IOS网络访问之NSURLSession

NSURLSession是IOS7中新添加的网络访问接口，作用与NSURLConnection一致，在程序在前台时，NSURLSession与NSURLConnection可以互为替代工作。如果用户强制将程序关闭，NSURLSession会断掉。

NSURLSession中关键类有下面几种

1：NSURLSessionConfiguration：用于配置NSURLSession工作模式以及网络设置

工作模式分为下面三种：

普通模式（default）：可以使用缓存

+ (NSURLSessionConfiguration *)defaultSessionConfiguration; 

瞬时模式（ephemeral）：不使用缓存

+ (NSURLSessionConfiguration *)ephemeralSessionConfiguration; 

后台模式（background）：当按下home键后仍然可以在后台进行上传下载操作，需要通过唯一的identity标示

+ (NSURLSessionConfiguration *)backgroundSessionConfiguration:(NSString *)identifier; 

网络设置：

@property BOOL allowsCellularAccess 　　允许使用蜂窝数据

@property (getter=isDiscretionary) BOOL discretionary　　YES时表示任务后台运行时自动选择最佳的网络选择（综合考虑网络，电量）

2：NSURLSession：获取NSURLSession有下面几种方式

+ (NSURLSession *)sharedSession　　共享的会话，使用全局的Cookie，缓存数据

+ (NSURLSession *)sessionWithConfiguration:(NSURLSessionConfiguration *)configuration　　使用配置好的NSURLSessionConfiguration获得会话

+ (NSURLSession *)sessionWithConfiguration:(NSURLSessionConfiguration *)configuration delegate:(id <NSURLSessionDelegate>)delegate delegateQueue:(NSOperationQueue *)queue　　与第2中相似，但是指定了委托

3：NSURLSessionTask：会话任务，通过NSURLSession创建，有下面3种常用子类

NSURLSessionDataTask：最普通的网络访问，与NSURLConnection使用方式相似，多用来获取xml或者json，可以通过NSURLSession的以下方法创建

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request;

- (NSURLSessionDataTask *)dataTaskWithURL:(NSURL *)url

同时还可以通过block指定回调

- (NSURLSessionDataTask *)dataTaskWithRequest:(NSURLRequest *)request completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler; 

- (NSURLSessionDataTask *)dataTaskWithURL:(NSURL *)url completionHandler:(void (^)(NSData *data, NSURLResponse *response, NSError *error))completionHandler; 

NSURLSessionDownLoadTask：用来处理下载任务

- (NSURLSessionDownloadTask *)downloadTaskWithRequest:(NSURLRequest *)request;

- (NSURLSessionDownloadTask *)downloadTaskWithURL:(NSURL *)url; 

- (NSURLSessionDownloadTask *)downloadTaskWithResumeData:(NSData *)resumeData; 

上面3种方法均可通过block设置回调，第三种方法可以实现断点续传

NSURLSessionUploadTask：用来处理上传任务，NSURLSessionDataTask的子类

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromFile:(NSURL *)fileURL

- (NSURLSessionUploadTask *)uploadTaskWithRequest:(NSURLRequest *)request fromData:(NSData *)bodyData

- (NSURLSessionUploadTask *)uploadTaskWithStreamedRequest:(NSURLRequest *)request

上面3种方法同样可以通过block设置回调

创建后的NSURLSessionTask需要调用resume才会执行

4：NSURLSessionDelegate和NSURLSessionTaskDelegate协议

在网络请求的各个阶段都会触发协议的不同方法，具体使用方法可以参照后面例子

示例1：通过NSSessionDataTask访问网络数据

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSString *urlStr = @"http://localhost/iostest.php";
    urlStr = [urlStr stringByAddingPercentEscapesUsingEncoding:NSUTF8StringEncoding];
    
    //创建request
    NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:urlStr]];
    request.HTTPMethod = @"POST";
    request.HTTPBody = [@"name=zanglitao&gender=male" dataUsingEncoding:NSUTF8StringEncoding];
    
    //创建NSURLSession
    NSURLSession *session = [NSURLSession sharedSession];
    
    //创建任务
    NSURLSessionDataTask *task = [session dataTaskWithRequest:request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        NSLog(@"%@",[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);
    }];
    
