多线程实际应用场景（dispatch_group_t、dispatch_semaphore_t）

核心知识点

• dispatch_group_t 通过dispatch_group_enter、dispatch_group_leave成对使用，并通过dispatch_group_notify监听所有加入到组中的任务完成状态
• dispatch_semaphore_t可以通过dispatch_semaphore_wait来阻塞当前线程、当接收到dispatch_semaphore_signal信号后继续执行下面代码，以此来控制任务执行的顺序

• dispatch_semaphore_wait

  等待信号量，信号量减1。当信号量< 0时会阻塞当前线程，根据传入的等待时间决定接下来的操作——如果永久等待将等到信号（signal）才执行下去

• dispatch_semaphore_signal<br />

  释放信号量，信号量加1。当信号量>= 0 会执行wait之后的代码

1. 异步任务请求，结束后返回主线程刷新UI（最常用、最简单）

- (void)dispatch_async_main_queue{
    NSURL *url = [NSURL URLWithString:@"https://www.baidu.com"];
    NSMutableURLRequest *mRequest = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:3];
    NSURLSessionDataTask *dataTask = [[NSURLSession sharedSession] dataTaskWithRequest:mRequest completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
        NSLog(@"请求结束线程：%@",[NSThread currentThread]);
        // 返回到主线程
        dispatch_async(dispatch_get_main_queue(), ^{
            NSLog(@"回到主线程：%@\n%@",response.URL,[NSThread currentThread]);
        });
    }];
    [dataTask resume];
}

2. 异步 有序/无序 执行组任务，全部执行完毕返回主线程刷新UI

/// 组任务无序异步执行
- (void)group_notify{
    NSArray *urls = @[@"https://www.sina.com",@"https://www.baidu.com",@"https://www.jd.com"];
    dispatch_group_t dispatchGroup = dispatch_group_create();
    for (NSString *urlString in urls) {
        NSURLSessionDataTask *dataTask = [self dataTaskWithURLString:urlString completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
            if (error) {
                NSLog(@"error:%@",error);
            }else{
                NSLog(@"任务：%@-%ld\n请求结束线程：%@",response.URL,httpResponse.statusCode,[NSThread currentThread]);
            }
            dispatch_group_leave(dispatchGroup);
        }];
        [dataTask resume];
        dispatch_group_enter(dispatchGroup);
    }
    // 设置组任务的超时时常(需要时可以设置)
    dispatch_time_t timeout = dispatch_group_wait(dispatchGroup, dispatch_time(DISPATCH_TIME_NOW, 10 *NSEC_PER_SEC));
    if (timeout != 0) {
        // 可以在此处取消正在进行的组任务
        [[NSURLSession sharedSession] getAllTasksWithCompletionHandler:^(NSArray * _Nonnull tasks) {
            [tasks enumerateObjectsUsingBlock:^(__kindof NSURLSessionTask * _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {
                if (obj.state != NSURLSessionTaskStateCompleted) {
                    [obj cancel];
                }
            }];
        }];
        NSLog(@"组任务执行超时_%llu",timeout);
    }else{
    }
    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^{
        NSLog(@"组任务执行完毕，返回主线程进行操作");
    });
}
/// 组任务有序异步执行
- (void)group_notify_semaphore{
    NSArray *urls = @[@"https://www.sina.com",@"https://www.baidu.com",@"https://www.jd.com"];
    dispatch_group_t dispatchGroup = dispatch_group_create();
    // 利用信号量来控制请求顺序
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    for (NSString *urlString in urls) {
        NSURLSessionDataTask *dataTask = [self dataTaskWithURLString:urlString completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
            NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
            if (error) {
                NSLog(@"error:%@",error);
            }else{
                NSLog(@"任务：%@-%ld\n请求结束线程：%@",response.URL,httpResponse.statusCode,[NSThread currentThread]);
            }
            dispatch_group_leave(dispatchGroup);
            // 释放信号量，信号量加1。当信号量>= 0 会执行wait之后的代码
            dispatch_semaphore_signal(sem);
        }];
        [dataTask resume];
        dispatch_group_enter(dispatchGroup);
        // 等待信号量，信号量减1。当信号量< 0时会阻塞当前线程，根据传入的等待时间决定接下来的操作——如果永久等待将等到信号（signal）才执行下去
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    }
    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^{
        NSLog(@"组任务执行完毕，返回主线程进行操作");
    });
    NSLog(@"...");
}
- (NSURLSessionDataTask *)dataTaskWithURLString:(NSString *)urlString completionHandler:(void (^)(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error))completionHandler{
    NSURL *url = [NSURL URLWithString:urlString];
    NSMutableURLRequest *mRequest = [NSMutableURLRequest requestWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:3];
    NSURLSessionDataTask *dataTask = [[NSURLSession sharedSession] dataTaskWithRequest:mRequest completionHandler:completionHandler];
    return dataTask;
}

group_notify

group_notify_semaphore

3. 异步分组执行多段代码

- (void)groups_semaphore{
    NSArray *urls1 = @[@"https://www.mi.com",@"https://www.qq.com",@"https://www.sougou.com"];
    NSArray *urls2 = @[@"https://www.sina.com",@"https://www.baidu.com",@"https://www.jd.com"];
    NSArray *> *groups = @[
        urls1,
        urls2
    ];
    dispatch_group_t sectionGroup = dispatch_group_create();
    // 利用信号量来控制请求顺序
    dispatch_semaphore_t sem = dispatch_semaphore_create(0);
    for (NSArray *urls in groups) {
        dispatch_group_t dispatchGroup = dispatch_group_create();
        for (NSString *urlString in urls) {
            NSURLSessionDataTask *dataTask = [self dataTaskWithURLString:urlString completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
                NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
                if (error) {
                    NSLog(@"error:%@",error);
                }else{
                    NSLog(@"任务：%@-%ld\n请求结束线程：%@",response.URL,httpResponse.statusCode,[NSThread currentThread]);
                }
                dispatch_group_leave(dispatchGroup);
            }];
            [dataTask resume];
            dispatch_group_enter(dispatchGroup);
        }
        dispatch_group_enter(sectionGroup);
        // 注意dispatch_queue_t 不能和dispatch_semaphore_wait在同一线程，不然会造成线程死锁
        dispatch_group_notify(dispatchGroup, dispatch_get_global_queue(0, 0), ^{
            NSLog(@"组任务执行完毕，返回主线程进行操作");
            dispatch_group_leave(sectionGroup);
            dispatch_semaphore_signal(sem);
        });
        NSLog(@"...");
        dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
    }
    dispatch_group_notify(sectionGroup, dispatch_get_main_queue(), ^{
        NSLog(@"多组任务都完成了...");
    });
}