GCD简单了解

	并发队列	串行队列	主队列
同步执行	未开启子线程；串行执行任务	未开启子线程；串行地执行任务	死锁
异步执行	开启了子线程；任务并行地执行	开启了子线程；串行执行任务	未开启子线程；任务串行地执行

2.1 同步 + 并发队列

说明：

该组合会在当前线程执行，不会开启新的线程，因为同步执行没有开启子线程的能力。
虽然队列指定为并发队列，但是执行方式为同步执行，同步执行不具备开启线程的能力，故任务只能在当前线程以串行方式一个接一个地执行。
可以确定该组合的代码执行顺序是从上至下的。

2.2 异步 + 并发队列

说明：

该组合会开启新的线程执行。因为异步具备开启新线程的能力，而并发队列确定了开启新线程的个数。
由于该组合是以异步方式执行，而该组合（编写的方法）内可能会存在异步执行队列之前的代码，这些代码会和异步队列同时/交替执行。

2.3 同步 + 串行队列

说明：

该组合不会开启新的线程去执行，因为执行方式是同步执行，该方式不具备开启新线程的能力；而串行队列决定了线程个数只有一个，故任务会挨个执行。
该组合（方法）内执行的代码顺序是从上至下的，但完成的进度依赖于任务本身的复杂度（耗时性）。

2.4 异步 + 串行队列

说明：

该组合会开启新的线程，但只会开启一个。但由于队列为串行队列，故只会在开启的新线程里挨个执行这些任务。
该组合（方法）内执行的顺序是从上至下的。即，组合内的非新线程代码和新线程代码同时执行。

2.5 同步 + 主队列

说明：

这种组合方式如果以方法形式直接在主线程里面调用会造成死锁。即主线程里面等待方法执行完毕，而方法里面的任务等待主线程处理完方法，二者互相等待，直接死锁。
但如果在组合形式是以子线程形式调用，则不会发生死锁。原因即：新开启的子线程等待方法执行完毕，而方法内部的任务是在主线程内执行，待主线程内的任务执行完毕，新开启的子线程也执行完毕，也不会发生死锁。

2.6 异步 + 主队列

说明：

不会开启新的线程。虽然异步具备开启新线程的能力，但队列为主队列，只能在此执行，故异步内的任务均在主队列里面执行。
主队列为运行在主线程的串行队列，任务挨个执行，也即异步任务在此会挨个执行。

三：常见异步业务 场景与编码

3.1 异步线程通信

异步线程通信的业务场景处处可见，以最常见的网络请求为例，我们通常会开启一个子线程用以请求网络，请求完毕后拿到请求数据回到主线程，供主线程刷新UI。

/**
 线程间通信
 - 在全局并发队列执行任务，执行完毕后会到主线程操作数据
 */
- (void)asyncDoAndAppendToMainQueue{
    dispatch_queue_t global_concurrent_queue = dispatch_queue_create("DIS", DISPATCH_QUEUE_PRIORITY_DEFAULT);
    
    dispatch_queue_t main_queue = dispatch_get_main_queue();
    
    dispatch_async(global_concurrent_queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"------&gt; THREAD: %@", [NSThread currentThread]);
        NSInteger a = 1;
        a++;
        
        dispatch_async(main_queue, ^{
            NSLog(@"------&gt; value: %d", a);
            NSLog(@"------>  THREAD: %@", [NSThread currentThread]);
        });
    });
}

代码说明：首先创建一个全局队列，接着定义一个主队列。在d i sp atch_async函数里面传入全局队列用以发起网络请求。在此模拟网络请求过程，耗时两秒使得a++，然后异步回到主线层并打印数据。结果如下：

2022-03-15 15:47:47.819016+0800 GCDios[17679:8316624] ------> THREAD: <NSThread: 0x2812ac380>{number = 9, name = (null)}
2022-03-15 15:47:47.819246+0800 GCDios[17679:8316613] ------> value: 2
2022-03-15 15:47:47.819345+0800 GCDios[17679:8316613] ------>  THREAD: <NSThread: 0x2812fc900>{number = 1, name = main}

