在创建一个同步的网链接的时候我们需要明白一点，并不是我们的这个同步连接一定会堵塞我们的主线程，如果这个同步的链接是创建在主线程上的，那么这种情况下是会堵塞我们的主线程的。其他情况下是不一定会堵塞我们的主线程的。

这个例子中，我们将会尝试取回Yahoo主页的内容：

-(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions{

    NSLog(@"We are here ... ");

    NSString *urlAsString = @"";

    NSURL *url = [NSURL URLWithString:urlAsString];

    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];

    NSURLResponse *response = nil;

    NSError *error = nil;

    NSLog(@"Firing synchronous url connection...");

    NSData *data = [NSURLConnection sendSynchronousRequest:urlRequest returningResponse:&responseerror:&error];

    if([data length]>0 && error == nil){

        NSLog(@"%lu bytes of data was returned.",(unsigned long)[data length]);

    }else if ([data length] == 0 && error == nil){

        NSLog(@"No data was returned.");

    }else if (error != ni){

        NSLog(@"Error happened = %@",error);

        NSLog(@"We are done.");

    }

    self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds]];

    self.window.backgroundColor = [UIColor whiteColor];

    [self.window makeKeyAndVisible];

    return YES;

}

打印效果:

We are here...
Firing synchronous url connection... 194472 bytes of data was returned. We are done.

-(BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary*)launchOptions{

    NSLog(@"We are here ... ");

    NSString *urlAsString = @"";

    NSLog(@"Firing synchronous url connection...");

    dispatch_queue_t dispatchQueue = dispatch_get_global_queue(DISPATCH_PRIORITY_DEFAULT, 0);

    dispatch_async(dispatchQueue, ^(void){

        NSURL *url = [NSURL URLWithString:urlAsString];

        NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];

        NSURLResponse *response = nil;

        NSError *error = nil;

        NSData *data = [NSURLConnection sendSynchronousRequest:urlRequestreturningResponse:&response error:&error];

        if([data length]>0 && error == nil){

            NSLog(@"%lu bytes of data was returned.",(unsigned long)[data length]);

        }else if ([data length] == 0 && error == nil){

            NSLog(@"No data was returned.");

        }else if (error != ni){

            NSLog(@"Error happened = %@",error);

        }

    });

    NSLog(@"We are done.");

    self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds]];

    self.window.backgroundColor = [UIColor whiteColor];

    [self.window makeKeyAndVisible];

    return YES;

}

打印效果：

We are here...
Firing synchronous url connection... We are done.
194326 bytes of data was returned.

看了如上的代码是不是感觉特别奇怪,怎么 we are done 还是放在最后面的,怎么现在打印到前面的了,这 就是我们要证明的,其实同步的程序并不一定会堵塞我们的主线程的。只要我们的这个同步的线程并不是建立 在我们的主线程上的,如果我们把他添加到 GCD 队列池中的话,那么我们的主线程也就没有被堵塞。这样同 步的概念实际上也可以看做是某种程度上的异步。

通过NSMutableURLRequest包装URL的请求形式

你想在传给一个请求链接前能适应一个URL的不同HTTP头及其设置。

通过NSMutableURLRequest代替NSURLRequest