Create functionality checking internet connection after clicking sign in and add permission ACCESS_NETWORK_STATE.

ui/src/main/AndroidManifest.xml

@@ -4,6 +4,7 @@
     package="com.wireguard.android"
     android:installLocation="internalOnly">
 
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
     <uses-permission android:name="android.permission.CAMERA" />
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />

ui/src/main/java/com/wireguard/android/activity/LoginActivity.java

@@ -1,10 +1,15 @@
 package com.wireguard.android.activity;
 
+import androidx.annotation.NonNull;
 import androidx.appcompat.app.AppCompatActivity;
 import androidx.lifecycle.Observer;
 import androidx.lifecycle.ViewModelProvider;
 
+import android.content.Context;
 import android.content.Intent;
+import android.net.ConnectivityManager;
+import android.net.Network;
+import android.net.NetworkRequest;
 import android.os.Bundle;
 import android.util.Log;
 import android.view.View;
@@ -31,6 +36,9 @@ public class LoginActivity extends AppCompatActivity {
     private EditText userName;
     private EditText password;
     private Button sign;
+    private ConnectivityManager connectivityManager;
+    private ConnectivityManager.NetworkCallback mCallback;
+    private boolean isConnected;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
@@ -38,13 +46,38 @@ public class LoginActivity extends AppCompatActivity {
         setContentView(R.layout.activity_login);
         initUI();
         loginViewModel = new ViewModelProvider(this).get(LoginViewModel.class);
+        connectivityManager = (ConnectivityManager)getSystemService(Context.CONNECTIVITY_SERVICE);
+        mCallback = new ConnectivityManager.NetworkCallback(){
+            @Override public void onAvailable(@NonNull final Network network) {
+                isConnected = true;
+            }
+
+            @Override public void onLost(@NonNull final Network network) {
+                isConnected = false;
+            }
+        };
         sign.setOnClickListener(new OnClickListener() {
             @Override public void onClick(final View v) {
-                login();
+                if (isConnected) {
+                    login();
+                } else {
+                    Toast.makeText(LoginActivity.this, getString(R.string.turnInternet), Toast.LENGTH_LONG).show();
+                }
             }
         });
     }
 
+    @Override protected void onStart() {
+        super.onStart();
+        final NetworkRequest request = new NetworkRequest.Builder().build();
+        connectivityManager.registerNetworkCallback(request,mCallback);
+    }
+
+    @Override protected void onPause() {
+        super.onPause();
+        connectivityManager.unregisterNetworkCallback(mCallback);
+    }
+
     private void initUI() {
         initViews();
     }

ui/src/main/res/values/strings.xml

@@ -240,4 +240,5 @@
     <string name="not_connected">Not Connected</string>
     <string name="connect">Connect</string>
     <string name="disable_apps">Disable Apps</string>
+    <string name="turnInternet">Please turn on internet connection</string>
 </resources>