3.2 异步线程组合请求并回调 通知

有时候一个VC或者模块发起的并不单单一个请求，而是多个请求，如何将多个请求结果包装为一个统一回调参数或对象是个问题，使用异步组合请求可以很好地解决(出现这样的业务场景是因为这个VC或模块在初始化的时候需要多个参数或特殊的模型，而初始化仅有一次，不可能在后续中再次请求刷新)。

/**
 异步执行全局并发队列，enter leave后notify回到主线程
 */
- (void)asyncDoGroupWithEnterLeaveAndAppendToMainQueue{
    dispatch_queue_t global_concurrent_queue = dispatch_queue_create("DIS", DISPATCH_QUEUE_PRIORITY_DEFAULT);
    
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_async(group, global_concurrent_queue, ^{
        NSInteger a = 0;
        a++;
        // 未完成任务数+1，标志着notify不会被调用
        dispatch_group_enter(group);
        [NSThread sleepForTimeInterval:2];
        NSLog(@"------> value: %d", a);
        NSLog(@"------> THREAD: %@n", [NSThread currentThread]);
        // 该耗时任务已完成，应该从group队列里面移除
        dispatch_group_leave(group);

        // 再次放入一个任务到group队列里
        dispatch_group_enter(group);
        [NSThread sleepForTimeInterval:2];
        a++;
        NSLog(@"------> value: %d", a);
        NSLog(@"------> THREAD: %@n", [NSThread currentThread]);
        // 任务结束，从group立main移除，则会通知notify调用
        dispatch_group_leave(group);
        
        // 将group队列任务操纵的数据带回到主线程
        dispatch_group_notify(group, dispatch_get_main_queue(), ^{
            NSLog(@"------> value: %d", a);
            NSLog(@"------> THREAD: %@n", [NSThread currentThread]);
        });
        
    });
}

可以看到，刚开始的时候group内任务数为0，然后调用dispatch_group_enter(group)，使得该group内任务数+1，而group内任务数为非0的情况下是不会执行dispatch_group_notify方法的。当第一个模拟网络请求结束时，调用dispatch_groyp_leave(group)使得任务数-1以便执行下一个模拟网络请求。待第二个网络请求结束后，group内任务数为0，此时便执行了dispatch_group_notify方法回到主线程。如果我们这个示例方法内包含一个b loc k 参数，并在notify内回调，那么外层模块就会等到所有请求结束后才能拿到参数或模型。

结果如下：

2022-03-15 15:54:35.889857+0800 GCDios[17681:8317881] ------> value: 1
2022-03-15 15:54:35.890880+0800 GCDios[17681:8317881] ------> THREAD: <NSThread: 0x281fa0680>{number = 8, name = (null)}
2022-03-15 15:54:37.896368+0800 GCDios[17681:8317881] ------> value: 2
2022-03-15 15:54:37.897212+0800 GCDios[17681:8317881] ------> THREAD: <NSThread: 0x281fa0680>{number = 8, name = (null)}
2022-03-15 15:54:37.898013+0800 GCDios[17681:8317865] ------> value: 2
2022-03-15 15:54:37.898510+0800 GCDios[17681:8317865] ------> THREAD: <NSThread: 0x281ff02c0>{number = 1, name = main}

3.3 异步任务通过dispatch_semaphore转同步

有时候业务为执行一个异步操作，但需要该异步操作的结果以供下面的代码调用，这就是异步任务转同步任务。

/**
 异步任务通过semaphore转同步任务
 */
- (void)asyncTransferSyncBySemaphore{
    // 1. 当前为主线程
    NSLog(@"------> currentThread: %@", [NSThread currentThread]);
    
    // 2. 创建全局队列用以执行子线程任务
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    // 3. 创建semaphore为0的信号量
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
    
    // 4. 定义资源
    __block NSInteger a = 20;
    dispatch_async(queue, ^{
        // 4.2 模拟耗时操作
        [NSThread sleepForTimeInterval:2];
        
        NSLog(@"------> Thread: %@", [NSThread currentThread]);
        a++;
        
        // 4.4 此时主线程已经卡住，执行这一步后semaphore+1，总和为0，主线程恢复
        dispatch_semaphore_signal(semaphore);
    });
    
