@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; // 位于前臺的 Activity 的數(shù)目
 @Override
 public void onActivityStarted(final Activity activity) {
  if (foregroundCount <= 0) {
   // TODO 這里處理從后臺恢復(fù)到前臺的邏輯
  }
  foregroundCount++;
 }
 @Override
 public void onActivityStopped(Activity activity) {
  foregroundCount--;
  if (foregroundCount <= 0) {
   // TODO 這里處理從前臺進入到后臺的邏輯
  }
 }
 /*
  * 下面回調(diào)，我們都不需要
  */
 @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; // 注意是個靜態(tài)變量
 @Override
 protected void onStart() {
  super.onStart();
  if (foregroundCount <= 0) {
   // TODO 這里處理從后臺恢復(fù)到前臺的邏輯
  }
  foregroundCount++;
 }
 @Override
 protected void onStop() {
  foregroundCount--;
  if (foregroundCount <= 0) {
   // TODO 這里處理從前臺進入到后臺的邏輯
  }
  super.onStop();
 }
}