断点下载

@interface ViewController ()<NSURLConnectionDataDelegate>
{
    // 文件的总大小
    long long _total;
    // 当前下载
    long long _current;
}

// 文件句柄
@property (nonatomic,strong)NSFileHandle *fileHandle;
// 连接对象
@property (nonatomic,strong)NSURLConnection *conn;
@property (weak, nonatomic) IBOutlet UIProgressView *progress;
@property (weak, nonatomic) IBOutlet UIButton *button;
@end

@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    self.progress.progress = 0;
    // Do any additional setup after loading the view, typically from a nib.
}

- (IBAction)buttonAction:(UIButton *)sender {
    if ([[sender titleForState:UIControlStateNormal]isEqualToString:@"开始"]) {
        [sender setTitle:@"暂停" forState:(UIControlStateNormal)];
        // 请求
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:kUrl]];
        // 设置请求参数
        NSString *rang = [NSString stringWithFormat:@"byte:%lld",_current];
        NSLog(@"request:%@end",request);
        [request setValue:rang forHTTPHeaderField:@"Range"];
         NSLog(@"request:%@end",request);
        // 建立连接
        self.conn = [NSURLConnection connectionWithRequest:request delegate:self];
        // 开始请求
        [_conn start];
    }else{
        [sender setTitle:@"开始" forState:(UIControlStateNormal)];
        // 取消
        [_conn cancel];
    }
}

- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark---delegate
// 收到响应
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{
    // 获取总文件大小(总文件的大小在响应里面可以获得)
    _total = response.expectedContentLength;
    // 创建文件
    NSFileManager *manager = [NSFileManager defaultManager];
    NSString *cachesPath  = NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES)[0];
    NSString *filePath = [cachesPath stringByAppendingPathComponent:@"music.mp3"];
    NSLog(@"%@",cachesPath);
    // 创建文件
    [manager createFileAtPath:filePath contents:nil attributes:nil];
    // 让文件句柄指向文件
    self.fileHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];
}
// 接收到数据
- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    // 计算当前总大小
    _current += data.length;
    // 更新 （在子线程中不能更新UI）
    [self performSelectorOnMainThread:@selector(updateProgress) withObject:self waitUntilDone:YES];
    // 指向文件末尾
    [self.fileHandle seekToEndOfFile];
    // 写数据
    [self.fileHandle writeData:data];
}
// 结束
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSLog(@"下载完毕");
}
// 更新progress
- (void)updateProgress
{
    self.progress.progress = (long double)_current / (long double)_total;
}

@end