快速复制completionHandler

Question

我一直在跟踪Appcoda的一个教程：http://www.appcoda.com/background-transfer-service-ios7/，但我使用的是快速编写它。我遇到了这一行代码，我无法快速地工作。

-(void)URLSessionDidFinishEventsForBackgroundURLSession:(NSURLSession *)session{
    AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;

    // Check if all download tasks have been finished.
    [self.session getTasksWithCompletionHandler:^(NSArray *dataTasks, NSArray *uploadTasks, NSArray *downloadTasks) {
        if ([downloadTasks count] == 0) {
            if (appDelegate.backgroundTransferCompletionHandler != nil) {
                // Copy locally the completion handler.
                void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler;

                // Make nil the backgroundTransferCompletionHandler.
                appDelegate.backgroundTransferCompletionHandler = nil;

                [[NSOperationQueue mainQueue] addOperationWithBlock:^{
                    // Call the completion handler to tell the system that there are no other background transfers.
                    completionHandler();

                    // Show a local notification when all downloads are over.
                    UILocalNotification *localNotification = [[UILocalNotification alloc] init];
                    localNotification.alertBody = @"All files have been downloaded!";
                    [[UIApplication sharedApplication] presentLocalNotificationNow:localNotification];
                }];
            }
        }
    }];
}

我不能说对的地方是：

void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler

我有appDelegate.backgroundTransferCompletionHandler变量，但是我不知道如何将它赋值给void(^completionHandler)()。void(^completionHandler)()未被斯威夫特所识别。

在这方面的帮助将不胜感激。

Answer 1

您可能应该处理应用程序委托，因为这是completionHandler闭包的定义所在。您可能会将backgroundTransferCompletionHandler属性定义为一个闭包，它是一个可选的，可能类似于：

var backgroundTransferCompletionHandler: (() -> ())?

func application(application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: () -> Void) {
    backgroundTransferCompletionHandler = completionHandler

    // do whatever else you want (e.g. reinstantiate background session, etc.)
}

然后，您在问题中引用的代码的快速呈现将获取该completionHandler的本地副本，如下所示：

let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate

let completionHandler = appDelegate.backgroundTransferCompletionHandler

然后称之为：

completionHandler?()