使用系统类NSURLSessionDownloadTask实现断点续传（音乐视频下载等）

断点续传原理：

#pragma mark - 开始
- (IBAction)startAction:(id)sender {
}

#pragma mark - 暂停
- (IBAction)stopAction:(id)sender {
}

#import <AVFoundation/AVFoundation.h>

@interface ViewController ()

@property (nonatomic, strong) NSURLSessionDownloadTask *downloadTask;   // 下载任务
@property (nonatomic, strong) NSURLSession *session;    // 网络
@property (nonatomic, strong) NSData *downloadData; // 下载的数据
@property(nonatomic, strong) AVAudioPlayer * player; // 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 player是为了下载完成后播放音乐

@end

- (void)viewDidLoad {
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
self.session = [NSURLSession sessionWithConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration] delegate:self delegateQueue:[NSOperationQueue mainQueue]];
}

#pragma mark - 开始
- (IBAction)startAction:(id)sender {
if (self.downloadTask) {
NSLog(@"继续下载");
NSLog(@"%lu", self.downloadData.length);
// 把存储的下载数据给通过网络传递给服务器继续下载
self.downloadTask = [self.session downloadTaskWithResumeData:self.downloadData];
}else{
NSLog(@"开始下载");
self.downloadTask = [self.session downloadTaskWithURL:[NSURL URLWithString:@"http://yinyueshiting.baidu.com/data2/music/244383700/244379959144425526164.mp3?xcode=ee4c4a527b584e3a794b86d808232fc4"]];
}

// 开始任务
[self.downloadTask resume];
}

#pragma mark - 暂停
- (IBAction)stopAction:(id)sender {
NSLog(@"暂停下载");
[self.downloadTask cancelByProducingResumeData:^(NSData * _Nullable resumeData) {
// 把暂停下载的时候的数据存储到全局变量中
NSLog(@"%lu", resumeData.length);
self.downloadData = resumeData;
}];
}

#pragma mark - 完成下载实现音乐播放
- (void)URLSession:(NSURLSession *)session downloadTask:(NSURLSessionDownloadTask *)downloadTask didFinishDownloadingToURL:(NSURL *)location {
NSLog(@"已下载");

// 存储到本地
NSURL * url=[NSURL fileURLWithPath:@"/Users/jiayuanfa/Desktop/music.mp3"];
NSFileManager * manager=[NSFileManager defaultManager];
[manager moveItemAtURL:location toURL:url error:nil];

// 播放音乐
dispatch_async(dispatch_get_main_queue(), ^{
NSData * data=[manager contentsAtPath:@"/Users/jiayuanfa/Desktop/music.mp3"];
self.player = [[AVAudioPlayer alloc] initWithData:data error:nil];
[self.player play];
});
}

#pragma mark - 监听下载 进度条等等
- (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);
}