// 所需要下载的文件的URL
＃define FileURL @"http://apk.r1.market.hiapk.com/data/upload/apkres/2016/3_23/10/com.dailyyoga.cn_102411.apk"

// 文件名（沙盒中的文件名）
＃define Filename [self md5String:FileURL]

// 文件的存放路径（caches）
＃define FileFullpath [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:Filename]

// 存储文件总长度的文件路径（caches）
＃define TotalLengthFullpath [[NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) lastObject] stringByAppendingPathComponent:@"totalLength.plist"]

// 文件的已下载长度
＃define DownloadLength [[[NSFileManager defaultManager] attributesOfItemAtPath:FileFullpath error:nil][NSFileSize] integerValue]

＃import "ViewController.h"
＃import "UIImageView+WebCache.h"
＃import <CommonCrypto/CommonDigest.h>
＃import <CommonCrypto/CommonHMAC.h>

@interface ViewController () <NSURLSessionDataDelegate>
/** 下载任务 */
@property (nonatomic, strong) NSURLSessionDataTask *task;
/** session */
@property (nonatomic, strong) NSURLSession *session;
/** 写文件的流对象 */
@property (nonatomic, strong) NSOutputStream *stream;
/** 文件的总长度 */
@property (nonatomic, assign) NSInteger totalLength;
@end

@implementation ViewController

- (NSURLSession *)session
{
    if (!_session) {
        _session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[[NSOperationQueue alloc] init]];
    }
    return _session;
}

- (NSOutputStream *)stream
{
    if (!_stream) {
        _stream = [NSOutputStream outputStreamToFileAtPath:FileFullpath append:YES];
    }
    return _stream;
}

- (NSURLSessionDataTask *)task
{
    if (!_task) {
        NSInteger totalLength = [[NSDictionary dictionaryWithContentsOfFile:TotalLengthFullpath][Filename] integerValue];
        if (totalLength && DownloadLength == totalLength) {
            NSLog(@"----文件已经下载过了");
            return nil;
        }
        
        // 创建请求
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:FileURL]];
        
        // 设置请求头
        // Range : bytes=xxx-xxx
        NSString *range = [NSString stringWithFormat:@"bytes=%zd-", DownloadLength];
        [request setValue:range forHTTPHeaderField:@"Range"];
        
        // 创建一个Data任务
        _task = [self.session dataTaskWithRequest:request];
    }
    return _task;
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    NSLog(@"%@", FileFullpath);
}

/**
 * 开始下载
 */
- (IBAction)start:(id)sender {
    // 启动任务
    [self.task resume];
}

/**
 * 暂停下载
 */
- (IBAction)pause:(id)sender {
    [self.task suspend];
}

＃pragma mark - <NSURLSessionDataDelegate>
/**
 * 1.接收到响应
 */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSHTTPURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler
{
    // 打开流
    [self.stream open];
    
    // 获得服务器这次请求 返回数据的总长度
    self.totalLength = [response.allHeaderFields[@"Content-Length"] integerValue] + DownloadLength;
    
    // 存储总长度
    NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithContentsOfFile:TotalLengthFullpath];
    if (dict == nil) dict = [NSMutableDictionary dictionary];
    dict[Filename] = @(self.totalLength);
    [dict writeToFile:TotalLengthFullpath atomically:YES];
    
    // 接收这个请求，允许接收服务器的数据
    completionHandler(NSURLSessionResponseAllow);
}

/**
 * 2.接收到服务器返回的数据（这个方法可能会被调用N次）
 */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data
{
    // 写入数据
    [self.stream write:data.bytes maxLength:data.length];
    
    // 下载进度
    NSLog(@"%f", 1.0 * DownloadLength / self.totalLength);
}

/**
 * 3.请求完毕（成功\失败）
 */
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error
{
    // 关闭流
    [self.stream close];
    self.stream = nil;
    
    // 清除任务
    self.task = nil;
}

- (NSString *)md5String:(NSString *)str
{
    const char *string = str.UTF8String;
    int length = (int)strlen(string);
    unsigned char bytes[CC_MD5_DIGEST_LENGTH];
    CC_MD5(string, length, bytes);
    return [self stringFromBytes:bytes length:CC_MD5_DIGEST_LENGTH];
}

- (NSString *)stringFromBytes:(unsigned char *)bytes length:(int)length
{
    NSMutableString *mutableString = @"".mutableCopy;
    for (int i = 0; i < length; i++)
        [mutableString appendFormat:@"%02x", bytes[i]];
    return [NSString stringWithString:mutableString];
}

@end