前言
在开发中，我们很容易遇到这样的需求，需要我们同时做多个网络请求，所有网络请求都完成后才能进行下一步的操作。
网络请求的任务是提交给子线程异步处理了，网络请求这样的任务也就快速执行完毕了，但是网络请求是一个任务，处理收到的网络响应又是一个任务，注意不要把这两个过程混为一谈。
如下载多个图片，下载完了才能展示，今天我们就来研究一下这个问题的解决方案。
解决方法
1.首先，我们创建一个项目，然后做一般性的做法，不做任何处理去连续请求一个接口10次：
先在viewDidLoad中创建第一种情况.
//1.无处理 UIButton *Btn1 = [UIButton buttonWithType:UIButtonTypeCustom]; Btn1.frame = CGRectMake(100, 100, 100, 40); Btn1.backgroundColor = [UIColor grayColor]; [Btn1 setTitle:@"noConduct" forState:UIControlStateNormal]; [Btn1 addTarget:self action:@selector(Btn1) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:Btn1];
实现第一种情况的方法
//1.noConduct-(void)Btn1{ NSString *str = @"http://www.jianshu.com/p/6930f335adba"; NSURL *url = [NSURL URLWithString:str]; NSURLRequest *request = [NSURLRequest requestWithURL:url]; NSURLSession *session = [NSURLSession sharedSession]; for (int i=0; i<10; i++) { NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) { NSLog(@"%d---%d",i,i); }]; [task resume]; } NSLog(@"end");}运行，看看我们的控制台输出:
2017-12-04 17:10:10.503 DownImage[3289:261033] end2017-12-04 17:10:10.676 DownImage[3289:261080] 0---02017-12-04 17:10:10.704 DownImage[3289:261080] 1---12017-12-04 17:10:10.754 DownImage[3289:261096] 4---42017-12-04 17:10:10.760 DownImage[3289:261080] 2---22017-12-04 17:10:10.800 DownImage[3289:261096] 5---52017-12-04 17:10:10.840 DownImage[3289:261080] 7---72017-12-04 17:10:10.844 DownImage[3289:261082] 6---62017-12-04 17:10:10.846 DownImage[3289:261096] 3---32017-12-04 17:10:10.888 DownImage[3289:261096] 8---82017-12-04 17:10:10.945 DownImage[3289:261080] 9---9
很明显，无任何处理情况下，end最先被打印出来，由于网络请求的异步回调，然后各个网络请求的回调顺序是无序的。
2.使用GCD的dispatch_group_t
viewDidLoad里:
//2.group UIButton *Btn2 = [UIButton buttonWithType:UIButtonTypeCustom]; Btn2.frame = CGRectMake(100, 200, 100, 40); Btn2.backgroundColor = [UIColor grayColor]; [Btn2 setTitle:@"group--" forState:UIControlStateNormal]; [Btn2 addTarget:self action:@selector(Btn2) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:Btn2];
实现:
//2.group---(void)Btn2{ NSString *str = @"http://www.jianshu.com/p/6930f335adba"; NSURL *url = [NSURL URLWithString:str]; NSURLRequest *request = [NSURLRequest requestWithURL:url]; NSURLSession *session = [NSURLSession sharedSession]; dispatch_group_t downloadGroup = dispatch_group_create(); for (int i=0; i<10; i++) { dispatch_group_enter(downloadGroup); NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) { NSLog(@"%d---%d",i,i); dispatch_group_leave(downloadGroup); }]; [task resume]; } dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ NSLog(@"end"); });}运行看看控制台输出:
2017-12-04 17:14:46.984 DownImage[3289:265374] 2---22017-12-04 17:14:46.987 DownImage[3289:265370] 1---12017-12-04 17:14:47.052 DownImage[3289:265383] 5---52017-12-04 17:14:47.065 DownImage[3289:265370] 4---42017-12-04 17:14:47.111 DownImage[3289:265379] 3---32017-12-04 17:14:47.121 DownImage[3289:265383] 6---62017-12-04 17:14:47.169 DownImage[3289:265383] 7---72017-12-04 17:14:47.192 DownImage[3289:265370] 9---92017-12-04 17:14:47.321 DownImage[3289:265383] 8---82017-12-04 17:14:47.747 DownImage[3289:265374] 0---02017-12-04 17:14:47.747 DownImage[3289:261033] end
