permission handler implemented

APED/app/src/main/AndroidManifest.xml

@@ -2,6 +2,9 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.example.aped">
 
+    <!-- permission to read and write the XML file -->
+    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
+
     <application
         android:allowBackup="true"
         android:icon="@mipmap/ic_launcher"

APED/app/src/main/java/com/example/aped/MainActivity.java

@@ -1,6 +1,9 @@
 package com.example.aped;
 
+import android.Manifest;
+import android.app.AlertDialog;
 import android.os.Bundle;
+import android.util.Log;
 import android.view.MenuItem;
 
 import android.view.Menu;
@@ -9,10 +12,11 @@ import android.widget.Toast;
 import com.example.aped.communication.IIO;
 import com.example.aped.utils.IXML;
 import com.example.aped.utils.TestXML;
-import com.example.aped.utils.XMLHandler;
 import com.google.android.material.navigation.NavigationView;
 
 import androidx.annotation.NonNull;
+import androidx.core.app.ActivityCompat;
+import androidx.core.content.ContextCompat;
 import androidx.navigation.NavController;
 import androidx.navigation.Navigation;
 import androidx.navigation.ui.AppBarConfiguration;
@@ -21,7 +25,11 @@ import androidx.drawerlayout.widget.DrawerLayout;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.appcompat.widget.Toolbar;
 
+import static android.content.pm.PackageManager.PERMISSION_GRANTED;
+
 public class MainActivity extends AppCompatActivity {
+    /** permission code for the storage permisson.*/
+    private final int STORAGE_PERMISSION_CODE = 42;
     /** was soll angezeigt werden in Navigation.*/
     private AppBarConfiguration mAppBarConfiguration;
     /** zur Verwendung von xml anstatt der direkten Einbindung.*/
@@ -32,6 +40,8 @@ public class MainActivity extends AppCompatActivity {
     @Override
     protected void onCreate(final Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
+        //checks that the permission to read and write the xml is granted
+        ensurePermissions();
         setContentView(R.layout.activity_main);
         Toolbar toolbar = findViewById(R.id.toolbar);
         setSupportActionBar(toolbar);
@@ -73,26 +83,97 @@ public class MainActivity extends AppCompatActivity {
             case R.id.action_settings:
                 Toast.makeText(this, "Settings",
                         Toast.LENGTH_SHORT).show();
-                return true;
+                break;
+
             case R.id.action_adjust_xml:
                 Toast.makeText(this, "Adjust .xml",
                         Toast.LENGTH_SHORT).show();
-                return true;
+                break;
+
             case R.id.action_download_xml:
                 if (xml.download() == 0) {
-                Toast.makeText(this, "Download .xml",
+                    Toast.makeText(this, "Download .xml",
                         Toast.LENGTH_SHORT).show();
-            }
+                }
-                return true;
+                break;
             case R.id.action_upload_xml:
                 if (xml.upload() == 0) {
-                Toast.makeText(this, "Upload .xml",
+                    Toast.makeText(this, "Upload .xml",
                         Toast.LENGTH_SHORT).show();
-            }
+                }
-                return true;
+                break;
             default:
+                Log.e("MainActivity", "unknown item: "
+                        + item.toString());
                 return super.onOptionsItemSelected(item);
         }
+        return true;
+    }
 
+    /**
+     * ensures that all the permissions needed by the application are granted.
+     */
+    private void ensurePermissions() {
+        if (ContextCompat.checkSelfPermission(
+                        this,
+                        Manifest.permission.WRITE_EXTERNAL_STORAGE)
+                == PERMISSION_GRANTED) {
+            Toast.makeText(
+                    this,
+                    "You have already granted this permission",
+                    Toast.LENGTH_LONG)
+                    .show();
+        } else {
+            if (ActivityCompat.shouldShowRequestPermissionRationale(
+                    this,
+                    Manifest.permission.READ_EXTERNAL_STORAGE)) {
+
+                new AlertDialog.Builder(this)
+                    .setTitle("Permission needed")
+                    .setMessage("This permission is needed to access the "
+                        + "XML configuration file")
+                    .setPositiveButton("ok", (dialog, which) ->
+                        ActivityCompat.requestPermissions(
+                            MainActivity.this,
+                            new String[]{
+                                Manifest.permission.WRITE_EXTERNAL_STORAGE},
+                                    STORAGE_PERMISSION_CODE))
+                    .setNegativeButton("cancel", (dialog, which) ->
+                        System.exit(1))
+                    .create()
+                    .show();
+
+            } else {
+                ActivityCompat.requestPermissions(
+                    this,
+                    new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE},
+                    STORAGE_PERMISSION_CODE);
+            }
+        }
+    }
+
+    /**
+     * handles request permission results.
+     * @param requestCode the code of the requested
+     * @param permissions the permissions requested
+     * @param grantResults the result of the permission request
+     */
+    @Override
+    public void onRequestPermissionsResult(
+            final int requestCode,
+            @NonNull final String[] permissions,
+            @NonNull final int[] grantResults) {
+        if (requestCode == STORAGE_PERMISSION_CODE) {
+            if (grantResults.length > 0
+                    && grantResults[0] == PERMISSION_GRANTED) {
+                Toast.makeText(
+                    this,
+                    "Permission GRANTED",
+                    Toast.LENGTH_LONG)
+                    .show();
+            } else {
+                System.exit(1);
+            }
+        }
     }
 }