    //开始任务
    [task resume];
}

示例2：通过NSSessionUploadTask上传文件，流程和NSURLConnection基本一致，关键点也是设置请求头参数和请求体拼接

//自定义一个boundary
#define boundary @"zanglitao"
@interface ZViewController()
@end

@implementation ZViewController

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSMutableURLRequest *request = [[NSMutableURLRequest alloc] initWithURL:[NSURL URLWithString:@"http://localhost/iostest.php"]];
    //必须使用POST
    request.HTTPMethod = @"POST";
    //设置请求体
    request.HTTPBody = [self getDataBody];
    //设置请求头
    [request setValue:[NSString stringWithFormat:@"%d",[self getDataBody].length] forHTTPHeaderField:@"Content-Length"];
    [request setValue:[NSString stringWithFormat:@"multipart/form-data; boundary=%@",boundary] forHTTPHeaderField:@"Content-Type"];
    
    
    NSURLSession *session = [NSURLSession sharedSession];
    NSURLSessionUploadTask *task = [session uploadTaskWithRequest:request fromData:[self getDataBody] completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
        NSLog(@"%@",[[NSString alloc] initWithData:data encoding:NSUTF8StringEncoding]);
    }];
    
    [task resume];
}

//获取请求体内容
-(NSData *)getDataBody {
    NSMutableData *data = [NSMutableData data];
    
    NSString *top = [NSString stringWithFormat:@"--%@
Content-Disposition: form-data; name="file"; filename="1.png"
Content-Type: image/png

",boundary];
    
    NSString *bottom = [NSString stringWithFormat:@"
--%@--

",boundary];
    
    NSData *content = [NSData dataWithContentsOfFile:[[NSBundle mainBundle] pathForResource:@"1" ofType:@"png"]];
    
    [data appendData:[top dataUsingEncoding:NSUTF8StringEncoding]];
    [data appendData:content];
    [data appendData:[bottom dataUsingEncoding:NSUTF8StringEncoding]];
    return data;
}

@end

  

示例3：使用NSURLSessionDownLoadTask下载文件

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSURLSession *session = [NSURLSession sharedSession];
    
    NSURLSessionDownloadTask *task = [session downloadTaskWithURL:[NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/e61190ef76c6a7efc0a0e1ebfffaaf51f2de667c.jpg"] completionHandler:^(NSURL *location, NSURLResponse *response, NSError *error) {
       //下载完成后文件位于location处，我们需要移到沙盒中
        NSString *dirPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
        NSString *path = [dirPath stringByAppendingPathComponent:@"1.jpg"];
        
        NSFileManager *manager = [NSFileManager defaultManager];
        if ([manager fileExistsAtPath:path isDirectory:NO]) {
            [manager removeItemAtPath:path error:nil];
        }
        
        [manager moveItemAtPath:[location path] toPath:path error:nil];
        
    }];
    
    //开始任务
    [task resume];
}

执行后我们看到沙盒中多了一张名为1.jpg的图片，这是我们下载的图片

上面的代码还有一点问题，就是下载过程中没有进度提示只能干等，我们可以通过设置委托获取下载进度

示例4：使用委托查看下载进度

@interface ZViewController()<NSURLSessionDelegate>
@end

@implementation ZViewController

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    
    //使用配置的NSURLSessionConfiguration获取NSSession，并且设置委托
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    
    NSURLSessionDownloadTask *task = [session downloadTaskWithURL:[NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/e61190ef76c6a7efc0a0e1ebfffaaf51f2de667c.jpg"] ];
    
    //开始任务
    [task resume];
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    float percent = (float)totalBytesWritten/totalBytesExpectedToWrite;
    NSLog(@"%f",percent);
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location {
    NSString *dirPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
    NSString *path = [dirPath stringByAppendingPathComponent:@"1.jpg"];
    
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:path isDirectory:NO]) {
        [manager removeItemAtPath:path error:nil];
    }
    [manager moveItemAtPath:[location path] toPath:path error:nil];
}

//任务结束
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    NSLog(@"%s",__func__);
}