2017-12-04 17:15:14.576 DownImage[3289:265942] 3---32017-12-04 17:15:14.626 DownImage[3289:265936] 2---22017-12-04 17:15:14.647 DownImage[3289:265944] 4---42017-12-04 17:15:14.648 DownImage[3289:265936] 0---02017-12-04 17:15:14.657 DownImage[3289:265943] 1---12017-12-04 17:15:14.709 DownImage[3289:265944] 5---52017-12-04 17:15:14.728 DownImage[3289:265944] 6---62017-12-04 17:15:14.734 DownImage[3289:265944] 7---72017-12-04 17:15:14.738 DownImage[3289:265943] 8---82017-12-04 17:15:14.816 DownImage[3289:265944] 9---92017-12-04 17:15:14.816 DownImage[3289:261033] end
从上两次输出可以看出，end确实是在所有网络请求之后才输出出来，这也是符合了我们的需求。
代码中我们只添加了4行代码
dispatch_group_t downloadGroup = dispatch_group_create();dispatch_group_enter(downloadGroup);dispatch_group_leave(downloadGroup);dispatch_group_notify(downloadGroup, dispatch_get_main_queue(), ^{ });对以上4行代码可理解为：创建一个dispatch_group_t， 每次网络请求前先dispatch_group_enter,请求回调后再dispatch_group_leave,对于enter和leave必须配合使用，有几次enter就要有几次leave，否则group会一直存在。当所有enter的block都leave后，会执行dispatch_group_notify的block。
3.使用GCD的信号量dispatch_semaphore_t
//3.semaphore UIButton *Btn3 = [UIButton buttonWithType:UIButtonTypeCustom]; Btn3.frame = CGRectMake(100, 300, 100, 40); Btn3.backgroundColor = [UIColor grayColor]; [Btn3 setTitle:@"semaphore" forState:UIControlStateNormal]; [Btn3 addTarget:self action:@selector(Btn3) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:Btn3];
//3.semaphore---(void)Btn3{ NSString *str = @"http://www.jianshu.com/p/6930f335adba"; NSURL *url = [NSURL URLWithString:str]; NSURLRequest *request = [NSURLRequest requestWithURL:url]; NSURLSession *session = [NSURLSession sharedSession]; dispatch_semaphore_t sem = dispatch_semaphore_create(0); for (int i=0; i<10; i++) { NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) { NSLog(@"%d---%d",i,i); count++; if (count==10) { dispatch_semaphore_signal(sem); count = 0; } }]; [task resume]; } dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER); dispatch_async(dispatch_get_main_queue(), ^{ NSLog(@"end"); });}运行看看控制台输出:
2017-12-04 17:36:49.098 DownImage[3428:283651] 2---22017-12-04 17:36:49.144 DownImage[3428:284210] 0---02017-12-04 17:36:49.152 DownImage[3428:284213] 3---32017-12-04 17:36:49.158 DownImage[3428:283651] 1---12017-12-04 17:36:49.167 DownImage[3428:284210] 4---42017-12-04 17:36:49.235 DownImage[3428:284213] 8---82017-12-04 17:36:49.249 DownImage[3428:283651] 5---52017-12-04 17:36:49.252 DownImage[3428:283651] 7---72017-12-04 17:36:49.324 DownImage[3428:283651] 9---92017-12-04 17:36:49.468 DownImage[3428:284214] 6---62017-12-04 17:36:49.469 DownImage[3428:283554] end
