iOS_多线程_GCD_顺序执行

多个任务同时执行，全部执行完毕再执行其他的操作

（1）dispatch_group_async、dispatch_group_notify结合使用

/// 队列组
dispatch_group_t group = dispatch_group_create();
/// 并发队列
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

dispatch_group_async(group, queue, ^{
    NSLog(@"任务一：%@",[NSThread currentThread]);
});

dispatch_group_async(group, queue, ^{
    NSLog(@"任务二：%@",[NSThread currentThread]);
});

dispatch_group_async(group, queue, ^{
    NSLog(@"任务三：%@",[NSThread currentThread]);
});

dispatch_group_notify(group, queue, ^{
    NSLog(@"所有任务执行完毕：%@",[NSThread currentThread]);
});

#打印结果：
任务一、任务二、任务三（顺序不一定，因为都是异步同时执行的）都执行完成，才执行：所有任务执行完毕

(2) 使用栅栏函数

/// 自定义并发队列
dispatch_queue_t queue = dispatch_queue_create("my_queue", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{
    NSLog(@"任务一：%@",[NSThread currentThread]);
});

dispatch_async(queue, ^{
    NSLog(@"任务二：%@",[NSThread currentThread]);
});

dispatch_async(queue, ^{
    NSLog(@"任务三：%@",[NSThread currentThread]);
});

dispatch_barrier_async(queue, ^{
    NSLog(@"所有任务执行完毕：%@",[NSThread currentThread]);
});

# 打印结果：
2020-10-13 16:24:17.926220+0800 ThreadingDemo[26902:6624494] 任务二：{number = 4, name = (null)}
2020-10-13 16:24:17.926227+0800 ThreadingDemo[26902:6624499] 任务一：{number = 6, name = (null)}
2020-10-13 16:24:17.926220+0800 ThreadingDemo[26902:6624495] 任务三：{number = 5, name = (null)}
2020-10-13 16:24:17.926396+0800 ThreadingDemo[26902:6624495] 所有任务执行完毕：{number = 5, name = (null)}

以上的两种写法对于任务是同步的，是没有问题的。
但是如果对于异步执行的任务，则会达不到顺序执行的，比如网络请求.
因为异步，请求还没回来，dispatch_group_notify或dispatch_barrier_async已经执行了

如果任务是异步的，可以使用信号量或者dispatch_group_enter、dispatch_group_leave事项

# 比如下方任务是异步的网络请求，那么栅栏函数也是控制不住执行顺序的

- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_queue_t queue = dispatch_queue_create("my_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        [self requestNetworkWithCallback:^{
            NSLog(@"任务1");
        }];
    });
    
    dispatch_async(queue, ^{
        [self requestNetworkWithCallback:^{
            NSLog(@"任务2");
        }];
    });
    
    dispatch_async(queue, ^{
        [self requestNetworkWithCallback:^{
            NSLog(@"任务3");
        }];
    });
    
    dispatch_barrier_async(queue, ^{
        NSLog(@"任务全部执行完成");
    });
    
}

/// 异步任务：模拟网络请求
- (void)requestNetworkWithCallback:(void(^)(void))callback {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:2];
        if (callback) {
            callback();
        }
    });
}

# 打印结果：（“任务全部执行完成”肯定都是先执行的）
2020-10-14 09:36:32.053314+0800 ThreadingDemo[45423:6822309] 任务全部执行完成
2020-10-14 09:36:34.053708+0800 ThreadingDemo[45423:6822308] 任务1
2020-10-14 09:36:34.053729+0800 ThreadingDemo[45423:6822310] 任务2
2020-10-14 09:36:34.053733+0800 ThreadingDemo[45423:6822307] 任务3

（3）信号量

信号量的作用：线程同步、线程加锁
dispatch_semaphore_t dispatch_semaphore_create(long value)：方法接收一个long类型的参数，返回一个dispatch_semaphore_t类型的信号量，值为传入的参数
long dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout)：接收一个信号和时间值，若信号的量为0，则会阻塞当前线程，知道信号量大于0或者经过输入的时间值；若信号量大于0，则会使信号量减1并返回，程序继续往下执行
long dispatch_semaphore_signal(dispatch_semaphore_t dsema)：使信号量加1并返回

