iOS信号量的使用场景

多请求结束后统一操作

//多请求结束后统一操作
//例：如一个页面多个网络请求后刷新UI
- (void)test1 {
    //模拟并发后统一操作数据
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //请求1
        NSLog(@"Request_1");
        dispatch_semaphore_t sema = dispatch_semaphore_create(0);
        //模拟网络请求
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            
            NSLog(@"请求开始");
            sleep(2);
            NSLog(@"请求完成");
            //请求完成信号量+1，信号量为1，通过
            dispatch_semaphore_signal(sema);
        });
        NSLog(@"我是测试");
        //信号量为0，进行等待
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    });
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //请求2
        NSLog(@"Request_2");
    });
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        //请求3
        NSLog(@"Request_3");
    });
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        //界面刷新
        NSLog(@"任务均完成，刷新界面");
    });
}

多请求顺序执行

//多请求顺序执行
//例：如第二个请求需要第一个请求的数据来操作
- (void)test2 {
    // 1.任务一：获取用户信息
    NSBlockOperation *operation1 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"Request_1");
        dispatch_semaphore_t sema = dispatch_semaphore_create(0);
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            
            NSLog(@"请求1开始");
            sleep(3);
            NSLog(@"请求1完成");
            //请求完成信号量+1，信号量为1，通过
            dispatch_semaphore_signal(sema);
        });
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    }];
    
    // 2.任务二：请求相关数据
    NSBlockOperation *operation2 = [NSBlockOperation blockOperationWithBlock:^{
        NSLog(@"Request_2");
        dispatch_semaphore_t sema = dispatch_semaphore_create(0);
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            
            NSLog(@"请求2开始");
            sleep(2);
            NSLog(@"请求2完成");
            //请求完成信号量+1，信号量为1，通过
            dispatch_semaphore_signal(sema);
        });
        dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    }];
    
    // 3.设置依赖
    [operation2 addDependency:operation1];// 任务二依赖任务一
    
    // 4.创建队列并加入任务
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [queue addOperations:@[operation2, operation1] waitUntilFinished:NO];
}

group的enter和leave可以达到和1同样的效果

//enter leave
- (void)test3 {
    
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"Request_1");
        sleep(3);
        NSLog(@"Request1完成");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"Request_2");
        sleep(1);
        NSLog(@"Request2完成");
        dispatch_group_leave(group);
    });
    
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        NSLog(@"Request_3");
        sleep(2);
        NSLog(@"Request3完成");
        dispatch_group_leave(group);
    });
    
    dispatch_group_notify(group,  dispatch_get_main_queue(), ^{
        
        NSLog(@"全部完成.%@",[NSThread currentThread]);
    });
}