iOS当多个网络请求完成后执行下一步的方法详解

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] end
2017-12-04 17:10:10.676 downimage[3289:261080] 0---0
2017-12-04 17:10:10.704 downimage[3289:261080] 1---1
2017-12-04 17:10:10.754 downimage[3289:261096] 4---4
2017-12-04 17:10:10.760 downimage[3289:261080] 2---2
2017-12-04 17:10:10.800 downimage[3289:261096] 5---5
2017-12-04 17:10:10.840 downimage[3289:261080] 7---7
2017-12-04 17:10:10.844 downimage[3289:261082] 6---6
2017-12-04 17:10:10.846 downimage[3289:261096] 3---3
2017-12-04 17:10:10.888 downimage[3289:261096] 8---8
2017-12-04 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---2
2017-12-04 17:14:46.987 downimage[3289:265370] 1---1
2017-12-04 17:14:47.052 downimage[3289:265383] 5---5
2017-12-04 17:14:47.065 downimage[3289:265370] 4---4
2017-12-04 17:14:47.111 downimage[3289:265379] 3---3
2017-12-04 17:14:47.121 downimage[3289:265383] 6---6
2017-12-04 17:14:47.169 downimage[3289:265383] 7---7
2017-12-04 17:14:47.192 downimage[3289:265370] 9---9
2017-12-04 17:14:47.321 downimage[3289:265383] 8---8
2017-12-04 17:14:47.747 downimage[3289:265374] 0---0
2017-12-04 17:14:47.747 downimage[3289:261033] end

2017-12-04 17:15:14.576 downimage[3289:265942] 3---3
2017-12-04 17:15:14.626 downimage[3289:265936] 2---2
2017-12-04 17:15:14.647 downimage[3289:265944] 4---4
2017-12-04 17:15:14.648 downimage[3289:265936] 0---0
2017-12-04 17:15:14.657 downimage[3289:265943] 1---1
2017-12-04 17:15:14.709 downimage[3289:265944] 5---5
2017-12-04 17:15:14.728 downimage[3289:265944] 6---6
2017-12-04 17:15:14.734 downimage[3289:265944] 7---7
2017-12-04 17:15:14.738 downimage[3289:265943] 8---8
2017-12-04 17:15:14.816 downimage[3289:265944] 9---9
2017-12-04 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---2
2017-12-04 17:36:49.144 downimage[3428:284210] 0---0
2017-12-04 17:36:49.152 downimage[3428:284213] 3---3
2017-12-04 17:36:49.158 downimage[3428:283651] 1---1
2017-12-04 17:36:49.167 downimage[3428:284210] 4---4
2017-12-04 17:36:49.235 downimage[3428:284213] 8---8
2017-12-04 17:36:49.249 downimage[3428:283651] 5---5
2017-12-04 17:36:49.252 downimage[3428:283651] 7---7
2017-12-04 17:36:49.324 downimage[3428:283651] 9---9
2017-12-04 17:36:49.468 downimage[3428:284214] 6---6
2017-12-04 17:36:49.469 downimage[3428:283554] end

2017-12-04 17:37:11.554 downimage[3428:284747] 0---0
2017-12-04 17:37:11.555 downimage[3428:284733] 1---1
2017-12-04 17:37:11.627 downimage[3428:284748] 5---5
2017-12-04 17:37:11.661 downimage[3428:284748] 2---2
2017-12-04 17:37:11.688 downimage[3428:284747] 4---4
2017-12-04 17:37:11.709 downimage[3428:284747] 6---6
2017-12-04 17:37:11.770 downimage[3428:284733] 7---7
2017-12-04 17:37:11.774 downimage[3428:284733] 8---8
2017-12-04 17:37:11.824 downimage[3428:284747] 9---9
2017-12-04 17:37:11.899 downimage[3428:284733] 3---3
2017-12-04 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-0
2017-12-04 18:03:10.226 downimage[3584:304362] norequest-1
2017-12-04 18:03:10.226 downimage[3584:304363] norequest-2
2017-12-04 18:03:10.231 downimage[3584:304363] norequest-3
2017-12-04 18:03:10.232 downimage[3584:304362] norequest-4
2017-12-04 18:03:10.233 downimage[3584:304362] norequest-5
2017-12-04 18:03:10.233 downimage[3584:304363] norequest-6
2017-12-04 18:03:10.234 downimage[3584:304363] norequest-7
2017-12-04 18:03:10.235 downimage[3584:304363] norequest-8
2017-12-04 18:03:10.236 downimage[3584:304363] norequest-9
2017-12-04 18:03:10.408 downimage[3584:304597] 2---2
2017-12-04 18:03:10.408 downimage[3584:304597] 0---0
2017-12-04 18:03:10.409 downimage[3584:304597] 1---1
2017-12-04 18:03:10.461 downimage[3584:304597] 5---5
2017-12-04 18:03:10.476 downimage[3584:304363] 4---4
2017-12-04 18:03:10.477 downimage[3584:304365] 6---6
2017-12-04 18:03:10.518 downimage[3584:304365] 7---7
2017-12-04 18:03:10.537 downimage[3584:304596] 8---8
2017-12-04 18:03:10.547 downimage[3584:304362] 9---9
2017-12-04 18:03:11.837 downimage[3584:304362] 3---3

