有什么方法可以检查iOS应用是否在后台？

[[UIApplication sharedApplication] applicationState]

UIApplicationState state = [[UIApplication sharedApplication] applicationState];
if (state == UIApplicationStateBackground || state == UIApplicationStateInactive)
{
   //Do checking here.
}

    let state = UIApplication.shared.applicationState
    if state == .background {
        print(\"App in Background\")
    }

let state = UIApplication.sharedApplication().applicationState
if state == .Background {
    print(\"App in Background\")
}

AppDelegate

- (void)applicationDidBecomeActive:(UIApplication *)application {
    NSLog(@\"app is actvie now\");
}


- (void)applicationWillResignActive:(UIApplication *)application {
    NSLog(@\"app is not actvie now\");
}

let state = UIApplication.shared.applicationState
    if state == .background {
        print(\"App in Background\")
        //MARK: - if you want to perform come action when app in background this will execute 
        //Handel you code here
    }
    else if state == .foreground{
        //MARK: - if you want to perform come action when app in foreground this will execute 
        //Handel you code here
    }

let appstate = UIApplication.shared.applicationState
        switch appstate {
        case .active:
            print(\"the app is in active state\")
        case .background:
            print(\"the app is in background state\")
        case .inactive:
            print(\"the app is in inactive state\")
        default:
            print(\"the default state\")
            break
        }

import UIKit

extension UIApplication {
    var isBackground: Bool {
        return UIApplication.shared.applicationState == .background
    }
}

let myAppIsInBackground = UIApplication.shared.isBackground

active

inactive

background