iOS 学习GCD（持续更新）

目的：执行完第一个任务再执行第二个任务

方法一：dispatch_barrier_async配合DISPATCH_QUEUE_CONCURRENT线程，先执行barrier前面的任务，再执行后面的任务。

- (void)barrier {
    // 必须用DISPATCH_QUEUE_CONCURRENT，dispatch_barrier_async才有作用
    dispatch_queue_t queue = dispatch_queue_create("concurrent", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        NSLog(@"1-----%@",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"2-----%@",[NSThread currentThread]);
    });
    // 执行完barrier之前的任务才能执行barrier之后的任务
    dispatch_barrier_async(queue, ^{
        NSLog(@"barrier------%@",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"3-----%@",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"4-----%@",[NSThread currentThread]);
    });
}

方法二：dispatch_group_enter和dispatch_group_leave

- (void)enterThenLeave {
    // 创建线程组
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        // 1、2并行
        dispatch_group_enter(group);// 添加任务开始标识
        for (NSInteger i = 0; i < 2; i ++) {
            NSLog(@"2-----%@",[NSThread currentThread]);
            if (i == 1) {
                dispatch_group_leave(group);// 添加任务结束标识
            }
        }
       
        dispatch_group_enter(group);
        for (NSInteger i = 0; i < 2; i ++) {
            NSLog(@"1-----%@",[NSThread currentThread]);
            if (i == 1) {
                dispatch_group_leave(group);
            }
        }
    });
    // 所有任务都执行dispatch_group_leave后才执行这个方法
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"3------");
    });
}