@end

2014-11-08 14:11:44.146 IOS网络请求[2366:3507] 0.020172

2014-11-08 14:11:44.147 IOS网络请求[2366:3507] 0.100307

2014-11-08 14:11:44.148 IOS网络请求[2366:3507] 0.153730

2014-11-08 14:11:44.148 IOS网络请求[2366:3507] 0.180442

2014-11-08 14:11:44.149 IOS网络请求[2366:3507] 0.207153

2014-11-08 14:11:44.150 IOS网络请求[2366:3507] 0.236461

2014-11-08 14:11:44.156 IOS网络请求[2366:3507] 0.263173

2014-11-08 14:11:44.160 IOS网络请求[2366:3507] 0.289884

2014-11-08 14:11:44.160 IOS网络请求[2366:3507] 0.316596

2014-11-08 14:11:44.161 IOS网络请求[2366:3507] 0.343307

2014-11-08 14:11:44.162 IOS网络请求[2366:3507] 0.370019

2014-11-08 14:11:44.162 IOS网络请求[2366:3507] 0.476865

2014-11-08 14:11:44.163 IOS网络请求[2366:3507] 0.557000

2014-11-08 14:11:44.169 IOS网络请求[2366:3803] 0.610423

2014-11-08 14:11:44.170 IOS网络请求[2366:3803] 0.663846

2014-11-08 14:11:44.171 IOS网络请求[2366:3803] 0.690558

2014-11-08 14:11:44.175 IOS网络请求[2366:3803] 0.717269

2014-11-08 14:11:44.176 IOS网络请求[2366:3803] 0.743981

2014-11-08 14:11:44.176 IOS网络请求[2366:3803] 0.797404

2014-11-08 14:11:44.177 IOS网络请求[2366:3803] 0.850827

2014-11-08 14:11:44.178 IOS网络请求[2366:3803] 0.877539

2014-11-08 14:11:44.179 IOS网络请求[2366:3803] 0.904250

2014-11-08 14:11:44.180 IOS网络请求[2366:3803] 0.930962

2014-11-08 14:11:44.180 IOS网络请求[2366:3803] 0.957674

2014-11-08 14:11:44.180 IOS网络请求[2366:3803] 0.984385

2014-11-08 14:11:44.181 IOS网络请求[2366:3803] 1.000000

2014-11-08 14:11:44.182 IOS网络请求[2366:3803] -[ZViewController URLSession:task:didCompleteWithError:]

 

下载是个漫长的过程，尤其是对于比较大的文件，所以我们还需要一个取消下载的按钮

示例5：示例4添加取消下载功能

@interface ZViewController()<NSURLSessionDelegate>
@property(nonatomic,strong)NSURLSessionDownloadTask *task;

@end

@implementation ZViewController

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    
    //使用配置的NSURLSessionConfiguration获取NSSession，并且设置委托
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    
    _task = [session downloadTaskWithURL:[NSURL URLWithString:@"http://yinyueshiting.baidu.com/data2/music/46729925/13125209147600128.mp3?xcode=10d30604371d01ea5cfa24abb2db23a644b52c81891b10c8"] ];
    
    //开始任务
    [_task resume];
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    float percent = (float)totalBytesWritten/totalBytesExpectedToWrite;
    NSLog(@"%f",percent);
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location {
    NSString *dirPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
    NSString *path = [dirPath stringByAppendingPathComponent:@"1.mp3"];
    
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:path isDirectory:NO]) {
        [manager removeItemAtPath:path error:nil];
    }
    
    [manager moveItemAtPath:[location path] toPath:path error:nil];
}

//任务结束
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    NSLog(@"%s",__func__);
}

- (IBAction)cancelDownload:(id)sender {
    [_task cancel];
}

@end

2014-11-08 14:25:28.351 IOS网络请求[2451:3803] 0.000917

2014-11-08 14:25:28.359 IOS网络请求[2451:3507] 0.008676

2014-11-08 14:25:28.370 IOS网络请求[2451:1303] 0.010250

2014-11-08 14:25:28.379 IOS网络请求[2451:5407] 0.018904

2014-11-08 14:25:28.393 IOS网络请求[2451:3507] 0.022838

2014-11-08 14:25:28.393 IOS网络请求[2451:3507] 0.023625

2014-11-08 14:25:28.405 IOS网络请求[2451:3507] 0.030705

2014-11-08 14:25:28.405 IOS网络请求[2451:3507] 0.034639

2014-11-08 14:25:28.409 IOS网络请求[2451:1303] 0.035032

2014-11-08 14:25:28.414 IOS网络请求[2451:5407] 0.035426

2014-11-08 14:25:28.436 IOS网络请求[2451:3507] 0.035819

2014-11-08 14:25:28.479 IOS网络请求[2451:3507] -[ZViewController URLSession:task:didCompleteWithError:]