2017-12-04 18:04:27.699 downimage[3584:306401] norequest-0
2017-12-04 18:04:27.700 downimage[3584:306405] norequest-1
2017-12-04 18:04:27.701 downimage[3584:306401] norequest-2
2017-12-04 18:04:27.701 downimage[3584:306405] norequest-3
2017-12-04 18:04:27.702 downimage[3584:306401] norequest-4
2017-12-04 18:04:27.702 downimage[3584:306405] norequest-5
2017-12-04 18:04:27.703 downimage[3584:306401] norequest-6
2017-12-04 18:04:27.703 downimage[3584:306401] norequest-7
2017-12-04 18:04:27.704 downimage[3584:306401] norequest-8
2017-12-04 18:04:27.704 downimage[3584:306401] norequest-9
2017-12-04 18:04:27.772 downimage[3584:306397] 2---2
2017-12-04 18:04:27.779 downimage[3584:306401] 0---0
2017-12-04 18:04:27.782 downimage[3584:306409] 1---1
2017-12-04 18:04:27.800 downimage[3584:306405] 3---3
2017-12-04 18:04:27.851 downimage[3584:306401] 6---6
2017-12-04 18:04:27.855 downimage[3584:306397] 5---5
2017-12-04 18:04:27.915 downimage[3584:306397] 7---7
2017-12-04 18:04:27.951 downimage[3584:306397] 9---9
2017-12-04 18:04:27.953 downimage[3584:306405] 8---8
2017-12-04 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---0
2017-12-05 10:17:28.331 downimage[938:51289] 1---1
2017-12-05 10:17:28.506 downimage[938:51289] 2---2
2017-12-05 10:17:28.563 downimage[938:51289] 3---3
2017-12-05 10:17:28.662 downimage[938:51289] 4---4
2017-12-05 10:17:28.733 downimage[938:51296] 5---5
2017-12-05 10:17:28.792 downimage[938:51296] 6---6
2017-12-05 10:17:28.856 downimage[938:51286] 7---7
2017-12-05 10:17:29.574 downimage[938:51289] 8---8
2017-12-05 10:17:29.652 downimage[938:51286] 9---9
2017-12-05 10:17:29.653 downimage[938:45252] end

2017-12-05 10:17:46.341 downimage[938:51608] 0---0
2017-12-05 10:17:47.967 downimage[938:51607] 1---1
2017-12-05 10:17:48.038 downimage[938:51603] 2---2
2017-12-05 10:17:48.132 downimage[938:51603] 3---3
2017-12-05 10:17:48.421 downimage[938:51608] 4---4
2017-12-05 10:17:48.537 downimage[938:51289] 5---5
2017-12-05 10:17:48.646 downimage[938:51289] 6---6
2017-12-05 10:17:48.939 downimage[938:51289] 7---7
2017-12-05 10:17:50.537 downimage[938:51607] 8---8
2017-12-05 10:17:50.615 downimage[938:51289] 9---9
2017-12-05 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---2
2017-12-05 10:29:51.193 downimage[938:57200] 0---0
2017-12-05 10:29:51.202 downimage[938:56631] 3---3
2017-12-05 10:29:51.248 downimage[938:56971] 1---1
2017-12-05 10:29:51.262 downimage[938:56971] 5---5
2017-12-05 10:29:51.291 downimage[938:56631] 6---6
2017-12-05 10:29:51.375 downimage[938:56631] 7---7
2017-12-05 10:29:51.384 downimage[938:56631] 4---4
2017-12-05 10:29:51.434 downimage[938:56971] 8---8
2017-12-05 10:29:51.487 downimage[938:57199] 9---9
2017-12-05 10:29:51.488 downimage[938:45252] end

5-------------5----------5-------
2017-12-05 10:29:52.190 downimage[938:56631] 0---0
2017-12-05 10:29:52.304 downimage[938:57199] 1---1
2017-12-05 10:29:52.432 downimage[938:56971] 2---2
2017-12-05 10:29:52.520 downimage[938:56971] 3---3
2017-12-05 10:29:52.576 downimage[938:56631] 4---4
2017-12-05 10:29:52.628 downimage[938:56971] 5---5
2017-12-05 10:29:52.706 downimage[938:56631] 6---6
2017-12-05 10:29:52.764 downimage[938:56971] 7---7
2017-12-05 10:29:52.853 downimage[938:56631] 8---8
2017-12-05 10:29:52.925 downimage[938:56971] 9---9
2017-12-05 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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