] 异步获取函数返回值 - 999
2021-04-10 12:04:02.670715+0800 GCD[2009:104065] 线程锁-begain
2021-04-10 12:04:05.671743+0800 GCD[2009:104065] 线程锁-end
2021-04-10 12:04:05.672013+0800 GCD[2009:104062] 线程锁-begain
2021-04-10 12:04:08.677301+0800 GCD[2009:104062] 线程锁-end
2021-04-10 12:04:08.677575+0800 GCD[2009:104064] 线程锁-begain
2021-04-10 12:04:11.678193+0800 GCD[2009:104064] 线程锁-end
2021-04-10 12:04:02.670569+0800 GCD[2009:103890] 异步获取函数返回值 - 999
2021-04-10 12:04:02.670715+0800 GCD[2009:104065] 线程锁-begain
2021-04-10 12:04:05.671743+0800 GCD[2009:104065] 线程锁-end
2021-04-10 12:04:05.672013+0800 GCD[2009:104062] 线程锁-begain
2021-04-10 12:04:08.677301+0800 GCD[2009:104062] 线程锁-end
2021-04-10 12:04:08.677575+0800 GCD[2009:104064] 线程锁-begain
2021-04-10 12:04:11.678193+0800 GCD[2009:104064] 线程锁-end

源码：https://github.com/gltwy/public

iOS多线程之GCD-同步、异步、并发、串行、线程组、栅栏函数、信号量等全网最全的总结

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