2017-12-04 17:37:11.554 DownImage[3428:284747] 0---02017-12-04 17:37:11.555 DownImage[3428:284733] 1---12017-12-04 17:37:11.627 DownImage[3428:284748] 5---52017-12-04 17:37:11.661 DownImage[3428:284748] 2---22017-12-04 17:37:11.688 DownImage[3428:284747] 4---42017-12-04 17:37:11.709 DownImage[3428:284747] 6---62017-12-04 17:37:11.770 DownImage[3428:284733] 7---72017-12-04 17:37:11.774 DownImage[3428:284733] 8---82017-12-04 17:37:11.824 DownImage[3428:284747] 9---92017-12-04 17:37:11.899 DownImage[3428:284733] 3---32017-12-04 17:37:11.900 DownImage[3428:283554] end
从输出可以看出，这样的方法也是满足我们的需求的，在这个方法中，我们使用了
dispatch_semaphore_t sem = dispatch_semaphore_create(0);dispatch_semaphore_signal(sem);dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);
对这三句代码可以这样理解：dispatch_semaphore信号量为基于计数器的一种多线程同步机制。如果semaphore计数大于等于1，计数-1，返回，程序继续运行。如果计数为0，则等待。dispatch_semaphore_signal(semaphore)为计数+1操作,dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER)为设置等待时间，这里设置的等待时间是一直等待。
对于以上代码通俗一点就是，开始为0，等待，等10个网络请求都完成了，dispatch_semaphore_signal(semaphore)为计数+1，然后计数-1返回，程序继续执行。 (这里也就是为什么有个count变量的原因，记录网络回调的次数，回调10次之后再发信号量，使后面程序继续运行)。
4.考虑新需求，10个网络请求顺序回调。
需求需要顺序回调，即执行完第一个网络请求后，第二个网络请求回调才可被执行，简单来讲就是输出得是0,1,2,3...9这种方式的。
对于这个需求我也是根据自己最近做的项目来提的，因为网络请求回调的异步性，我们虽可以控制网络请求的顺序执行，却不能控制它的完成回调顺序。这就有点伤了，目前我项目是找到了解决方案，但这个问题还没有找到解决办法，提出来跟大家讨论一下。(请忽略网络请求执行，回调，在回调里请求下一个接口的办法，讨论还有没有别的方法,最好show the code).
最后，贴点NSOperation的代码，为了解决新需求所写，由于网络请求回调异步性不能满足需求，但若不是网络请求等异步回调的方式，这样的做法是可以的，大家可以试试.
//4.NSOperation UIButton *Btn4 = [UIButton buttonWithType:UIButtonTypeCustom]; Btn4.frame = CGRectMake(100, 400, 100, 40); Btn4.backgroundColor = [UIColor grayColor]; [Btn4 setTitle:@"NSOperation" forState:UIControlStateNormal]; [Btn4 addTarget:self action:@selector(Btn4) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:Btn4];
//4.NSOperation-(void)Btn4{ NSString *str = @"http://www.jianshu.com/p/6930f335adba"; NSURL *url = [NSURL URLWithString:str]; NSURLRequest *request = [NSURLRequest requestWithURL:url]; NSURLSession *session = [NSURLSession sharedSession]; NSMutableArray *operationArr = [[NSMutableArray alloc]init]; for (int i=0; i<10; i++) { NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{ NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) { NSLog(@"%d---%d",i,i); }]; [task resume]; //非网络请求 NSLog(@"noRequest-%d",i); }]; [operationArr addObject:operation]; if (i>0) { NSBlockOperation *operation1 = operationArr[i-1]; NSBlockOperation *operation2 = operationArr[i]; [operation2 addDependency:operation1]; } } NSOperationQueue *queue = [[NSOperationQueue alloc]init]; [queue addOperations:operationArr waitUntilFinished:NO]; //YES会阻塞当前线程#warning - 绝对不要在应用主线程中等待一个Operation,只能在第二或次要线程中等待。阻塞主线程将导致应用无法响应用户事件,应用也将表现为无响应。 }运行结果：
