Swift

Question

我一直在关注 Appcoda 的教程：http://www.appcoda.com/background-transfer-service-ios7/ 但写在 swift 中。我遇到了无法在 swift

中工作的这行代码

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

我不对的部分是：

void(^completionHandler)() = appDelegate.backgroundTransferCompletionHandler

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

无法识别 void(^completionHandler)()

我们将不胜感激。

Answer 1

您可能应该处理应用程序委托，因为那是 completionHandler 闭包的定义所在。您可能会将 backgroundTransferCompletionHandler 属性定义为可选的闭包，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?()

Swift - 复制完成处理程序

Swift - Copy completionHandler

background

objective-c

completionhandler

appdelegate