使用NSURLSession封装的断点续传

伴随着iOS的发展，iOS系统的网络请求类在逐渐的完善，iOS9以后弃用了NSURLConnection，用NSURLSession来封装网络请求。接下来，就NSURLSession中常用的类来做下解读。

NSURLSession会涉及两个重要的类NSURLSessionConfiguration和NSURLSessionTask

使用NSURLSession创建网络请求分为两步：

• 通过NSURLSession实例创建task（NSURLSessionTask）
• 启动task

NSURLSessionTask可以简单理解为任务，如下载，上传等。
我们一般会使用NSURLSessionTask的子类：
NSURLSessionDataTask NSURLSessionUploadTask NSURLSessionDownloadTask
NSURLSessionDataTask 从字面上看是与数据相关的任务，其实完全可以胜任下载和上传的任务，平时程序中用的最多。
以下就用下载作为例子讲解：
1.初始化session

 #pragma mark - init 
- (instancetype)initWithURL:(NSURL *)url downloadPath:(NSString *)pathToDL delegate:(id<YYDownloaderDelegate>)delegateOrNil
{
    if (self = [super init]) {
        self.downloadURL = url;
        self.delegate = delegateOrNil;
        self.pathToDownloadDirectory = pathToDL;
        self.fileRequest = [[NSMutableURLRequest alloc] initWithURL:url cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:kDefaultRequestTimeout];
        self.sessionConfiguration = [NSURLSessionConfiguration defaultSessionConfiguration];
        self.sessionConfiguration.timeoutIntervalForRequest = kDefaultRequestTimeout;
        self.operationQueue = [[NSOperationQueue alloc] init];
        self.operationQueue.maxConcurrentOperationCount = 5;
        self.session = [NSURLSession sessionWithConfiguration:self.sessionConfiguration delegate:self delegateQueue:self.operationQueue];
    }
    return self;
}

2.开始下载

#pragma mark - NSOperation override
- (void)start
{
    if (![NSURLConnection canHandleRequest:self.fileRequest]) {
        NSError *error = [NSError errorWithDomain:YYDownloadErrorDomain code:YYDownloadErrorInvalidURL userInfo:@{NSLocalizedDescriptionKey:[NSString stringWithFormat:@"Invaild URL provided: %@",self.fileRequest.URL]}];
        [self notifyFromCompletionWithError:error pathToFile:nil];
        return;
    }
    
    NSFileManager *fm = [NSFileManager defaultManager];
    //创建一个下载目录
    NSError *err = nil;
    if (![fm createDirectoryAtPath:self.pathToDownloadDirectory withIntermediateDirectories:YES attributes:nil error:&err]) {
        [self notifyFromCompletionWithError:err pathToFile:nil];
        return;
    }
    //检查文件是否已经存在，如果存在设置HTTP `bytes` header
    if (![fm fileExistsAtPath:self.pathToFile]) {
        [fm createFileAtPath:self.pathToFile contents:nil attributes:nil];
    }else{
        uint64_t fileSize = [[fm attributesOfItemAtPath:self.pathToFile error:nil] fileSize];
        NSString *range = [NSString stringWithFormat:@"byte=%lld-",fileSize];
        [self.fileRequest setValue:range forHTTPHeaderField:@"Range"];
        self.receivedDataLength += fileSize;
    }
    
    self.fileHandle = [NSFileHandle fileHandleForWritingAtPath:self.pathToFile];
    self.receivedDataBuffer = [[NSMutableData alloc] init];
    self.samplesOfDownloadedBytes = [NSMutableArray array];
    self.dataTask = [self.session dataTaskWithRequest:self.fileRequest];
    if (self.dataTask) {
        [self willChangeValueForKey:@"isExecuting"];
        self.state = YYDownloadStateDownloading;
        [self didChangeValueForKey:@"isExecuting"];
        
        [self.fileHandle seekToEndOfFile];
        //调用resume，才开始请求
        [self.dataTask resume];
    }
}

3.NSURLSessionDataTask代理方法处理

// 1.接收到服务器的响应
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveResponse:(NSURLResponse *)response completionHandler:(void (^)(NSURLSessionResponseDisposition))completionHandler {
    // 允许处理服务器的响应，才会继续接收服务器返回的数据
    completionHandler(NSURLSessionResponseAllow);
    //计算期望接收数据长度
    self.expectedDataLength = self.receivedDataLength + [response expectedContentLength];
    //写数据准备工作
}

// 2.接收到服务器的数据（可能调用多次）
- (void)URLSession:(NSURLSession *)session dataTask:(NSURLSessionDataTask *)dataTask didReceiveData:(NSData *)data {
    // 处理每次接收的数据
 [self.receivedDataBuffer appendData:data];
    self.receivedDataLength += data.length;
    if (self.receivedDataBuffer && self.receivedDataBuffer.length > kBufferSize && [self isExecuting]) {
        //数据写入
        [self.fileHandle writeData:self.receivedDataBuffer];
        //清缓存
        [self.receivedDataBuffer setData:[NSData data]];
    }
    dispatch_async(dispatch_get_main_queue(), ^{
        //下载进度block或代理方法（二选一）
        if (self.progressBlock) {
            self.progressBlock(self.receivedDataLength, self.expectedDataLength, self.remainingTime, self.progress);
        }
        if ([self.delegate respondsToSelector:@selector(download:didReceiveData:onTotal:progress:)]) {
            [self.delegate download:self
                     didReceiveData:self.receivedDataLength
                            onTotal:self.expectedDataLength
                           progress:self.progress];
        }
    });
}

// 3.请求成功或者失败（如果失败，error有值）
- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didCompleteWithError:(NSError *)error {
    // 请求完成,成功或者失败的处理
}

ps这里只抽取一些方法做讲解，详细请进入 https://github.com/caoyuanyuan/YYDownload.git查看。