2017-12-04 18:03:10.224 DownImage[3584:304363] noRequest-02017-12-04 18:03:10.226 DownImage[3584:304362] noRequest-12017-12-04 18:03:10.226 DownImage[3584:304363] noRequest-22017-12-04 18:03:10.231 DownImage[3584:304363] noRequest-32017-12-04 18:03:10.232 DownImage[3584:304362] noRequest-42017-12-04 18:03:10.233 DownImage[3584:304362] noRequest-52017-12-04 18:03:10.233 DownImage[3584:304363] noRequest-62017-12-04 18:03:10.234 DownImage[3584:304363] noRequest-72017-12-04 18:03:10.235 DownImage[3584:304363] noRequest-82017-12-04 18:03:10.236 DownImage[3584:304363] noRequest-92017-12-04 18:03:10.408 DownImage[3584:304597] 2---22017-12-04 18:03:10.408 DownImage[3584:304597] 0---02017-12-04 18:03:10.409 DownImage[3584:304597] 1---12017-12-04 18:03:10.461 DownImage[3584:304597] 5---52017-12-04 18:03:10.476 DownImage[3584:304363] 4---42017-12-04 18:03:10.477 DownImage[3584:304365] 6---62017-12-04 18:03:10.518 DownImage[3584:304365] 7---72017-12-04 18:03:10.537 DownImage[3584:304596] 8---82017-12-04 18:03:10.547 DownImage[3584:304362] 9---92017-12-04 18:03:11.837 DownImage[3584:304362] 3---3
2017-12-04 18:04:27.699 DownImage[3584:306401] noRequest-02017-12-04 18:04:27.700 DownImage[3584:306405] noRequest-12017-12-04 18:04:27.701 DownImage[3584:306401] noRequest-22017-12-04 18:04:27.701 DownImage[3584:306405] noRequest-32017-12-04 18:04:27.702 DownImage[3584:306401] noRequest-42017-12-04 18:04:27.702 DownImage[3584:306405] noRequest-52017-12-04 18:04:27.703 DownImage[3584:306401] noRequest-62017-12-04 18:04:27.703 DownImage[3584:306401] noRequest-72017-12-04 18:04:27.704 DownImage[3584:306401] noRequest-82017-12-04 18:04:27.704 DownImage[3584:306401] noRequest-92017-12-04 18:04:27.772 DownImage[3584:306397] 2---22017-12-04 18:04:27.779 DownImage[3584:306401] 0---02017-12-04 18:04:27.782 DownImage[3584:306409] 1---12017-12-04 18:04:27.800 DownImage[3584:306405] 3---32017-12-04 18:04:27.851 DownImage[3584:306401] 6---62017-12-04 18:04:27.855 DownImage[3584:306397] 5---52017-12-04 18:04:27.915 DownImage[3584:306397] 7---72017-12-04 18:04:27.951 DownImage[3584:306397] 9---92017-12-04 18:04:27.953 DownImage[3584:306405] 8---82017-12-04 18:04:28.476 DownImage[3584:306409] 4---4
5.还是使用信号量semaphore完成4的需求
//5.semaphore---order UIButton *Btn5 = [UIButton buttonWithType:UIButtonTypeCustom]; Btn5.frame = CGRectMake(100, 500, 100, 40); Btn5.backgroundColor = [UIColor grayColor]; [Btn5 setTitle:@"order" forState:UIControlStateNormal]; [Btn5 addTarget:self action:@selector(Btn5) forControlEvents:UIControlEventTouchUpInside]; [self.view addSubview:Btn5];
//5.semaphore--order-(void)Btn5{ NSString *str = @"http://www.jianshu.com/p/6930f335adba"; NSURL *url = [NSURL URLWithString:str]; NSURLRequest *request = [NSURLRequest requestWithURL:url]; NSURLSession *session = [NSURLSession sharedSession]; dispatch_semaphore_t sem = dispatch_semaphore_create(0); for (int i=0; i<10; i++) { NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) { NSLog(@"%d---%d",i,i); dispatch_semaphore_signal(sem); }]; [task resume]; dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER); } dispatch_async(dispatch_get_main_queue(), ^{ NSLog(@"end"); });}我们看看运行结果:
