Show an error message if VPN is not supported

Some devices have Android 4 installed but the system images still seem to
lack the components that are required for VPN support. One such
component is the dialog used to grant permission to create .

src/frontends/android/res/values-de/strings.xml

@@ -23,6 +23,8 @@
     <string name="reload_trusted_certs">CA-Zertifikate neu laden</string>
     <string name="show_log">Log anzeigen</string>
     <string name="search">Suchen</string>
+    <string name="vpn_not_supported_title">VPN nicht unterstützt</string>
+    <string name="vpn_not_supported">Ihr Gerät unterstützt keine VPN Anwendungen.\nBitte kontaktieren Sie den Hersteller.</string>
 
     <!-- Log view -->
     <string name="log_title">Log</string>

src/frontends/android/res/values/strings.xml

@@ -23,6 +23,8 @@
     <string name="reload_trusted_certs">Reload CA certificates</string>
     <string name="show_log">View log</string>
     <string name="search">Search</string>
+    <string name="vpn_not_supported_title">VPN not supported</string>
+    <string name="vpn_not_supported">Your device does not support VPN applications.\nPlease contact the manufacturer.</string>
 
     <!-- Log view -->
     <string name="log_title">Log</string>

src/frontends/android/src/org/strongswan/android/ui/MainActivity.java

@@ -30,6 +30,7 @@ import android.app.AlertDialog;
 import android.app.AlertDialog.Builder;
 import android.app.Dialog;
 import android.app.DialogFragment;
+import android.content.ActivityNotFoundException;
 import android.content.Context;
 import android.content.DialogInterface;
 import android.content.Intent;
@@ -46,8 +47,11 @@ import android.widget.EditText;
 public class MainActivity extends Activity implements OnVpnProfileSelectedListener
 {
 	public static final String CONTACT_EMAIL = "[email protected]";
+	private static final String SHOW_ERROR_DIALOG = "errordialog";
 	private static final int PREPARE_VPN_SERVICE = 0;
+
 	private VpnProfile activeProfile;
+	private AlertDialog mErrorDialog;
 
 	@Override
 	public void onCreate(Bundle savedInstanceState)
@@ -59,10 +63,32 @@ public class MainActivity extends Activity implements OnVpnProfileSelectedListen
 		ActionBar bar = getActionBar();
 		bar.setDisplayShowTitleEnabled(false);
 
+		if (savedInstanceState != null && savedInstanceState.getBoolean(SHOW_ERROR_DIALOG))
+		{
+			showVpnNotSupportedError();
+		}
+
 		/* load CA certificates in a background task */
 		new CertificateLoadTask().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, false);
 	}
 
+	@Override
+	protected void onSaveInstanceState(Bundle outState)
+	{
+		super.onSaveInstanceState(outState);
+		outState.putBoolean(SHOW_ERROR_DIALOG, mErrorDialog != null);
+	}
+
+	@Override
+	protected void onDestroy()
+	{
+		super.onDestroy();
+		if (mErrorDialog != null)
+		{	/* avoid any errors about leaked windows */
+			mErrorDialog.dismiss();
+		}
+	}
+
 	@Override
 	public boolean onCreateOptionsMenu(Menu menu)
 	{
@@ -96,7 +122,18 @@ public class MainActivity extends Activity implements OnVpnProfileSelectedListen
 		Intent intent = VpnService.prepare(this);
 		if (intent != null)
 		{
-			startActivityForResult(intent, PREPARE_VPN_SERVICE);
+			try
+			{
+				startActivityForResult(intent, PREPARE_VPN_SERVICE);
+			}
+			catch (ActivityNotFoundException ex)
+			{
+				/* it seems some devices, even though they come with Android 4,
+				 * don't have the VPN components built into the system image.
+				 * com.android.vpndialogs/com.android.vpndialogs.ConfirmDialog
+				 * will not be found then */
+				showVpnNotSupportedError();
+			}
 		}
 		else
 		{
@@ -138,6 +175,25 @@ public class MainActivity extends Activity implements OnVpnProfileSelectedListen
 		}
 	}
 
+	/**
+	 * Show an error dialog if case the device lacks VPN support.
+	 */
+	private void showVpnNotSupportedError()
+	{
+		mErrorDialog = new AlertDialog.Builder(this)
+			.setTitle(R.string.vpn_not_supported_title)
+			.setMessage(getString(R.string.vpn_not_supported))
+			.setCancelable(false)
+			.setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
+				@Override
+				public void onClick(DialogInterface dialog, int id)
+				{
+					mErrorDialog = null;
+					dialog.dismiss();
+				}
+			}).show();
+	}
+
 	/**
 	 * Class that loads or reloads the cached CA certificates.
 	 */