    // 4.1 上述子线程与这里的主线程会同步执行，但是子线程里和耗时操作尚未操纵资源a，此时a为20
    NSLog(@"------> Before wait Thread: %@ AND PARA A:%d", [NSThread currentThread], a);
    // 4.3 信号量-1，主线程堵塞，当过不了多久子线程里耗时操作就会完成，其后操纵资源a，并使semaphore+1，使主线程开放
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    // 4.5 主线程通路后资源a已被子线程通过耗时操作操纵，此时a为21
    NSLog(@"------> After wait Thread: %@ AND PARA A:%d", [NSThread currentThread], a);
}

如果最后一个Log语句打印的参数a为21，即证明了我们成功地将异步任务转换为了同步任务，而下面的结果也恰恰佐证了这一方法。

结果如下：

2022-03-15 16:55:45.889530+0800 GCDios[17704:8329687] ------> currentThread: <NSThread: 0x2818ac900>{number = 1, name = main}
2022-03-15 16:55:45.889676+0800 GCDios[17704:8329687] ------> Before wait Thread: <NSThread: 0x2818ac900>{number = 1, name = main} AND PARA A:20
2022-03-15 16:55:47.894895+0800 GCDios[17704:8329702] ------> Thread: <NSThread: 0x2818e9f80>{number = 6, name = (null)}
2022-03-15 16:55:47.895247+0800 GCDios[17704:8329687] ------> After wait Thread: <NSThread: 0x2818ac900>{number = 1, name = main} AND PARA A:21

3.4 异步线程安全之加锁 解锁

竞争性的资源在多线程修改时永远不安全，可能会出现各式各样奇怪的问题。在此，模拟出一种常见的业务，使用semaphore信号量加锁来保证竞争资源的安全。

我们此时拥有车辆为20辆，拥有两家门店 store_1_queue与store_2_queue，每家门店当顾客预订车辆时会使得carCount自减1，当carCount为0时不再提供预订服务，Log没有车辆。

/// 模拟门店预约车辆服务
- (void)bookCar{
    NSLog(@"------> FIRST: %@ AT THREAD: %@", self.semaphore, [NSThread currentThread]);
    
    while (1) {

        if (self.carCount > 0) {
            self.carCount--;
            NSLog(@"------> Store: %@, Used Count: %d", [NSThread currentThread], self.carCount);
            [NSThread sleepForTimeInterval:0.2];
        } else {
            NSLog(@"------> Store: %@ has no car.", [NSThread currentThread]);

            break;
        }

    }
}

上面使用了self.semaphore与self.carCount，在viewDidLoad方法内不初始化一下即可，如下：

    self.carCount = 20;
    self.semaphore = dispatch_semaphore_create(1);

门店的代码如下：

- (void)threadSafe{
    // 创建门店1、门店2串行队列用以提供购买方式
    dispatch_queue_t store_1_queue = dispatch_queue_create("queue.dayueceng", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t store_2_queue = dispatch_queue_create("queue.changfenggongyuan", DISPATCH_QUEUE_SERIAL);
    
    // 门店1异步执行订车服务
    dispatch_async(store_1_queue, ^{
        [self bookCar];
    });
    // 门店2异步执行订车服务
    dispatch_async(store_2_queue, ^{
        [self bookCar];
    });
}

当在viewDidLoad内调用threadSafe方法看似没问题，每个门店在各自的线程去执行bookCar方法直至self.carCount为0然后Log没有车辆。但是问题严重在于每个门店线程可能在同一时刻访问self.carCount，即会出现store_1_queue预订的时候车辆总数为16，而接下来当store_2_queue预订的时候车辆总数为17，这明显不缝合逻辑的。因此需要使用semaphore信号量加锁，使得在同一时刻只有唯一一个线程去访问修改self.carCount。完整代码如下：

@interface ViewController ()

@property (nonatomic, assign) NSInteger carCount;

@property (nonatomic, strong) dispatch_semaphore_t semaphore;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    [self initData];
    [self threadSafe];
}

- (void)initData{
    self.carCount = 20;
    self.semaphore = dispatch_semaphore_create(1);
}

/**
 线程安全示例
 - 异步访问竞争资源并更新
 */
- (void)threadSafe{
    // 创建门店1、门店2串行队列用以提供购买方式
    dispatch_queue_t store_1_queue = dispatch_queue_create("queue.dayueceng", DISPATCH_QUEUE_SERIAL);
    dispatch_queue_t store_2_queue = dispatch_queue_create("queue.changfenggongyuan", DISPATCH_QUEUE_SERIAL);
    
