1.使用GCD的dispatch_group_t
```
-(void)Btn{
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(inti=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");
});
}
```
2..使用GCD的信号量dispatch_semaphore_t
```
-(void)Btn{
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(inti=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");
});
}
```
dispatch_semaphore_t sem = dispatch_semaphore_create(0);
dispatch_semaphore_signal(sem);
dispatch_semaphore_wait(sem, DISPATCH_TIME_FOREVER);复制代码
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)。
3.用GCD信号量实现顺序执行，待所有请求完成后再进行相应操作。
```
-(void)Btn{
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(inti=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");
});
}
```
注：1、2、3均为网络请求执行完后再执行相应操作，但1、2的网络请求为无序，3的请网络请求是按顺序执行。3是最耗时的！！！
```