当按下取消按钮时下载停止，同时任务结束的方法被触发

当然现实中更常见的方式是任务停止下载后还能恢复下载

示例6：暂停与恢复

- (IBAction)pauseDownload:(id)sender {
    [_task suspend];
}

- (IBAction)continueDownload:(id)sender {
    [_task resume];
}

有时候我们暂停了任务然后退出了程序，如果希望下次进入程序后还能继续下载，那就需要实现断点下载功能，在NSURLConnection中可以利用请求头的Range参数实现断点下载，在NSURLSession中实现断点下载更加简单

示例7：断点下载

@interface ZViewController()<NSURLSessionDelegate>

@property (strong, nonatomic)NSURLSessionDownloadTask *task;
//存储已下载的数据
@property(nonatomic,strong)NSData *data;
@end

@implementation ZViewController

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    
    //使用配置的NSURLSessionConfiguration获取NSSession，并且设置委托
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    _task = [session downloadTaskWithURL:[NSURL URLWithString:@"http://yinyueshiting.baidu.com/data2/music/46729925/13125209147600128.mp3?xcode=10d30604371d01ea5cfa24abb2db23a644b52c81891b10c8"] ];
    
    //开始任务
    [_task resume];
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    float percent = (float)totalBytesWritten/totalBytesExpectedToWrite;
    NSLog(@"%f",percent);
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location {
    NSString *dirPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
    NSString *path = [dirPath stringByAppendingPathComponent:@"1.mp3"];
    
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:path isDirectory:NO]) {
        [manager removeItemAtPath:path error:nil];
    }
    
    [manager moveItemAtPath:[location path] toPath:path error:nil];
}

//任务结束
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    NSLog(@"%s",__func__);
}

- (IBAction)pauseDownload:(id)sender {
    [_task cancelByProducingResumeData:^(NSData *resumeData) {
        _task = nil;
        _data = resumeData;
    }];
}

- (IBAction)continueDownload:(id)sender {
    NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration defaultSessionConfiguration];
    NSURLSession *session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];

    _task = [session downloadTaskWithResumeData:_data];
    [_task resume];
}

@end

 

示例8：后台下载

@implementation ZViewController

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSURLSession *session = [self session];
    NSURLSessionDownloadTask *task = [session downloadTaskWithURL:[NSURL URLWithString:@"http://yinyueshiting.baidu.com/data2/music/46729925/13125209147600128.mp3?xcode=10d30604371d01ea5cfa24abb2db23a644b52c81891b10c8"] ];
    
    //开始任务
    [task resume];
}

//后台会话(保证只有一个后台会话)
-(NSURLSession *)session {
    static NSURLSession *session;
    static dispatch_once_t token;
    dispatch_once(&token, ^{
        
        //后台下载
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"zang"];
        configuration.discretionary = YES;
        
        //使用配置的NSURLSessionConfiguration获取NSSession，并且设置委托
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    });
    
    return session;
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    float percent = (float)totalBytesWritten/totalBytesExpectedToWrite;
    NSLog(@"%f",percent);
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location {
    NSString *dirPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
    NSString *path = [dirPath stringByAppendingPathComponent:@"1.mp3"];
    
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:path isDirectory:NO]) {
        [manager removeItemAtPath:path error:nil];
    }
    
    [manager moveItemAtPath:[location path] toPath:path error:nil];
}



//任务结束
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    NSLog(@"%s",__func__);
}
@end

启动程序后按下home键发现下载进度信息不再更新，静候片刻再进入程序发现沙盒中多了1.mp3，使用backgroundSessionConfiguration可以使我们的下载任务运行在后台