    // 门店1异步执行订车服务
    dispatch_async(store_1_queue, ^{
        [self bookCar];
    });
    // 门店2异步执行订车服务
    dispatch_async(store_2_queue, ^{
        [self bookCar];
    });
}

/// 模拟门店预约车辆服务
- (void)bookCar{
    
    // 对于每一个门店线程而言，其操纵预定的是同一个竞争资源self.carCount，只要保证在同一时刻只有唯一一个线程去访问修改self.carCount即可保证线程安全
    // 则使用wait与singal保证某个门店线程在预订期间semaphore为唯一访问者即可。
    NSLog(@"------> FIRST: %@ AT THREAD: %@", self.semaphore, [NSThread currentThread]);
    
    while (1) {
        // 加锁，semaphore
        dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
        
        NSLog(@"------> WAIT: %@ AT THREAD: %@", self.semaphore, [NSThread currentThread]);
        if (self.carCount > 0) {
            self.carCount--;
            NSLog(@"------> Store: %@, Used Count: %d", [NSThread currentThread], self.carCount);
            [NSThread sleepForTimeInterval:0.2];
        } else {
            NSLog(@"------> Store: %@ has no car.", [NSThread currentThread]);
            
            dispatch_semaphore_signal(self.semaphore);
            break;
        }
        
        // 解锁
        dispatch_semaphore_signal(self.semaphore);
        NSLog(@"------> SIGNAL: %@ AT THREAD: %@", self.semaphore, [NSThread currentThread]);
    }
}

可以看到虽然每个门店线程都访问了bookCar方法，但是通过wait使得每个门店线程在访问期间从加锁-防止其他门店预订-本门店预订-解锁这个流程一直走下去，直到无车辆可供调度。

结果如下：

2022-03-15 16:20:19.170210+0800 GCDios[17690:8322635] ------> FIRST: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:19.170332+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:19.170381+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 19
2022-03-15 16:20:19.170467+0800 GCDios[17690:8322634] ------> FIRST: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:19.371599+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:19.371779+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:19.371850+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 18
2022-03-15 16:20:19.572499+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:19.572502+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:19.572929+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 17
2022-03-15 16:20:19.778504+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:19.778827+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:19.779001+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 16
2022-03-15 16:20:19.984579+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:19.984587+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:19.985258+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 15
2022-03-15 16:20:20.191370+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:20.191516+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:20.192384+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 14
2022-03-15 16:20:20.393759+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:20.393768+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:20.394415+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 13
2022-03-15 16:20:20.597513+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:20.597750+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:20.598420+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 12
2022-03-15 16:20:20.804358+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:20.804525+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:20.805148+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 11
2022-03-15 16:20:21.011005+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:21.011204+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:21.011824+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 10
2022-03-15 16:20:21.217719+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:21.218465+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:21.218938+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 9
2022-03-15 16:20:21.424744+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:21.424779+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:21.425369+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 8
2022-03-15 16:20:21.631025+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:21.631465+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:21.631723+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 7
2022-03-15 16:20:21.834804+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:21.834888+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:21.835452+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 6
2022-03-15 16:20:22.041551+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:22.042263+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:22.042735+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 5
2022-03-15 16:20:22.248610+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:22.248646+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:22.249263+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 4
2022-03-15 16:20:22.455297+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:22.455332+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:22.455917+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 3
2022-03-15 16:20:22.661461+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:22.661466+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:22.661857+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 2
2022-03-15 16:20:22.867473+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:22.867513+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:22.867893+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)}, Used Count: 1
2022-03-15 16:20:23.073678+0800 GCDios[17690:8322635] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:23.073685+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:23.074345+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)}, Used Count: 0
2022-03-15 16:20:23.278227+0800 GCDios[17690:8322634] ------> SIGNAL: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:23.278236+0800 GCDios[17690:8322635] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d86ac0>{number = 7, name = (null)}
2022-03-15 16:20:23.279220+0800 GCDios[17690:8322635] ------> Store: <NSThread: 0x280d86ac0>{number = 7, name = (null)} has no car.
2022-03-15 16:20:23.280028+0800 GCDios[17690:8322634] ------> WAIT: <OS_dispatch_semaphore: 0x283bdf390> AT THREAD: <NSThread: 0x280d84b00>{number = 8, name = (null)}
2022-03-15 16:20:23.280499+0800 GCDios[17690:8322634] ------> Store: <NSThread: 0x280d84b00>{number = 8, name = (null)} has no car.

