1.通過NSURLSessionDelegate來下載

#import "URLSessionViewController.h"

@interface URLSessionViewController ()<NSURLSessionDataDelegate>
@property (nonatomic, strong) NSMutableData *responseData;
@end

@implementation URLSessionViewController

-(NSMutableData *)responseData
{
    if (_responseData == nil) {
        _responseData = [NSMutableData data];
    }
    return _responseData;
}

//當(dāng)點(diǎn)擊控制器View的時(shí)候會(huì)調(diào)用該方法
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
    [self delegateTest];
}

2.創(chuàng)建NSURLSession對(duì)象并設(shè)置代理

-(void)delegateTest
{
    //1.確定請(qǐng)求路徑
    NSURL *url = [NSURL URLWithString:@"http://120.25.226.186:32812/login?username=520it&pwd=520it&type=JSON"];
    
    //2.創(chuàng)建請(qǐng)求對(duì)象
    //請(qǐng)求對(duì)象內(nèi)部默認(rèn)已經(jīng)包含了請(qǐng)求頭和請(qǐng)求方法（GET）
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    
    //3.獲得會(huì)話對(duì)象,并設(shè)置代理
    /*
     第一個(gè)參數(shù)：會(huì)話對(duì)象的配置信息defaultSessionConfiguration 表示默認(rèn)配置
     第二個(gè)參數(shù)：誰成為代理藏澳，此處為控制器本身即self
     第三個(gè)參數(shù)：隊(duì)列曼尊，該隊(duì)列決定代理方法在哪個(gè)線程中調(diào)用重挑，可以傳主隊(duì)列|非主隊(duì)列
     [NSOperationQueue mainQueue]   主隊(duì)列：   代理方法在主線程中調(diào)用
     [[NSOperationQueue alloc]init] 非主隊(duì)列： 代理方法在子線程中調(diào)用
     */
    NSURLSession *session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[NSOperationQueue mainQueue]];
    
    //4.根據(jù)會(huì)話對(duì)象創(chuàng)建一個(gè)Task(發(fā)送請(qǐng)求）
    NSURLSessionDataTask *dataTask = [session dataTaskWithRequest:request];
    
    //5.執(zhí)行任務(wù)
    [dataTask resume];
}

3.代理方法實(shí)現(xiàn)

//1.接收到服務(wù)器響應(yīng)的時(shí)候調(diào)用該方法
-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler
{
    //在該方法中可以得到響應(yīng)頭信息，即response
    NSLog(@"didReceiveResponse--%@",[NSThread currentThread]);
    
    //注意：需要使用completionHandler回調(diào)告訴系統(tǒng)應(yīng)該如何處理服務(wù)器返回的數(shù)據(jù)
    //默認(rèn)是取消的
    /*
     NSURLSessionResponseCancel = 0,        默認(rèn)的處理方式踪央，取消
     NSURLSessionResponseAllow = 1,         接收服務(wù)器返回的數(shù)據(jù)
     NSURLSessionResponseBecomeDownload = 2,變成一個(gè)下載請(qǐng)求
     NSURLSessionResponseBecomeStream        變成一個(gè)流
     */
    
    completionHandler(NSURLSessionResponseAllow);
}

//2.接收到服務(wù)器返回?cái)?shù)據(jù)的時(shí)候會(huì)調(diào)用該方法，如果數(shù)據(jù)較大那么該方法可能會(huì)調(diào)用多次
-(void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data
{
    NSLog(@"didReceiveData--%@",[NSThread currentThread]);
    
    //拼接服務(wù)器返回的數(shù)據(jù)
    [self.responseData appendData:data];
}

//3.當(dāng)請(qǐng)求完成(成功|失敗)的時(shí)候會(huì)調(diào)用該方法，如果請(qǐng)求失敗祈坠，則error有值
-(void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    NSLog(@"didCompleteWithError--%@",[NSThread currentThread]);
    
    if(error == nil)
    {
        //解析數(shù)據(jù),JSON解析請(qǐng)參考http://www.cnblogs.com/wendingding/p/3815303.html
        NSDictionary *dict = [NSJSONSerialization JSONObjectWithData:self.responseData options:kNilOptions error:nil];
        NSLog(@"%@",dict);
    }
}