2017-12-05 10:17:28.175 DownImage[938:51296] 0---02017-12-05 10:17:28.331 DownImage[938:51289] 1---12017-12-05 10:17:28.506 DownImage[938:51289] 2---22017-12-05 10:17:28.563 DownImage[938:51289] 3---32017-12-05 10:17:28.662 DownImage[938:51289] 4---42017-12-05 10:17:28.733 DownImage[938:51296] 5---52017-12-05 10:17:28.792 DownImage[938:51296] 6---62017-12-05 10:17:28.856 DownImage[938:51286] 7---72017-12-05 10:17:29.574 DownImage[938:51289] 8---82017-12-05 10:17:29.652 DownImage[938:51286] 9---92017-12-05 10:17:29.653 DownImage[938:45252] end
2017-12-05 10:17:46.341 DownImage[938:51608] 0---02017-12-05 10:17:47.967 DownImage[938:51607] 1---12017-12-05 10:17:48.038 DownImage[938:51603] 2---22017-12-05 10:17:48.132 DownImage[938:51603] 3---32017-12-05 10:17:48.421 DownImage[938:51608] 4---42017-12-05 10:17:48.537 DownImage[938:51289] 5---52017-12-05 10:17:48.646 DownImage[938:51289] 6---62017-12-05 10:17:48.939 DownImage[938:51289] 7---72017-12-05 10:17:50.537 DownImage[938:51607] 8---82017-12-05 10:17:50.615 DownImage[938:51289] 9---92017-12-05 10:17:50.616 DownImage[938:45252] end
我们对比 3 的代码，3 中我们只在最后也就是循环结束后调用dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER) ，循环中当网络请求回调10次(也就是都回调完)后，使传入的信号量加1:( dispatch_semaphore_signal(sem) ) ,这时等待结束，然后进行后续的操作。
在这个方法里，我们每一次遍历，都让其dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER) ，这个时候线程会等待，阻塞当前线程，直到dispatch_semaphore_signal(sem)调用之后，而我们dispatch_semaphore_signal(sem)是在网络请求的回调里调用的，所以这个方法的逻辑是：
遍历—>发起任务—>等待—>任务完成信号量加1—>等待结束,开始下一个任务
发起任务—>等待—>任务完成信号量加1—>等待结束,开始下一个任务
发起任务—>等待—>任务完成信号量加1—>等待结束,开始下一个任务
这样循环的模式,一个任务完成才能接着去做下面的任务，满足我们的需求。
但我们也要发现这样一个问题，我们使用这种方式，可以明显感觉出整个过程需要花费的时间大大增加了，不像我们 3 中同时(几乎)开启任务等待完成回调，这里是一个网络请求发出，等待，完成后发出第二个网络请求，等待，完成后再发出第三个，这样我们等待的时间是10个网络请求每一个回调时间的和，在时间上大大增加了消耗，而且对于dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER) ，它是会阻塞线程的，我们如果需要在网络请求完成后修改UI，那这种方式会影响我们的界面交互,接下来我们对比一下两者时间消耗:
3-------------3----------3-------2017-12-05 10:29:51.178 DownImage[938:56971] 2---22017-12-05 10:29:51.193 DownImage[938:57200] 0---02017-12-05 10:29:51.202 DownImage[938:56631] 3---32017-12-05 10:29:51.248 DownImage[938:56971] 1---12017-12-05 10:29:51.262 DownImage[938:56971] 5---52017-12-05 10:29:51.291 DownImage[938:56631] 6---62017-12-05 10:29:51.375 DownImage[938:56631] 7---72017-12-05 10:29:51.384 DownImage[938:56631] 4---42017-12-05 10:29:51.434 DownImage[938:56971] 8---82017-12-05 10:29:51.487 DownImage[938:57199] 9---92017-12-05 10:29:51.488 DownImage[938:45252] end5-------------5----------5-------2017-12-05 10:29:52.190 DownImage[938:56631] 0---02017-12-05 10:29:52.304 DownImage[938:57199] 1---12017-12-05 10:29:52.432 DownImage[938:56971] 2---22017-12-05 10:29:52.520 DownImage[938:56971] 3---32017-12-05 10:29:52.576 DownImage[938:56631] 4---42017-12-05 10:29:52.628 DownImage[938:56971] 5---52017-12-05 10:29:52.706 DownImage[938:56631] 6---62017-12-05 10:29:52.764 DownImage[938:56971] 7---72017-12-05 10:29:52.853 DownImage[938:56631] 8---82017-12-05 10:29:52.925 DownImage[938:56971] 9---92017-12-05 10:29:52.926 DownImage[938:45252] end
看得出3花费时间为51.488 - 51.178约300多ms
--- ---5花费时间为52.926 - 52.190约700多ms
所以大家还请谨慎使用。
总结
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