我们也可以通过委托在文件下载完后进行界面更新

示例9：后台下载任务监控

当在后台下载完成后会触发AppDelegate的

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler方法

- (void)application:(UIApplication *)application handleEventsForBackgroundURLSession:(NSString *)identifier completionHandler:(void (^)())completionHandler {
    NSLog(@"%@:download complete",identifier);
   
    self.handler = completionHandler;
}

此方法包含一个competionHandler（此操作表示应用完成所有处理工作），通常我们会保存此对象；直到最后一个任务完成，此时会重新通过会话标识（上面sessionConfig中设置的）找到对应的会话并调用NSURLSession的-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session代理方法，在这个方法中通常可以进行UI更新，并调用completionHandler通知系统已经完成所有操作

@implementation ZViewController

-(void)viewDidLoad {
    [super viewDidLoad];
    
    NSURLSession *session = [self session];
    NSURLSessionDownloadTask *task = [session downloadTaskWithURL:[NSURL URLWithString:@"http://yinyueshiting.baidu.com/data2/music/46729925/13125209147600128.mp3?xcode=10d30604371d01ea5cfa24abb2db23a644b52c81891b10c8"] ];
    
    //开始任务
    [task resume];
}

//后台会话(保证只有一个后台会话)
-(NSURLSession *)session {
    static NSURLSession *session;
    static dispatch_once_t token;
    dispatch_once(&token, ^{
        
        //后台下载
        NSURLSessionConfiguration *configuration = [NSURLSessionConfiguration backgroundSessionConfiguration:@"zang"];
        configuration.discretionary = YES;
        
        //使用配置的NSURLSessionConfiguration获取NSSession，并且设置委托
        session = [NSURLSession sessionWithConfiguration:configuration delegate:self delegateQueue:nil];
    });
    
    return session;
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
      didWriteData:(int64_t)bytesWritten
 totalBytesWritten:(int64_t)totalBytesWritten
totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    float percent = (float)totalBytesWritten/totalBytesExpectedToWrite;
    NSLog(@"%f",percent);
}

- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask
didFinishDownloadingToURL:(NSURL *)location {
    NSLog(@"%s",__func__);
    NSString *dirPath = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) firstObject];
    NSString *path = [dirPath stringByAppendingPathComponent:@"1.mp3"];
    
    NSFileManager *manager = [NSFileManager defaultManager];
    if ([manager fileExistsAtPath:path isDirectory:NO]) {
        [manager removeItemAtPath:path error:nil];
    }
    
    [manager moveItemAtPath:[location path] toPath:path error:nil];
}

//任务结束
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task
didCompleteWithError:(NSError *)error {
    NSLog(@"%s",__func__);
}

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session {
    NSLog(@"%s",__func__);
    
    ZLTAppDelegate *appDelegate = (ZLTAppDelegate *)[UIApplication sharedApplication].delegate;
    void(^handler)() = appDelegate.handler;
    appDelegate.handler = nil;
    handler();
}

@end

2014-11-08 16:35:38.553 IOS网络请求[5149:1303] 0.018046

2014-11-08 16:35:38.554 IOS网络请求[5149:1303] 0.018117

2014-11-08 16:35:38.632 IOS网络请求[5149:3d07] 0.036427

2014-11-08 16:35:38.633 IOS网络请求[5149:3d07] 0.036606

2014-11-08 16:35:38.719 IOS网络请求[5149:4707] 0.054916

2014-11-08 16:35:38.719 IOS网络请求[5149:4707] 0.055094

2014-11-08 16:35:38.813 IOS网络请求[5149:1303] 0.073583

2014-11-08 16:35:38.814 IOS网络请求[5149:1303] 0.092072

2014-11-08 16:35:38.881 IOS网络请求[5149:3d07] 0.110560

2014-11-08 16:35:46.697 IOS网络请求[5149:607] zang:download complete

2014-11-08 16:35:46.701 IOS网络请求[5149:1303] -[ZViewController URLSession:downloadTask:didFinishDownloadingToURL:]

2014-11-08 16:35:46.703 IOS网络请求[5149:1303] -[ZViewController URLSession:task:didCompleteWithError:]

2014-11-08 16:35:46.703 IOS网络请求[5149:1303] -[ZViewController URLSessionDidFinishEventsForBackgroundURLSession:]

执行程序后按下home键，静静等待下载完成后发现虽然程序处于后台，但委托的3个方法均得到了执行