AFNetworking实现程序重新启动时的断点续传

用AFNetworking实现断点续传的功能，研究后，发现AFNetworking虽然支持下载文件的暂停和继续， 但是程序重新启动后再次下载无法进行续传。网上有说可以通过AFDownloadRequestOperation这个AFNetworking的扩展库 来实现重新启动后的续传，但是经过本人测试，这个库在最新的AFNetworking上会报错，无奈之下，参考他的代码，自己实现了一个，在这里分享给大 家。

实现的代码如下：

    //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。

转载自 http://blog.csdn.net/zhaoxy_thu/article/details/21383515