- (void)viewDidLoad {

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  [super viewDidLoad];

  NSLog(@"%@",NSHomeDirectory());

  //取得已下载数据大小

  NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];

  receiveTotal = [[userDefaults objectForKey:@"ReceiveTotal"] doubleValue];

  total = [[userDefaults objectForKey:@"Total"] doubleValue];

  //显示上一次下载的进度

  if (total > 0) {

    CGFloat progress = receiveTotal/total;

    self.progressView.progress = progress;

    self.progressLabel.text = [NSString stringWithFormat:@"%.2f%%", progress * 100];

  }

}

- (IBAction)downLoadClick:(UIButton *)sender {

  if (!isDownLoad) {

    //1.构建URL

    NSURL *url = [NSURL URLWithString:@"http://s.qdcdn.com/lovebizhi/LoveWallpaper4Mac.dmg"];

    //2.构建Request

    NSMutableURLRequest *mRequest = [NSMutableURLRequest requestWithURL:url];

    //断点续传

    if (receiveTotal > 0) {

      //设置续传位置

      NSString *position = [NSString stringWithFormat:@"bytes=%d-",(int)receiveTotal];

      [mRequest setValue:position forHTTPHeaderField:@"Range"];

    }

    //3.发送异步请求

    connection = [NSURLConnection connectionWithRequest:mRequest delegate:self];

    isDownLoad = YES;

    //获取字符串

    NSString *urlStr = url.absoluteString;

    //获取urlStr的最后一部分

    NSString *fileName = [urlStr lastPathComponent];

    //写入文件

    filePath = [NSHomeDirectory() stringByAppendingFormat:@"/Documents/%@",fileName];

    //创建文件

    [[NSFileManager defaultManager] createFileAtPath:filePath contents:nil attributes:nil];

  }

}

//暂停下载

- (IBAction)pauseClick:(UIButton *)sender {

  //取消下载连接

  [connection cancel];

  connection = nil;

  //将缓冲数据写入文件

  [self appendFileData:self.receiveData];

  //在本地保存已下载文件的大小和文件总大小

  NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];

  [userDefaults setObject:@(receiveTotal) forKey:@"ReceiveTotal"];

  [userDefaults setObject:@(total) forKey:@"Total"];

  //将数据同步写入文件

  [userDefaults synchronize];

  isDownLoad = NO;

}

#pragma mark-NSURLConnectionDataDelegate

//服务器响应

- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSHTTPURLResponse *)response{

  self.receiveData = [[NSMutableData alloc]init];

  //判断总大小是否为0，如果为0,说明从起始位置开始下，获取文件总大小

  if (total == 0) {

    //获取所有的响应头

    NSDictionary *fields = response.allHeaderFields;

    NSLog(@"fields is %@",fields);

    //获取数据的总大小

    NSNumber *length = [fields objectForKey:@"Content-Length"];

    total = [length doubleValue];

    NSLog(@"total is %.2f",total);

  }

}

//接收数据

- (void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{

  [self.receiveData appendData:data];

  receiveTotal += data.length;

  //计算进度

  double progress = receiveTotal / total;

  //刷新UI

  self.progressView.progress = progress;

  self.progressLabel.text = [NSString stringWithFormat:@"%.2f%%",progress * 100];

  //判断缓冲的数据是否大于500KB

  if (self.receiveData.length >= 500 * 1024) {

    //写入数据

    [self appendFileData:self.receiveData];

    //清空

    self.receiveData.data = nil;

  }

}

//数据传输完成

- (void)connectionDidFinishLoading:(NSURLConnection *)connection{

  //将最后一个缓冲文件写入

  if (self.receiveData.length < 500 * 1024) {

    //写入数据

    [self appendFileData:self.receiveData];

    //清空

    self.receiveData.data = nil;

  }

  _progressLabel.text = @"下载完成";

  self.progressView.progress = 0;

  isDownLoad = NO;

}

- (void)appendFileData:(NSData *)data{

  if (data.length == 0) {

    return;

  }

  //使用NSFileHandle写文件，此文件必须已经创建，NSFileHandle不会创建文件

  NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:filePath];

  //将数据插入到写入点

  [fileHandle seekToEndOfFile];

  [fileHandle writeData:data];

  //关闭文件，确保写入完成

  [fileHandle closeFile];

}

- (void)didReceiveMemoryWarning {

  [super didReceiveMemoryWarning];

  // Dispose of any resources that can be recreated.

}

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文章题目：网络通信中关于请求数据、断点续传和写入本地文件-创新互联
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