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，但是在进行了一番研究之后，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：

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//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。