3.5 dispatch_barr ier_async

有时候业务场景为先执行一组异步任务，接着再执行另外一组异步任务。这时候便可使用dispoatch_barr ier_async进行栅栏分割操作。

- (void)barrierAysnc{
    dispatch_queue_t concurrent_queue = dispatch_queue_create("com.barrier", DISPATCH_QUEUE_CONCURRENT);
    __block NSInteger a = 0;
    
    dispatch_async(concurrent_queue, ^{
        [NSThread sleepForTimeInterval:2];
        a++;
        NSLog(@"------> 1 CURRENT THREAD: %@", [NSThread currentThread]);
    });
    dispatch_async(concurrent_queue, ^{
        [NSThread sleepForTimeInterval:2];
        a++;
        NSLog(@"------> 2 CURRENT THREAD: %@", [NSThread currentThread]);
    });
    dispatch_async(concurrent_queue, ^{
        [NSThread sleepForTimeInterval:5];
        a++;
        NSLog(@"------> 3 CURRENT THREAD: %@", [NSThread currentThread]);
    });
    
    dispatch_barrier_sync(concurrent_queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"------> value:%ld barrier CURRENT THREAD: %@", (long)a, [NSThread currentThread]);
    });
    
    dispatch_async(concurrent_queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"------> 4 CURRENT THREAD: %@", [NSThread currentThread]);
    });
    dispatch_async(concurrent_queue, ^{
        [NSThread sleepForTimeInterval:2];
        NSLog(@"------> 5 CURRENT THREAD: %@", [NSThread currentThread]);
    });
}

在dispatch_barrier_async执行之前，开启的子线层会对被操作资源（a）进行操作，dispatch_barrier_async执行的时候，会利用资源进行进一步操作；此过程完毕，会执行第二组异步操作。

结果如下：

2022-03-21 15:28:08.445063+0800 GCDios[18226:9521504] ------> 1 CURRENT THREAD: <NSThread: 0x2838c0140>{number = 5, name = (null)}
2022-03-21 15:28:08.445146+0800 GCDios[18226:9521499] ------> 2 CURRENT THREAD: <NSThread: 0x2838c0d40>{number = 6, name = (null)}
2022-03-21 15:28:11.445473+0800 GCDios[18226:9521503] ------> 3 CURRENT THREAD: <NSThread: 0x2838c0b80>{number = 7, name = (null)}
2022-03-21 15:28:13.447573+0800 GCDios[18226:9521489] ------> value:3 barrier CURRENT THREAD: <NSThread: 0x2838808c0>{number = 1, name = main}
2022-03-21 15:28:15.452746+0800 GCDios[18226:9521503] ------> 4 CURRENT THREAD: <NSThread: 0x2838c0b80>{number = 7, name = (null)}
2022-03-21 15:28:15.452800+0800 GCDios[18226:9521499] ------> 5 CURRENT THREAD: <NSThread: 0x2838c0d40>{number = 6, name = (null)}

四：dispatch_semaphore浅谈

4.1 dispatch_semaphore_t

dispatch_semaphore_t用来声明dispatch_semaphore函数，创建则使用dispatch_semaphore_create函数，该函数依赖于结构体dispatch_semaphore_s，后者结构体声明如下：

struct dispatch_semaphore_s {
    DISPATCH_STRUCT_HEADER(semaphore);
    semaphore_t dsema_port;    //等同于mach_port_t信号
    long dsema_orig;           //初始化的信号量值
    long volatile dsema_value; //当前信号量值
    union {
        long volatile dsema_sent_ksignals;
        long volatile dsema_group_waiters;
    };
    struct dispatch_continuation_s *volatile dsema_notify_head; //notify的链表头部
    struct dispatch_continuation_s *volatile dsema_notify_tail; //notify的链表尾部
};

