@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
 func applicationWillResignActive(_ application: UIApplication) {
 // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
 // Use this method to pause ongoing tasks, disable timers, and invalidate graphics rendering callbacks. Games should use this method to pause the game.
 }
 func applicationDidEnterBackground(_ application: UIApplication) {
 // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
 // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
 }
 func applicationWillEnterForeground(_ application: UIApplication) {
 // Called as part of the transition from the background to the active state; here you can undo many of the changes made on entering the background.
 }
 func applicationDidBecomeActive(_ application: UIApplication) {
 // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
 }
}

applicationWillResignActive() -> applicationDidEnterBackground()

applicationWillEnterForeground() -> applicationDidBecomeActive()

A.onStart()
A.onResume()

A.onPause()
B.onStart()
B.onResume()
A.onStop()

B.onPause()
A.onStart()
A.onResume()
B.onStop()

A.onPause()
A.onStop()

public class ApplicationListener implements Application.ActivityLifecycleCallbacks {
 private int foregroundCount = 0; // 位于前台的 Activity 的数目
 @Override
 public void onActivityStarted(final Activity activity) {
  if (foregroundCount <= 0) {
   // TODO 这里处理从后台恢复到前台的逻辑
  }
  foregroundCount++;
 }
 @Override
 public void onActivityStopped(Activity activity) {
  foregroundCount--;
  if (foregroundCount <= 0) {
   // TODO 这里处理从前台进入到后台的逻辑
  }
 }
 /*
  * 下面回调，我们都不需要
  */
 @Override
 public void onActivityCreated(Activity activity, Bundle savedInstanceState) {}
 @Override
 public void onActivityResumed(Activity activity) {}
 @Override
 public void onActivityPaused(Activity activity) {}
 @Override
 public void onActivitySaveInstanceState(Activity activity, Bundle outState) {}
 @Override
 public void onActivityDestroyed(Activity activity) {}
}

public class MyApplication extends Application {
 @Override
 public void onCreate() {
  super.onCreate();
  registerActivityLifecycleCallbacks(new ApplicationListener());
 }
}

public class BaseActivity extends AppCompatActivity {
 private static int foregroundCount = 0; // 注意是个静态变量
 @Override
 protected void onStart() {
  super.onStart();
  if (foregroundCount <= 0) {
   // TODO 这里处理从后台恢复到前台的逻辑
  }
  foregroundCount++;
 }
 @Override
 protected void onStop() {
  foregroundCount--;
  if (foregroundCount <= 0) {
   // TODO 这里处理从前台进入到后台的逻辑
  }
  super.onStop();
 }
}