表示头500个字节：bytes=0-499

表示第二个500字节：bytes=500-999

表示最后500个字节：bytes=-500

表示500字节以后的范围：bytes=500-

第一个和最后一个字节：bytes=0-0,-1

同时指定几个范围：bytes=500-600,601-999

#import "ViewController.h"
#import "NSString+Md5.h"
//要下载的文件
#define fileURL @"http://120.25.226.186:32812/resources/videos/minion_01.mp4"

//通过md5加密生成唯一的文件名
#define fileName fileURL.md5String

//用来存储文件总长度的plist,key:是URL md5加密后的文件名 value:文件的长度
#define downFilePlist [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:@"downFile.plist"]

//下载文件的全路径
#define downFileFullPath [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES).firstObject stringByAppendingPathComponent:fileName]

//已经下载文件的长度
#define downFileLength [[[NSFileManager defaultManager]  attributesOfItemAtPath:downFileFullPath error:nil][@"NSFileSize"] integerValue]


@interface ViewController ()<NSURLSessionDataDelegate>
//下载分发任务
@property(nonatomic, strong)NSURLSessionDataTask *dataTask;
//下载使用
@property(nonatomic, strong)NSURLSession  *session;
//输出流
@property(nonatomic, strong)NSOutputStream *outputStream;
//文件的总长度
@property(nonatomic, assign)NSInteger totalLength;
//下载文件的长度
@property(nonatomic, assign)NSInteger downLength;
@end
@implementation ViewController
//重新下载或开始下载
- (IBAction)starDown:(UIButton *)sender {
    [self.dataTask resume];
}
//暂停下载
- (IBAction)pauseDown:(UIButton *)sender {
    [self.dataTask suspend];
}
#pragma mark --- getters and setters
- (NSURLSession *)session{
    if (_session == nil) {
        _session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[[NSOperationQueue alloc] init]];
    }
    return _session;
}
- (NSURLSessionDataTask *)dataTask{
    if (_dataTask == nil) {
        
        //获取文件的总长度
        NSInteger totalLength = [[NSDictionary dictionaryWithContentsOfFile:downFilePlist][fileName] integerValue];
        
        //请求同一个文件，判断文件是否下载过，如果已下载的文件长度和文件长度相等或有总长度说明下载过
        if(totalLength && totalLength == downFileLength){
            NSLog(@"已经下载过");
            return nil;  
        }
        //否则没有下载过或没有下载完，格式符z和整数转换说明符一起使用,表示对应数字是一个size_t值
        //请求这个URL
        //设置Range
        //设置请求头
        NSMutableURLRequest *request  = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:fileURL]];
        NSString *range = [NSString stringWithFormat:@"bytes=%zd-",downFileLength];
        NSLog(@"%@",range);
        [request setValue:range forHTTPHeaderField:@"Range"];
        _dataTask = [self.session dataTaskWithRequest:request];
    }
    return _dataTask;
}
#pragma mark --- NSURLDataDelegate
/**
 92  * 接收到响应的时候调用
 93  */
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
didReceiveResponse:(NSURLResponse *)response
 completionHandler:(void (^)(NSURLSessionResponseDisposition disposition))completionHandler{
    //调用block才能获取到数据
    completionHandler(NSURLSessionResponseAllow);
    //初始化数据流
    self.outputStream = [NSOutputStream outputStreamToFileAtPath:downFileFullPath append:YES];
    [self.outputStream open]; //打开数据流
    
    //获取文件总长度
//    self.totalLength = [response.allHeaderFields[@"Content-Length"] integerValue] + downFileLength;
    self.totalLength = [((NSHTTPURLResponse*)response).allHeaderFields[@"Content-Length"] integerValue] + downFileLength;
    
    // 接收到服务器响应的时候存储文件的总长度到plist,实现多文件下载，先取出字典，给字典赋值最后写入。
    // 错误做法：直接写入文件，会用这次写入的信息覆盖原来所有的信息
    NSMutableDictionary *dic = [NSMutableDictionary dictionaryWithContentsOfFile:downFilePlist];
    
    dic[fileName] = @(self.totalLength);
    [dic writeToFile:downFilePlist atomically:YES];
}
//接收服务器发送的数据
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask
    didReceiveData:(NSData *)data{
    
    //通过输出流写出数据，获取当前下载的长度/总长度，来获取下载进度
    [self.outputStream write:[data bytes] maxLength:data.length];
    self.downLength = downFileLength;
    
    NSLog(@"%f",1.0*self.downLength/self.totalLength);
}
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error{
    
    //关闭输出流，并设置为空，清空任务，一个任务对应一个文件
    [self.outputStream close];
    self.outputStream = nil;
    self.dataTask = nil;
    NSLog(@"下载完