通过NSURLConnection进行异步下载   

      NSURLConnection提供了两种方式来实现连接,一种是同步的另一种是异步的,异步的连接将会创建一个新的线程,这个线程将会来负责下载的动作。而对于同步连接,在下载连接和处理通讯时,则会阻塞当前调用线程。

     许多开发者都会认为同步的连接将会堵塞主线程,其实这种观点是错误的。一个同步的连接是会阻塞调用它的线程。如果你在主线程中创建一个同步连接,没错,主线程会阻塞。但是如果你并不是从主线程开启的一个同步的连接,它将会类似异步的连接一样。因此这种情况并不会堵塞你的主线程。事实上,同步和异步的主要区别就是运行runtime 为会异步连接创建一个线程,而同步连接则不会。

//asynchronousRequest connection
-(void)fetchAppleHtml{
    NSString *urlString = @"http://www.apple.com";
    NSURL *url = [NSURL URLWithString:urlString];
//    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url cachePolicy:NSURLRequestReloadIgnoringLocalAndRemoteCacheData timeoutInterval:30.0f]; //maximal timeout is 30s
    
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [NSURLConnection sendAsynchronousRequest:urlRequest queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
        if ([data length] > 0 && connectionError == nil) {
            NSString *documentsDir = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
            NSString *filePath = [documentsDir stringByAppendingPathComponent:@"apple.html"];
            [data writeToFile:filePath atomically:YES];
            NSLog(@"Successfully saved the file to %@",filePath);
            NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            NSLog(@"HTML = %@",html);
        }else if ([data length] == 0 && connectionError == nil){
            NSLog(@"Nothing was downloaded.");
        }else if (connectionError != nil){
            NSLog(@"Error happened = %@",connectionError);
        }
    }];
}

      通过NSURLConnection进行同步下载   

//synchronousRequest connection
-(void)fetchYahooData{
    NSLog(@"We are here...");
    NSString *urlString = @"http://www.yahoo.com";
    NSURL *url = [NSURL URLWithString:urlString];
    NSURLRequest *urlRequest = [NSURLRequest requestWithURL:url];
    NSURLResponse *response = nil;
    NSError *error = nil;
    NSLog(@"Firing synchronous url connection...");
    NSData *data = http://www.mamicode.com/[NSURLConnection sendSynchronousRequest:urlRequest returningResponse:&response error:&error];>
查看运行输出结果，分别为：
synchronous download on main thread without GCD

synchronous download on main thread with GCD

    可以看到在主线程上调用同步下载会阻塞当前线程，而使用GCD则不会。
    通过NSURLConnection发送一个HTTP GET请求
//send a GET request to server with some params
-(void)httpGetWithParams{
    NSString *urlString = @"http://chaoyuan.sinaapp.com";
    urlString = [urlString stringByAppendingString:@"?p=1059"];
    NSURL *url = [NSURL URLWithString:urlString];
    NSMutableURLRequest *urlRequest = [NSMutableURLRequest requestWithURL:url];
    [urlRequest setTimeoutInterval:30.0f];
    [urlRequest setHTTPMethod:@"GET"];
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [NSURLConnection sendAsynchronousRequest:urlRequest queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
        if ([data length] > 0 && connectionError == nil) {
            NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding];
            NSLog(@"HTML = %@",html);
        }else if([data length] == 0 && connectionError == nil){
            NSLog(@"nothing was download.");
        }else if(connectionError != nil){
            NSLog(@"Error happened = %@",connectionError);
        }
    }];
}


  通过NSURLConnection发送一个HTTP POST请求
//send a POST request to a server with some params
-(void)httpPostWithParams{
    NSString *urlAsString = @"http://chaoyuan.sinaapp.com";
    urlAsString = [urlAsString stringByAppendingString:@"?param1=First"];
    urlAsString = [urlAsString stringByAppendingString:@"¶m2=Second"];
    NSURL *url = [NSURL URLWithString:urlAsString];
    NSMutableURLRequest *urlRequest = [NSMutableURLRequest requestWithURL:url]; [urlRequest setTimeoutInterval:30.0f];
    [urlRequest setHTTPMethod:@"POST"];
    NSString *body = @"bodyParam1=BodyValue1&bodyParam2=BodyValue2"; [urlRequest setHTTPBody:[body dataUsingEncoding:NSUTF8StringEncoding]]; NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    [NSURLConnection
     sendAsynchronousRequest:urlRequest
     queue:queue completionHandler:^(NSURLResponse *response, NSData *data,
                                     NSError *error) {
         if ([data length] >0 &&
             error == nil){
             NSString *html = [[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]; NSLog(@"HTML = %@", html);
         }
         else if ([data length] == 0 &&
                  error == nil){
             NSLog(@"Nothing was downloaded.");
         }
         else if (error != nil){
             NSLog(@"Error happened = %@", error);
         }
     }];
}

tips:
    except http get and post there are http delete and put and something else, if you are crazy about http, please GOOGLE!
完整项目代码在这里。