ios - 传递一个委托(delegate)作为 NSURLSession 的参数

Question

我有一个名为 RestService 的类，我在我的整个应用程序中使用它来执行对 Web 服务的多个同步请求。我向此类添加了一个新方法来执行异步请求，我想再次在我的整个应用程序中重复使用该请求。这是该新方法的代码:

- (void)backgroundExecutionOfService:(NSString *)serviceName 
                      withParameters:(NSDictionary *)parameters
                              inView:(UIView *)view
                        withDelegate:(UIViewController *)delegate
{
    NSString *serviceUrl = @"http://MyWebServer/public/api/clients/5";
    NSURLSessionConfiguration *sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
    sessionConfig.allowsCellularAccess          = YES;
    sessionConfig.timeoutIntervalForRequest     = 10;
    sessionConfig.timeoutIntervalForResource    = 10;
    sessionConfig.HTTPMaximumConnectionsPerHost =  1;

    NSURLSession *session;
    session = [NSURLSession sessionWithConfiguration:sessionConfig
                                            delegate:delegate
                                       delegateQueue:nil];

    NSURLSessionDownloadTask *getFileTask;
    getFileTask = [session downloadTaskWithURL:[NSURL URLWithString:serviceUrl]];
    [getFileTask resume];
}

但 XCode 警告我使用该参数作为委托(delegate)(将 UIViewController * __strong' 发送到不兼容类型 'id< NSURLSessionDelegate > _Nullable' 的参数)。我确保我作为参数发送的 View Controller 已在 .h 中声明了 < NSURLSessionDelegate >，并且我在 ViewController 的实现文件中创建了委托(delegate)方法。

- (void)URLSession:(NSURLSession *)session didBecomeInvalidWithError:(nullable NSError *)error{
    NSLog(@"Became invalid with error: %@", [error localizedDescription]);
}

- (void)URLSession:(NSURLSession *)session didReceiveChallenge:(NSURLAuthenticationChallenge *)challenge
 completionHandler:(void (^)(NSURLSessionAuthChallengeDisposition disposition, NSURLCredential * __nullable credential))completionHandler{
    NSLog(@"Received challenge");
}

- (void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
    NSLog(@"Did finish events for session: %@", session);
}

应用程序不会崩溃，但永远不会调用委托(delegate)方法。同步 Web 服务按预期工作。

Answer 1

发生这种情况是因为 UIViewController 类不符合 NSURLSessionDelegate 协议(protocol)。要解决该差异，只需将方法的签名更改为如下所示:

- (void)backgroundExecutionOfService:(NSString *)serviceName withParameters:(NSDictionary *)parameters inView:(UIView *)view withDelegate:(id<NSURLSessionDelegate>)delegate{
//... your code
}

并“阅读代表的基础知识。”