<h2>断点续传原理：
1.断点：记录点击暂停时已经下载的字节数
2.续传：将上次记录的字节数通过HTTP请求头传给服务器，继续下载
</h2>

具体实现：
系统自带的NSURLSessionDownloadTask非常的简单

创建属性

@property(nonatomic, strong) NSURLSessionDownloadTask * task;
@property(nonatomic, strong) NSURLSession * session;
@property(nonatomic, strong) NSData * summate;
// 将player写入属性，用模拟器start项目时必须用属性，否则是没有声音的
@property(nonatomic, strong) AVAudioPlayer * player;

P.s: player为了实现最后下载完音乐资源的播放

先初始化session
写的比较懒，见谅

    self.session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[NSOperationQueue mainQueue]];

遵守代理

@interface ViewController ()<NSURLSessionDownloadDelegate>

开始下载&继续下载

- (IBAction)StartWork:(id)sender {
    
    if (self.task) {
        NSLog(@"继续下载");
        NSLog(@"%lu",(unsigned long)self.sumaDate.length);
        self.task = [self.session downloadTaskWithResumeData:self.sumaDate];
    } else {
        NSLog(@"开始下载");
        self.task=  [self.session downloadTaskWithURL:[NSURL URLWithString:@"http://yinyueshiting.baidu.com/data2/music/244383700/244379959144425526164.mp3?xcode=ee4c4a527b584e3a794b86d808232fc4"]];
    }
    [self.task resume];
}

暂停下载

- (IBAction)PauseWork:(id)sender {
    NSLog(@"暂停下载");
    [self.task cancelByProducingResumeData:^(NSData * _Nullable resumeData) {
        NSLog(@"%lu",(unsigned long)resumeData.length);
        self.sumaDate = [NSData dataWithData:resumeData];
    }];
}

实现主要的两个代理方法

#pragma NSURLSessionDownloadDelegate
// 完成下载（实现音乐播放）
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
    NSLog(@"已下载");
    NSURL * url=[NSURL fileURLWithPath:@"/Users/apple/Desktop/music.mp3"];
    NSFileManager * manager=[NSFileManager defaultManager];
    [manager moveItemAtURL:location toURL:url error:nil];
    dispatch_async(dispatch_get_main_queue(), ^{
        NSData * data=[manager contentsAtPath:@"/Users/apple/Desktop/music.mp3"];
        self.player = [[AVAudioPlayer alloc] initWithData:data error:nil];
        [self.player play];
    });
}
// 监听下载时的状态
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite {
    double downloadProgress = totalBytesWritten / (double)totalBytesExpectedToWrite;
    NSLog(@"%f",downloadProgress);
}