simple-email/app/src/main/java/eu/faircode/email/ActivityBase.java

package eu.faircode.email;

/*
    This file is part of Safe email.

    Safe email is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.

    NetGuard is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with NetGuard.  If not, see <http://www.gnu.org/licenses/>.

    Copyright 2018 by Marcel Bokhorst (M66B)
*/

import android.content.SharedPreferences;
import android.content.res.Configuration;
import android.os.Bundle;
import android.preference.PreferenceManager;
import android.util.Log;

import androidx.appcompat.app.AppCompatActivity;

abstract class ActivityBase extends AppCompatActivity implements SharedPreferences.OnSharedPreferenceChangeListener {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Log.i(Helper.TAG, "Create " + this.getClass().getName());
        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
        String theme = prefs.getString("theme", "light");
        setTheme("light".equals(theme) ? R.style.AppThemeLight : R.style.AppThemeDark);
        PreferenceManager.getDefaultSharedPreferences(this).registerOnSharedPreferenceChangeListener(this);
        super.onCreate(savedInstanceState);
    }

    @Override
    protected void onResume() {
        Log.i(Helper.TAG, "Resume " + this.getClass().getName());
        super.onResume();
    }

    @Override
    protected void onPause() {
        Log.i(Helper.TAG, "Pause " + this.getClass().getName());
        super.onPause();
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        Log.i(Helper.TAG, "Config " + this.getClass().getName());
        super.onConfigurationChanged(newConfig);
    }

    @Override
    protected void onDestroy() {
        Log.i(Helper.TAG, "Destroy " + this.getClass().getName());
        PreferenceManager.getDefaultSharedPreferences(this).unregisterOnSharedPreferenceChangeListener(this);
        super.onDestroy();
    }

    @Override
    public void onSharedPreferenceChanged(SharedPreferences prefs, String key) {
        Log.i(Helper.TAG, "Preference " + key + "=" + prefs.getAll().get(key));
        if ("theme".equals(key))
            recreate();
    }
}