我要为iOS应用程序设置后台提取。我愿意:funcapplication(application:UIApplication,didFinishLaunchingWithOptionslaunchOptions:[NSObject:AnyObject]?)->Bool{...application.setMinimumBackgroundFetchInterval(UIApplicationBackgroundFetchIntervalMinimum)...如果我在调试器中检查并打印UIApplicationBackgroundFetchIntervalMinimum，它会显示0-实际