NetConnector是自定义的一个类，该类使用代理的方法实现异步下载特定url页面的内容。

HyNetConnector.h

//
//  HyNetConnector.h
//  HyNetConnector
//
//  Created by kinds on 15/7/1.
//  Copyright (c) 2015年 hopy. All rights reserved.
//

#import <Foundation/Foundation.h>

@interface HyNetConnector : NSObject
@property NSURLConnection *conn;
@property (readonly) BOOL is_finished;
@end

HyNetConnector.m

//
//  HyNetConnector.m
//  HyNetConnector
//
//  Created by kinds on 15/7/1.
//  Copyright (c) 2015年 hopy. All rights reserved.
//

#import "HyNetConnector.h"

@implementation HyNetConnector
-(id)init{
    if(self = [super init]){
        NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
        NSURLRequest *req = [NSURLRequest requestWithURL:url];
        _is_finished = false;
        _conn = [[NSURLConnection alloc] initWithRequest:req delegate:self
                                        startImmediately:YES];
    }
    return self;
}

-(void)connectionDidFinishLoading:(NSURLConnection *)connection{
    NSLog(@"%s:finish loading!",__func__);
    _is_finished = true;
}

-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
    //NSLog(@"%s:recv %@(%lu)",__func__,data,data.length);
    NSLog(@"%s:recv len %lu",__func__,data.length);
}
@end

main.m

//
//  main.m
//  HyNetConnector
//
//  Created by kinds on 15/7/1.
//  Copyright (c) 2015年 hopy. All rights reserved.
//

#import <Foundation/Foundation.h>
#import "HyNetConnector.h"

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        HyNetConnector *nc = [HyNetConnector new];
        NSLog(@"%@",nc);

        while(!nc.is_finished){
            NSDate *date = [NSDate dateWithTimeIntervalSinceNow:.5];
            [[NSRunLoop currentRunLoop]runUntilDate:date];
        }
        NSLog(@"It's over...");
    }
    return 0;
}