androidApp/app/src/main/java/com/localtransfer/SettingsActivity.java

package com.localtransfer;

import android.Manifest;
import android.app.Activity;
import android.content.Intent;
import android.content.SharedPreferences;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.MenuItem;

import androidx.appcompat.app.ActionBar;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import androidx.preference.Preference;
import androidx.preference.PreferenceFragmentCompat;
import androidx.preference.PreferenceManager;
import androidx.preference.SwitchPreference;

public class SettingsActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.settings_activity);
        getSupportFragmentManager()
                .beginTransaction()
                .replace(R.id.settings, new SettingsFragment())
                .commit();
        ActionBar actionBar = getSupportActionBar();
        if (actionBar != null) {
            actionBar.setDisplayHomeAsUpEnabled(true);
        }
    }

    public static class SettingsFragment extends PreferenceFragmentCompat {

        private final static int REQUEST_DIRECTORY_PICKER = 4000;
        static Preference directory;

        @Override
        public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
            setPreferencesFromResource(R.xml.root_preferences, rootKey);

            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());

            directory = findPreference("shared_storage");
            directory.setSummary(prefs.getString("shared_storage", null));

            SwitchPreference shared;
            shared = findPreference("use_shared_storage");
            if(shared.isChecked()) {
                if(checkAndRequestPermissions())
                    directory.setVisible(true);
                else
                    shared.setChecked(false);
            }
            else
                directory.setVisible(false);

            shared.setOnPreferenceClickListener(preference -> {
                if(shared.isChecked()) {
                    if(checkAndRequestPermissions())
                        directory.setVisible(true);
                    else
                        shared.setChecked(false);
                }
                else
                    directory.setVisible(false);
                return false;
            });
            directory.setOnPreferenceClickListener(new Preference.OnPreferenceClickListener() {
                @Override
                public boolean onPreferenceClick(Preference preference) {
                    Intent intent = new Intent(Intent.ACTION_OPEN_DOCUMENT_TREE);
                    intent.addCategory(Intent.CATEGORY_DEFAULT);
                    startActivityForResult(intent, REQUEST_DIRECTORY_PICKER);
                    return false;
                }
            });
        }

        private boolean checkAndRequestPermissions() {

            if (ContextCompat.checkSelfPermission(
                    getActivity(), Manifest.permission.READ_EXTERNAL_STORAGE) ==
                    PackageManager.PERMISSION_GRANTED) {
                Log.d("READ_EXTERNAL_STORAGE", "already granted");
                return true;
            }
            else {
                ActivityCompat.requestPermissions(getActivity(), new String[] {android.Manifest.permission.READ_EXTERNAL_STORAGE}, MainActivity.REQUEST_ID_READ_EXTERNAL_STORAGE);
            }
            return false;
        }
        @Override
        public void onRequestPermissionsResult(int requestCode,
                                               String permissions[],
                                               int[] grantResults) {
            switch (requestCode) {
                case MainActivity.REQUEST_ID_READ_EXTERNAL_STORAGE:
                    if (grantResults.length > 0 &&
                            grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                        Log.d("READ_EXTERNAL_STORAGE", "granted");
                    }  else {
                        Log.d("READ_EXTERNAL_STORAGE", "No granted");
                    }
                    return;
            }
        }

        @Override
        public void onActivityResult(int requestCode, int resultCode, Intent data) {
            super.onActivityResult(requestCode, resultCode, data);
            switch (requestCode) {
                case REQUEST_DIRECTORY_PICKER:
                    if (resultCode == Activity.RESULT_OK) {
                        String currentPath = data.getExtras().getString("data");
                        if(currentPath.contains(Environment.getExternalStorageDirectory() + "/")) {
                            String save_location = currentPath.replace(Environment.getExternalStorageDirectory() + "/", "");
                            Log.d("Path", save_location);
                            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(getActivity());
                            prefs.edit().putString("shared_storage", save_location).apply();
                            directory.setSummary(save_location);
                        }
                    }
                    break;
            }
        }
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        Transfer.parameter(this);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();

        if (id == android.R.id.home) {
            onBackPressed();  return true;
        }

        return super.onOptionsItemSelected(item);
    }
}