iOS GCD (一) 任务+队列 基础组合
iOS GCD (二 ) dispatch_group 队列组
iOS GCD(三) dispatch_barrier_async 栅栏方法
iOS GCD (四) dispatch_semaphore 信号量
iOS GCD(五) 死锁案例分析
iOS GCD(六)线程加锁

有时候我们会有这样的需求：分别异步执行2个耗时任务，然后当2个耗时任务都执行完毕后再回到主线程执行任务。这时候我们可以用到 GCD 的队列组。

1.调用队列组的 dispatch_group_async 先把任务放到队列中，然后将队列放入队列组中。或者使用队列组的 dispatch_group_enter、dispatch_group_leave 组合 来实现
2.dispatch_group_async。
调用队列组的 dispatch_group_notify 回到指定线程执行任务。或者使用 dispatch_group_wait 回到当前线程继续向下执行（会阻塞当前线程）。

3.dispatch_group_sync这个幻想函数是不存在的～

dispatch_group_notify

/**
 * 队列组 dispatch_group_notify
 */
- (void)groupNotify {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin");
    
    dispatch_group_t group =  dispatch_group_create();
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务1
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 等前面的异步任务1、任务2都执行完毕后，回到主线程执行下边任务
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"3---%@",[NSThread currentThread]);      // 打印当前线程
        }
        NSLog(@"group---end");
    });
}

输出结果：
2018-02-23 22:05:03.790035+0800 YSC-GCD-demo[20494:5183349] currentThread---<NSThread: 0x604000072040>{number = 1, name = main}
2018-02-23 22:05:03.790237+0800 YSC-GCD-demo[20494:5183349] group---begin
2018-02-23 22:05:05.792721+0800 YSC-GCD-demo[20494:5183654] 1---<NSThread: 0x60000026f280>{number = 4, name = (null)}
2018-02-23 22:05:05.792725+0800 YSC-GCD-demo[20494:5183656] 2---<NSThread: 0x60000026f240>{number = 3, name = (null)}
2018-02-23 22:05:07.797408+0800 YSC-GCD-demo[20494:5183656] 2---<NSThread: 0x60000026f240>{number = 3, name = (null)}
2018-02-23 22:05:07.797408+0800 YSC-GCD-demo[20494:5183654] 1---<NSThread: 0x60000026f280>{number = 4, name = (null)}
2018-02-23 22:05:09.798717+0800 YSC-GCD-demo[20494:5183349] 3---<NSThread: 0x604000072040>{number = 1, name = main}
2018-02-23 22:05:11.799827+0800 YSC-GCD-demo[20494:5183349] 3---<NSThread: 0x604000072040>{number = 1, name = main}
2018-02-23 22:05:11.799977+0800 YSC-GCD-demo[20494:5183349] group---end

从dispatch_group_notify相关代码运行输出结果可以看出： 当所有任务都执行完成之后，才执行dispatch_group_notify block 中的任务。

dispatch_group_wait

暂停当前线程（阻塞当前线程），等待指定的 group 中的任务执行完成后，才会往下继续执行。

/**
 * 队列组 dispatch_group_wait
 */
- (void)groupWait {
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin");
    
    dispatch_group_t group =  dispatch_group_create();
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务1
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 追加任务2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
    });
    
    // 等待上面的任务全部完成后，会往下继续执行（会阻塞当前线程）
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    
    NSLog(@"group---end");
}

输出结果：
2018-02-23 22:10:16.939258+0800 YSC-GCD-demo[20538:5198871] currentThread---<NSThread: 0x600000066780>{number = 1, name = main}
2018-02-23 22:10:16.939455+0800 YSC-GCD-demo[20538:5198871] group---begin
2018-02-23 22:10:18.943862+0800 YSC-GCD-demo[20538:5199137] 2---<NSThread: 0x600000464b80>{number = 4, name = (null)}
2018-02-23 22:10:18.943861+0800 YSC-GCD-demo[20538:5199138] 1---<NSThread: 0x604000076640>{number = 3, name = (null)}
2018-02-23 22:10:20.947787+0800 YSC-GCD-demo[20538:5199137] 2---<NSThread: 0x600000464b80>{number = 4, name = (null)}
2018-02-23 22:10:20.947790+0800 YSC-GCD-demo[20538:5199138] 1---<NSThread: 0x604000076640>{number = 3, name = (null)}
2018-02-23 22:10:20.948134+0800 YSC-GCD-demo[20538:5198871] group---end

从dispatch_group_wait相关代码运行输出结果可以看出： 当所有任务执行完成之后，才执行 dispatch_group_wait 之后的操作。但是，使用dispatch_group_wait 会阻塞当前线程。

dispatch_group_enter、dispatch_group_leave

/**
 * 队列组 dispatch_group_enter、dispatch_group_leave
 */
- (void)groupEnterAndLeave
{
    NSLog(@"currentThread---%@",[NSThread currentThread]);  // 打印当前线程
    NSLog(@"group---begin");
    
    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        // 追加任务1
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"1---%@",[NSThread currentThread]);      // 打印当前线程
        }
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(queue, ^{
        // 追加任务2
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"2---%@",[NSThread currentThread]);      // 打印当前线程
        }
        dispatch_group_leave(group);
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        // 等前面的异步操作都执行完毕后，回到主线程.
        for (int i = 0; i < 2; ++i) {
            [NSThread sleepForTimeInterval:2];              // 模拟耗时操作
            NSLog(@"3---%@",[NSThread currentThread]);      // 打印当前线程
        }
        NSLog(@"group---end");
    });
    
//    // 等待上面的任务全部完成后，会往下继续执行（会阻塞当前线程）
//    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
//
//    NSLog(@"group---end");
}

输出结果：
2018-02-23 22:14:17.997667+0800 YSC-GCD-demo[20592:5214830] currentThread---<NSThread: 0x604000066600>{number = 1, name = main}
2018-02-23 22:14:17.997839+0800 YSC-GCD-demo[20592:5214830] group---begin
2018-02-23 22:14:20.000298+0800 YSC-GCD-demo[20592:5215094] 1---<NSThread: 0x600000277c80>{number = 4, name = (null)}
2018-02-23 22:14:20.000305+0800 YSC-GCD-demo[20592:5215095] 2---<NSThread: 0x600000277c40>{number = 3, name = (null)}
2018-02-23 22:14:22.001323+0800 YSC-GCD-demo[20592:5215094] 1---<NSThread: 0x600000277c80>{number = 4, name = (null)}
2018-02-23 22:14:22.001339+0800 YSC-GCD-demo[20592:5215095] 2---<NSThread: 0x600000277c40>{number = 3, name = (null)}
2018-02-23 22:14:24.002321+0800 YSC-GCD-demo[20592:5214830] 3---<NSThread: 0x604000066600>{number = 1, name = main}
2018-02-23 22:14:26.002852+0800 YSC-GCD-demo[20592:5214830] 3---<NSThread: 0x604000066600>{number = 1, name = main}
2018-02-23 22:14:26.003116+0800 YSC-GCD-demo[20592:5214830] group---end

从dispatch_group_enter、dispatch_group_leave相关代码运行结果中可以看出：当所有任务执行完成之后，才执行 dispatch_group_notify 中的任务。这里的dispatch_group_enter、dispatch_group_leave组合，其实等同于dispatch_group_async。