下面是当应用程序处于活动状态(前台或打开)时接收推送通知的代码。UNUserNotificationCenter文档

@available(iOS 10.0, *)
func userNotificationCenter(center: UNUserNotificationCenter, willPresentNotification notification: UNNotification, withCompletionHandler completionHandler: (UNNotificationPresentationOptions) -> Void)
{
     completionHandler([UNNotificationPresentationOptions.Alert,UNNotificationPresentationOptions.Sound,UNNotificationPresentationOptions.Badge])
}

如果您需要访问通知的userInfo，请使用代码:notification.request.content.userInfo

如果你的应用程序处于前台状态，这意味着你正在使用同一个应用程序。所以通常没有必要在顶部显示通知。

但如果你想显示通知在那种情况下你必须创建你的自定义警报视图或自定义视图像Toast或其他东西来向用户显示你已经收到通知。

如果你的应用中有这样的功能，你也可以在顶部显示一个徽章。

如果应用程序在前台运行，iOS不会显示通知横幅/警报。这是故意的。你必须编写一些代码来处理应用程序在前台接收通知的情况。您应该以最合适的方式显示通知(例如，向UITabBar图标添加徽章号，模拟notification Center横幅等)。

iOS 14 +

下面这个答案有一个不同之处，.alert已弃用，使用.banner:

class AppDelegate: NSObject, UIApplicationDelegate, UNUserNotificationCenterDelegate {
    func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil
    ) -> Bool {
        UNUserNotificationCenter.current().delegate = self

        return true
    }

    func userNotificationCenter(
        _ center: UNUserNotificationCenter,
        willPresent notification: UNNotification,
        withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void
    ) {
        completionHandler([.banner, .badge, .sound])
    }
}

正如@Danial Martine所说，iOS不会显示通知横幅/提醒。这是故意的。但如果真要这么做，还有一个办法。我也用同样的方法做到了这一点。

1.从parse FrameWork下载解析框架

2.导入# Import <Parse/Parse.h> .h

3.将以下代码添加到您的didReceiveRemoteNotification方法

 - (void)application:(UIApplication *)application
didReceiveRemoteNotification:(NSDictionary *)userInfo
{
    [PFPush handlePush:userInfo];
}

PFPush会注意如何处理远程通知。如果App在前台，这显示警报，否则它显示在顶部的通知。

在你的应用委托中使用下面的代码

import UIKit
import UserNotifications
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate, UNUserNotificationCenterDelegate {
 var currentToken: String?
 var window: UIWindow?
 func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.
        application.registerForRemoteNotifications()
        let center = UNUserNotificationCenter.current()
        center.requestAuthorization(options: [.alert, .sound, .badge]) { (granted, error) in

            // Enable or disable features based on authorization.
            if granted == true
            {
                print("Allow")
                UIApplication.shared.registerForRemoteNotifications()
            }
            else
            {
                print("Don't Allow")
            }
        }
        UNUserNotificationCenter.current().delegate = self

        return true
    }
func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data){
        let tokenParts = deviceToken.map { data -> String in
            return String(format: "%02.2hhx", data)
        }
        let token = tokenParts.joined()
        currentToken = token  //get device token to delegate variable

    }
 public class var shared: AppDelegate {
        return UIApplication.shared.delegate as! AppDelegate
    }
 func userNotificationCenter(_ center: UNUserNotificationCenter, willPresent notification: UNNotification, withCompletionHandler completionHandler: @escaping (UNNotificationPresentationOptions) -> Void) {
         completionHandler([.alert, .badge, .sound])
    }
}