dispatch_semaphore_create函数内部申请地址的时候会用到dispatch_semaphore_s结构体，其dsma 变量会根据dispatch_semaphore计算出合适的空间。dispatcH-semaphore_t的函数内部如下：

dispatch_semaphore_t dispatch_semaphore_create(long value) {
    dispatch_semaphore_t dsema;
    if (value < 0) {
       //value值需大于或等于0
        return NULL;
    }
  //申请dispatch_semaphore_t的内存
    dsema = (dispatch_semaphore_t)_dispatch_alloc(DISPATCH_VTABLE(semaphore),
            sizeof(struct dispatch_semaphore_s) -
            sizeof(dsema->dsema_notify_head) -
            sizeof(dsema->dsema_notify_tail));
    //调用初始化函数
    _dispatch_semaphore_init(value, dsema);
    return dsema;
}
//初始化结构体信息
static void _dispatch_semaphore_init(long value, dispatch_object_t dou) {
    dispatch_semaphore_t dsema = dou._dsema;
    dsema->do_next = (dispatch_semaphore_t)DISPATCH_OBJECT_LISTLESS;
    dsema->do_targetq = dispatch_get_global_queue(
            DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dsema->dsema_value = value; //设置信号量的当前value值
    dsema->dsema_orig = value;  //设置信号量的初始value值
}

从这里我们可以看到，当调用dispatch_semaphore_create()函数时，若初始化值小于1则会返回空，这是我们需要注意的地方。而另一个需要注意的地方为dispatch_semaphore销毁的时候。销毁时调用函数如下：

//释放信号量的函数
void _dispatch_semaphore_dispose(dispatch_object_t dou) {
    dispatch_semaphore_t dsema = dou._dsema;

    if (dsema->dsema_value < dsema->dsema_orig) {
       //Warning:信号量还在使用的时候销毁会造成崩溃
        DISPATCH_CLIENT_CRASH(
                "Semaphore/group object deallocated while in use");
    }
    kern_return_t kr;
    if (dsema->dsema_port) {
        kr = semaphore_destroy(mach_task_self(), dsema->dsema_port);
        DISPATCH_SEMAPHORE_VERIFY_KR(kr);
    }
}

可以看到，若当前信号量值小于初始化值时会崩溃，因此一定要慎重注意到设置dispatch_semaphore的值在信号量尚在使用时不可设置其为nil或重新赋值。

4.2 dispatch_semaphore_wait

long dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout){
    long value = dispatch_atomic_dec2o(dsema, dsema_value, acquire);
    if (fastpath(value >= 0)) {
        return 0;
    }
    return _dispatch_semaphore_wait_slow(dsema, timeout);
}

dispatch_semaphore_wait先将信号量的dsema值原子性减一，并将新值赋给value。如果value大于等于0就立即返回，否则调用_dispatch_semaphore_wait_slow函数，等待信号量唤醒或者timeout 超时。_dispatch_semaphore_wait_slow函数的实现较为复杂，这里不再展开赘述。

4.3 dispatch_semaphore_signal

long dispatch_semaphore_signal(dispatch_semaphore_t dsema) {
    long value = dispatch_atomic_inc2o(dsema, dsema_value, release);
    if (fastpath(value > 0)) {
        return 0;
    }
    if (slowpath(value == LONG_MIN)) {
        DISPATCH_CLIENT_CRASH("Unbalanced call to dispatch_semaphore_signal()");
    }
    return _dispatch_semaphore_signal_slow(dsema);
}

首先将dsema_value调用原子方法加1，如果大于零就立即返回0，否则进入_dispatch_semaphore_signal_slow函数，该函数会调用semaphore_signal函数唤醒在dispatch_semaphore_wait中等待的线程。_dispatch_semaphore_signal_slow同样不再赘述。

五：dispatch_group浅谈

dispatch_group的创建实际上是依赖于dispatch_semaphore，下面就常用函数进行浅谈。

5.1 dispatch_group_create

dispatch_group_t dispatch_group_create(void) { 
	return (dispatch_group_t)dispatch_semaphore_create(LONG_MAX); 
}

可以看到，dispatch_group_create就是创造了以LONG_MAX为信号量的dispatch_semaphore，至于为什么信号量为LONG_MAX，猜测可能是为了防止使用者过多地调用而崩溃特意将信号量调大。

