objective-c - Swift - 复制完成处理程序

Question

我一直在关注 Appcoda 的教程:http://www.appcoda.com/background-transfer-service-ios7/但写得很快。我遇到了这行代码，我无法 swift 开始工作

-(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];
                }];
            }
        }
    }];
}

我不能正确的部分是:

void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler

我有 appDelegate.backgroundTransferCompletionHandler 变量，但我不知道如何将它分配给 void(^completionHandler)()。void(^completionHandler)() 无法被 swift 识别。

在此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(delegate)，因为这是 completionHandler 闭包的定义所在。您可能会将 backgroundTransferCompletionHandler 属性定义为一个可选的闭包，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 Swift 再现将获取该 completionHandler 的本地副本，如下所示:

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

let completionHandler = appDelegate.backgroundTransferCompletionHandler

然后调用它:

completionHandler?()