Open FirebaseUI Auth sign in screen if user is not logged in.

app/build.gradle

@@ -43,6 +43,10 @@ dependencies {
 
     // Firebase
     compile 'com.google.firebase:firebase-database:9.6.0'
+    compile 'com.google.firebase:firebase-auth:9.6.0'
+
+    // FirebaseUI
+    compile 'com.firebaseui:firebase-ui-auth:0.6.0'
 }
 
 apply plugin: 'com.google.gms.google-services'

app/src/main/java/com/google/firebase/udacity/friendlychat/MainActivity.java

@@ -16,6 +16,7 @@
 package com.google.firebase.udacity.friendlychat;
 
 import android.os.Bundle;
+import android.support.annotation.NonNull;
 import android.support.v7.app.AppCompatActivity;
 import android.text.Editable;
 import android.text.InputFilter;
@@ -29,7 +30,11 @@
 import android.widget.ImageButton;
 import android.widget.ListView;
 import android.widget.ProgressBar;
+import android.widget.Toast;
 
+import com.firebase.ui.auth.AuthUI;
+import com.google.firebase.auth.FirebaseAuth;
+import com.google.firebase.auth.FirebaseUser;
 import com.google.firebase.database.ChildEventListener;
 import com.google.firebase.database.DataSnapshot;
 import com.google.firebase.database.DatabaseError;
@@ -46,6 +51,8 @@ public class MainActivity extends AppCompatActivity {
     public static final String ANONYMOUS = "anonymous";
     public static final int DEFAULT_MSG_LENGTH_LIMIT = 1000;
 
+    public static final int RC_SIGN_IN = 1;
+
     private ListView mMessageListView;
     private MessageAdapter mMessageAdapter;
     private ProgressBar mProgressBar;
@@ -59,6 +66,8 @@ public class MainActivity extends AppCompatActivity {
     private FirebaseDatabase mFirebaseDatabase;
     private DatabaseReference mMessagesDatabaseReference;
     private ChildEventListener mChildEventListener;
+    private FirebaseAuth mFirebaseAuth;
+    private FirebaseAuth.AuthStateListener mAuthStateListener;
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
@@ -69,6 +78,7 @@ protected void onCreate(Bundle savedInstanceState) {
 
         // Initialize Firebase components
         mFirebaseDatabase = FirebaseDatabase.getInstance();
+        mFirebaseAuth = FirebaseAuth.getInstance();
 
         mMessagesDatabaseReference = mFirebaseDatabase.getReference().child("messages");
 
@@ -141,6 +151,42 @@ public void onChildMoved(DataSnapshot dataSnapshot, String s) {}
             public void onCancelled(DatabaseError databaseError) {}
         };
         mMessagesDatabaseReference.addChildEventListener(mChildEventListener);
+
+        mAuthStateListener = new FirebaseAuth.AuthStateListener() {
+            @Override
+            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
+                FirebaseUser user = firebaseAuth.getCurrentUser();
+                if (user != null) {
+                    // User is signed in
+                    Toast.makeText(MainActivity.this, "You're now signed in. Welcome to FriendlyChat.", Toast.LENGTH_SHORT).show();
+                } else {
+                    // User is signed out
+                    startActivityForResult(
+                            AuthUI.getInstance()
+                                    .createSignInIntentBuilder()
+                                    .setIsSmartLockEnabled(false)
+                                    .setProviders(
+                                            AuthUI.EMAIL_PROVIDER,
+                                            AuthUI.GOOGLE_PROVIDER)
+                                    .build(),
+                            RC_SIGN_IN);
+                }
+            }
+        };
+    }
+
+    @Override
+    protected void onResume() {
+        super.onResume();
+        mFirebaseAuth.addAuthStateListener(mAuthStateListener);
+    }
+
+    @Override
+    protected void onPause() {
+        super.onPause();
+        if (mAuthStateListener != null) {
+            mFirebaseAuth.removeAuthStateListener(mAuthStateListener);
+        }
     }
 
     @Override