AFNetworking实现程序重新启动时的断点续传

//获取已下载的文件大小
- (unsigned long long)fileSizeForPath:(NSString *)path {
    signed long long fileSize = 0;
    NSFileManager *fileManager = [NSFileManager new]; // default is not thread safe
    if ([fileManager fileExistsAtPath:path]) {
        NSError *error = nil;
        NSDictionary *fileDict = [fileManager attributesOfItemAtPath:path error:&error];
        if (!error && fileDict) {
            fileSize = [fileDict fileSize];
        }
    }
    return fileSize;
}
//开始下载
- (void)startDownload {
    NSString *downloadUrl = @"http://www.xxx.com/xxx.zip";
    NSString *cacheDirectory = [NSSearchPathForDirectoriesInDomains(NSCachesDirectory, NSUserDomainMask, YES) objectAtIndex:0];
    NSString *downloadPath = [cacheDirectory stringByAppendingPathComponent:@"xxx.zip"];
    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:downloadUrl]];
    //检查文件是否已经下载了一部分
    unsigned long long downloadedBytes = 0;
    if ([[NSFileManager defaultManager] fileExistsAtPath:downloadPath]) {
    //获取已下载的文件长度
        downloadedBytes = [self fileSizeForPath:downloadPath];
        if (downloadedBytes > 0) {
            NSMutableURLRequest *mutableURLRequest = [request mutableCopy];
            NSString *requestRange = [NSString stringWithFormat:@"bytes=%llu-", downloadedBytes];
            [mutableURLRequest setValue:requestRange forHTTPHeaderField:@"Range"];
            request = mutableURLRequest;
        }
    }
    //不使用缓存，避免断点续传出现问题
    [[NSURLCache sharedURLCache] removeCachedResponseForRequest:request];
    //下载请求
    AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    //下载路径
    operation.outputStream = [NSOutputStream outputStreamToFileAtPath:downloadPath append:YES];
    //下载进度回调
    [operation setDownloadProgressBlock:^(NSUInteger bytesRead, long long totalBytesRead, long long totalBytesExpectedToRead) {
        //下载进度
        float progress = ((float)totalBytesRead + downloadedBytes) / (totalBytesExpectedToRead + downloadedBytes);
    }];
    //成功和失败回调
    [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {

    } failure:^(AFHTTPRequestOperation *operation, NSError *error) {

    }];
    [operation start];
}

需要注意的是，此种写法仅适用于下载zip包，因为下载其他格式的文件有可能出现数据过多的情况。当文件已经下载完成时，再次调用该函数，无法判断文件是否已经下载完整，于是会再次下载，此时服务器会报416错，同时返回也会输出到文件中，使得文件大小异常。但是zip格式不受影响。