NSURLConnection下载

@interface AppDelegate () <NSURLConnectionDataDelegate> {
    NSMutableData *mData;
}

@end

@implementation AppDelegate

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    
    NSURL *url = [NSURL URLWithString:@"http://www.baidu.com"];
    NSURLRequest *req = [NSURLRequest requestWithURL:url];
 
    // 第二个参数指定的委托用于处理服务器的响应
    // 当服务器返回一个响应、发送数据以及完成响应时会回调委托中的方法
    NSURLConnection *conn = [[NSURLConnection alloc] initWithRequest:req delegate:self];
    
    // 执行start方法时才真正连接服务器
    [conn start];
    
    return YES;
}

#pragma mark NSURLConnectionDataDelegate回调方法
// 收到服务器的响应
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response {
    
    if (!mData) {
        mData = [NSMutableData data];
    }
    else {
        mData.length = 0;
    }
    
}

// 收到服务器发送的数据(该方法可能被调用很多次)
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data {
    // 将收到的数据拼接起来
    [mData appendData:data];
}

// 服务器响应完成(结束从服务器加载数据)
- (void)connectionDidFinishLoading:(NSURLConnection *)connection {
    NSString *str = [[NSString alloc] initWithData:mData encoding:NSUTF8StringEncoding];
    
    // 获得应用程序的沙箱路径
    NSString *homeDir = NSHomeDirectory();
    NSLog(@"%@", homeDir);
    
    NSString *filename = [homeDir stringByAppendingPathComponent:@"Documents/index.html"];
    
    [str writeToFile:filename atomically:NO encoding:NSUTF8StringEncoding error:nil];
    
    NSLog(@"文件已保存!!!");
}