# 对于多个任务同时进行，执行完成后再执行其他任务，用单个信号量完成不了
# 只能是一个个任务执行完成再执行另外一个任务
# 使用场景：多张图片，一张张上传
- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_queue_t queue = dispatch_queue_create("my_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        /// 创建信号量，并设置为0
        dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
        
        [self requestNetworkWithTime:1 callback:^{
            NSLog(@"任务1");
            /// 信号量 + 1
            dispatch_semaphore_signal(semaphore);
        }];
        
        /// 判断如果信号量为0，则阻塞等待
        /// 信号量大于1，则会使信号量减 1，程序继续往下执行
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"任务2");
        
    });
    
}

/// 异步任务：模拟网络请求
- (void)requestNetworkWithTime:(NSInteger )time callback:(void(^)(void))callback {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:time];
        if (callback) {
            callback();
        }
    });
}

# 打印结果：
2020-10-14 13:51:20.956845+0800 ThreadingDemo[59313:7012870] 任务1
2020-10-14 13:51:20.957029+0800 ThreadingDemo[59313:7012871] 任务2

# 创建多个信号，异步同时执行任务，都执行完成再执行其他操作

- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_group_async(group, queue, ^{
        /// 创建信号1
        dispatch_semaphore_t semap = dispatch_semaphore_create(0);
        [self requestNetworkWithTime:2 callback:^{
            NSLog(@"任务1");
            dispatch_semaphore_signal(semap);
        }];
        dispatch_semaphore_wait(semap, DISPATCH_TIME_FOREVER);
    });
    
    dispatch_group_async(group, queue, ^{
        /// 创建信号2
        dispatch_semaphore_t semap = dispatch_semaphore_create(0);
        [self requestNetworkWithTime:1 callback:^{
            NSLog(@"任务2");
            dispatch_semaphore_signal(semap);
        }];
        dispatch_semaphore_wait(semap, DISPATCH_TIME_FOREVER);
    });
    
    dispatch_group_notify(group, queue, ^{
        NSLog(@"全部执行完成");
    });
}

/// 异步任务：模拟网络请求
- (void)requestNetworkWithTime:(NSInteger )time callback:(void(^)(void))callback {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:time];
        if (callback) {
            callback();
        }
    });
}

# 打印结果：
2020-10-14 15:42:34.781421+0800 ThreadingDemo[76163:7169489] 任务2
2020-10-14 15:42:35.781343+0800 ThreadingDemo[76163:7169490] 任务1
2020-10-14 15:42:35.781643+0800 ThreadingDemo[76163:7169495] 全部执行完成

（4）dispatch_group_enter/leave

dispatch_group_enter 和 dispatch_group_leave 必须成对出现，否则会造成“死锁”
dispatch_group_wait：如果enter数不等于leave数，则线程阻塞等待，若enter数与leave数相等则继续往下执行
dispatch_group_notify：只有enter数和leave数相等，才会执行

- (void)viewDidLoad {
    [super viewDidLoad];
    
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_enter(group);
    [self requestNetworkWithTime:1 callback:^{
        NSLog(@"任务1");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_enter(group);
    [self requestNetworkWithTime:2 callback:^{
        NSLog(@"任务2");
        dispatch_group_leave(group);
    }];
    
    /// 等待任务1、任务2执行完毕
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    
    dispatch_group_enter(group);
    [self requestNetworkWithTime:2 callback:^{
        NSLog(@"任务3");
        dispatch_group_leave(group);
    }];
    
    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
        NSLog(@"全部执行完成");
    });
}

/// 异步任务：模拟网络请求
- (void)requestNetworkWithTime:(NSInteger )time callback:(void(^)(void))callback {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:time];
        if (callback) {
            callback();
        }
    });
}

