如何在切换夜间模式时从SettingActivity重建MainActivity？

Hey there! I get exactly what you're dealing with—you toggle night mode in your SettingActivity, but when you head back to MainActivity, the layout doesn't update because onCreate() isn't triggered. Let's fix this with a few straightforward approaches:

Why this happens #

When you call AppCompatDelegate.setDefaultNightMode(), the current activity (in this case, SettingActivity) will automatically recreate itself to apply the theme change. But your MainActivity is sitting in the background, so it doesn't get the memo to refresh. We need to manually tell it to rebuild.

Approach 1: Use Broadcasts to Trigger Recreate #

This method sends a "heads-up" to MainActivity whenever the night mode changes, prompting it to rebuild.

1. Register a Broadcast Receiver in MainActivity
   Add this code to your MainActivity to listen for the mode change broadcast:

   private BroadcastReceiver nightModeUpdateReceiver = new BroadcastReceiver() {
       @Override
       public void onReceive(Context context, Intent intent) {
           // Rebuild the activity to apply new theme
           recreate();
       }
   };
   
   @Override
   protected void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);
   
       // Register the receiver with a custom action
       IntentFilter filter = new IntentFilter("com.yourapp.NIGHT_MODE_UPDATED");
       registerReceiver(nightModeUpdateReceiver, filter);
   }
   
   @Override
   protected void onDestroy() {
       super.onDestroy();
       // Don't forget to unregister to avoid memory leaks
       unregisterReceiver(nightModeUpdateReceiver);
   }
   

2. Send Broadcast from SettingActivity
   After toggling the mode, send the broadcast before finishing the activity:

   // Toggle night mode using your ApplicationClass method
   ApplicationClass.setNightMode(); // or setDayMode()
   
   // Send broadcast to notify MainActivity
   Intent updateIntent = new Intent("com.yourapp.NIGHT_MODE_UPDATED");
   sendBroadcast(updateIntent);
   
   // Go back to MainActivity
   finish();
   

Approach 2: Use Activity Result API (AndroidX) #

If you're using AndroidX, this is a cleaner alternative to startActivityForResult.

1. Register Launcher in MainActivity
   When opening SettingActivity, use this launcher to listen for a result:

   private ActivityResultLauncher<Intent> settingsLauncher = registerForActivityResult(
       new ActivityResultContracts.StartActivityForResult(),
       result -> {
           // If we get a successful result, rebuild MainActivity
           if (result.getResultCode() == RESULT_OK) {
               recreate();
           }
       }
   );
   
   // Call this when you want to open SettingActivity
   public void openSettings() {
       settingsLauncher.launch(new Intent(this, SettingActivity.class));
   }
   

2. Set Result in SettingActivity
   After toggling mode, set a result and finish:

   ApplicationClass.setNightMode(); // or setDayMode()
   setResult(RESULT_OK);
   finish();
   

Approach 3: Check Mode in onResume() #

This simpler method checks if the night mode has changed every time MainActivity comes back to the foreground.

Add this to your MainActivity:

private int lastNightMode;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    // Save initial night mode state
    lastNightMode = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
}

@Override
protected void onResume() {
    super.onResume();
    // Check current night mode
    int currentNightMode = getResources().getConfiguration().uiMode & Configuration.UI_MODE_NIGHT_MASK;
    
    // If mode changed, rebuild activity
    if (currentNightMode != lastNightMode) {
        lastNightMode = currentNightMode;
        recreate();
    }
}

All these methods will make sure MainActivity rebuilds and applies the new night mode theme. Your existing ApplicationClass code for setting night mode is totally correct—we just needed to trigger the refresh for the background activity.

内容的提问来源于stack exchange，提问作者Faisal Shaikh