5.2 dispatch_group_enter

void dispatch_group_enter(dispatch_group_t dg) { 	
	dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg; 
	(void)dispatch_semaphore_wait(dsema, DISPATCH_TIME_FOREVER); 
}

可以看到，dispatch_group实际上就是以当前group为参数，发出wait信号量。

5.3 dispatch_group_leave

void dispatch_group_leave(dispatch_group_t dg) { 
	dispatch_semaphore_t dsema = (dispatch_semaphore_t)dg; 
	dispatch_atomic_release_barrier(); 
	long value = dispatch_atomic_inc2o(dsema, dsema_value);
	//dsema_value原子性加1 
	if (slowpath(value == LONG_MIN)) {
		//内存溢出，由于dispatch_group_leave在dispatch_group_enter之前调用 
		DISPATCH_CLIENT_CRASH("Unbalanced call to dispatch_group_leave()"); 
	} 
	if (slowpath(value == dsema->dsema_orig)) {
  	  //表示所有任务已经完成，唤醒group 
  	  (void)_dispatch_group_wake(dsema); 
    } 
}

可以看到 dispatch_group_leave将 dispatch_group_t 转换成 dispatch_semaphore_t 后将 dsema_value 的值原子性加1。如果 value 为 LONG_MIN 则崩溃；如果 value 等于 dsema_orig 表示所有任务已完成，调用 _dispatch_group_wake 唤醒group（即唤醒notify函数）

原文地址:https://blog.csdn.net/kic inio/article/details/123497594

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任务执行队列

一：基础名称

1.1 dispatch _queue _t

1.2 dispatch _group _t

1.3 dispatch _group _async

1.4 di s patch _group_ent e r

1.5 di s patch_group_leave

1.6 di s patch_g ro up_no tify

1.7 di s patch_semaphore_wai t

1.8 di s patch_semaphore_signal

二：排列方式

2.1 同步 + 并发队列

2.2 异步 + 并发队列

2.3 同步 + 串行队列

2.4 异步 + 串行队列

2.5 同步 + 主队列

2.6 异步 + 主队列

三：常见异步业务 场景与编码

3.1 异步线程通信

3.2 异步线程组合请求并回调 通知

3.3 异步任务通过dispatch_semaphore转同步

3.4 异步线程安全之加锁 解锁

3.5 dispatch_barr ier_async

四：dispatch_semaphore浅谈

4.1 dispatch_semaphore_t

4.2 dispatch_semaphore_wait

4.3 dispatch_semaphore_signal

五：dispatch_group浅谈

5.1 dispatch_group_create

5.2 dispatch_group_enter

5.3 dispatch_group_leave

发表回复取消回复

一：基础名称

1.1 dispatch_queue_t

1.2 dispatch_group_t

1.3 dispatch_group_async

1.4 dispatch_group_enter

1.5 dispatch_group_leave

1.6 dispatch_group_notify

1.7 dispatch_semaphore_wait

1.8 dispatch_semaphore_signal

二： 排列方式

2.1 同步 + 并发队列

2.2 异步 + 并发队列

2.3 同步 + 串行队列

2.4 异步 + 串行队列

2.5 同步 + 主队列

2.6 异步 + 主队列

三：常见异步业务场景与编码

3.1 异步线程通信

3.2 异步线程组合请求并回调通知

3.3 异步任务通过dispatch_semaphore转同步

3.4 异步线程安全之加锁解锁

3.5 dispatch_barrier_async

四：dispatch_semaphore浅谈

4.1 dispatch_semaphore_t

4.2 dispatch_semaphore_wait

4.3 dispatch_semaphore_signal

五：dispatch_group浅谈

5.1 dispatch_group_create

5.2 dispatch_group_enter

5.3 dispatch_group_leave

相关文章

发表回复 取消回复

1.1 dispatch _queue _t

1.2 dispatch _group _t

1.3 dispatch _group _async

1.4 di s patch _group_ent e r

1.5 di s patch_group_leave

1.6 di s patch_g ro up_no tify

1.7 di s patch_semaphore_wai t

1.8 di s patch_semaphore_signal

二：排列方式

3.5 dispatch_barr ier_async

发表回复取消回复