# 打印结果：
2020-10-14 15:18:03.405899+0800 ThreadingDemo[74573:7126913] 任务1
2020-10-14 15:18:04.407338+0800 ThreadingDemo[74573:7126917] 任务2
2020-10-14 15:18:06.410904+0800 ThreadingDemo[74573:7126917] 任务3
2020-10-14 15:18:06.411069+0800 ThreadingDemo[74573:7126917] 全部执行完成

补充：栅栏函数dispatch_barrier使用教程

dispatch_barrier_async：无需等待栅栏函数执行完成，会继续往下走（任务执行顺序：栅栏前的任务===》栅栏函数 ====》 栅栏函数后的任务）
dispatch_barrier_sync：需要等待栅栏函数执行完才会执行栅栏后面的任务
注意：
（1）栅栏必须在并发队列执行 ，并且不能使用全局队列，否则栅栏函数就不起作用;
dispatch_barrier_async 等同于 dispatch_async , dispatch_barrier_sync 等同于 dispatch_sync
（2）理解的时候，必须要区分哪些是任务，哪些是普通代码

# dispatch_barrier_async

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSLog(@"start");
    dispatch_queue_t queue = dispatch_queue_create("my_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"任务1");
    });
   
    dispatch_async(queue, ^{
        NSLog(@"任务2");
    });
    
    ///  因为是栅栏异步，所以栅栏函数和栅栏函数后面的任务都
    dispatch_barrier_async(queue, ^{
        NSLog(@"===== barrier ====");
    });
    NSLog(@"mid");
    dispatch_async(queue, ^{
        NSLog(@"任务3");
    });
    dispatch_async(queue, ^{
        NSLog(@"任务4");  
    });
    NSLog(@"last");
}

打印结果：
2020-10-13 16:02:03.843934+0800 ThreadingDemo[26494:6576066] start
2020-10-13 16:02:03.844143+0800 ThreadingDemo[26494:6576066] mid
2020-10-13 16:02:03.844189+0800 ThreadingDemo[26494:6576341] 任务1
2020-10-13 16:02:03.844182+0800 ThreadingDemo[26494:6576343] 任务2
2020-10-13 16:02:03.844231+0800 ThreadingDemo[26494:6576066] last
2020-10-13 16:02:03.844300+0800 ThreadingDemo[26494:6576343] ===== barrier ====
2020-10-13 16:02:03.844375+0800 ThreadingDemo[26494:6576343] 任务3
2020-10-13 16:02:03.844381+0800 ThreadingDemo[26494:6576341] 任务4

# dispatch_barrier_sync

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSLog(@"start");
    dispatch_queue_t queue = dispatch_queue_create("my_queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"任务1");
    });
    
    dispatch_async(queue, ^{
        NSLog(@"任务2");
    });
    
    /// 因为是同步栅栏，执行顺序：栅栏前 ===》栅栏函数 ====》 栅栏后
    dispatch_barrier_sync(queue, ^{
        NSLog(@"===== barrier ====");
    });
    NSLog(@"mid");
    dispatch_async(queue, ^{
        NSLog(@"任务3");
    });
    dispatch_async(queue, ^{
        NSLog(@"任务4");
    });
    NSLog(@"last");
}

打印结果：
2020-10-13 16:02:42.629822+0800 ThreadingDemo[26527:6577198] start
2020-10-13 16:02:42.630035+0800 ThreadingDemo[26527:6577356] 任务1
2020-10-13 16:02:42.630037+0800 ThreadingDemo[26527:6577360] 任务2
2020-10-13 16:02:42.630155+0800 ThreadingDemo[26527:6577198] ===== barrier ====
2020-10-13 16:02:42.630224+0800 ThreadingDemo[26527:6577198] mid
2020-10-13 16:02:42.630310+0800 ThreadingDemo[26527:6577198] last
2020-10-13 16:02:42.630327+0800 ThreadingDemo[26527:6577360] 任务3
2020-10-13 16:02:42.630374+0800 ThreadingDemo[26527:6577356] 任务4