Добавил защиту от неправильного URL, удалил фрагмент (вместо этого TreeView находится на MainActivity (SwipeRefreshLayout), добавил SwipeRefreshLayout.

app/src/main/java/ru/volgorobot/vrcatalog/MainActivity.java

@@ -1,8 +1,5 @@
 package ru.volgorobot.vrcatalog;
 
-import android.app.Fragment;
-import android.app.FragmentManager;
-import android.app.FragmentTransaction;
 import android.content.Intent;
 import android.content.SharedPreferences;
 import android.os.Bundle;
@@ -10,14 +7,36 @@ import android.preference.PreferenceManager;
 import android.support.design.widget.NavigationView;
 import android.support.v4.view.GravityCompat;
 import android.support.v4.widget.DrawerLayout;
+import android.support.v4.widget.SwipeRefreshLayout;
 import android.support.v7.app.ActionBarDrawerToggle;
 import android.support.v7.app.AppCompatActivity;
 import android.support.v7.widget.Toolbar;
+import android.util.Log;
 import android.view.MenuItem;
-import android.view.Menu;
+import android.view.ViewGroup;
+import android.widget.Toast;
+
+import com.unnamed.b.atv.model.TreeNode;
+import com.unnamed.b.atv.view.AndroidTreeView;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import retrofit2.Call;
+import retrofit2.Callback;
+import retrofit2.Response;
+import ru.volgorobot.vrcatalog.api.Controller;
+import ru.volgorobot.vrcatalog.api.VRApi;
+import ru.volgorobot.vrcatalog.model.FirstLevelModel;
 
 public class MainActivity extends AppCompatActivity
-        implements NavigationView.OnNavigationItemSelectedListener {
+        implements NavigationView.OnNavigationItemSelectedListener, SwipeRefreshLayout.OnRefreshListener {
+
+    private AndroidTreeView treeView;
+    private VRApi vrApi;
+    private List<FirstLevelModel> firstLevelObjects = new ArrayList<>();
+    private List<TreeNode> firstLevelNodes = new ArrayList<>();
+    private TreeNode root = TreeNode.root();
 
     @Override
     protected void onCreate(Bundle savedInstanceState) {
@@ -35,17 +54,22 @@ public class MainActivity extends AppCompatActivity
 
         NavigationView navigationView = findViewById(R.id.nav_view);
         navigationView.setNavigationItemSelectedListener(this);
-        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);
-        String baseURL = sharedPreferences.getString("addressOfServer", "");
 
-        FragmentManager fragmentManager = getFragmentManager();
+        SwipeRefreshLayout mSwipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swipeRefreshLayout);
-        FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
+        mSwipeRefreshLayout.setOnRefreshListener(MainActivity.this);
-        TreeViewFragment treeViewFragment = new TreeViewFragment();
+        mSwipeRefreshLayout.setColorSchemeResources(R.color.colorPrimary);
+
+        treeView = new AndroidTreeView(MainActivity.this, root);
+        treeView.setDefaultAnimation(true);
+        ViewGroup containerView = (ViewGroup) findViewById(R.id.treeViewContainer);
+        containerView.addView(treeView.getView());
+
+        try{
+            refreshTree();
+        } catch (IllegalStateException e) {
+            //
+        }
 
-        Bundle bundle = new Bundle();
-        bundle.putString("baseURL", baseURL);
-        treeViewFragment.setArguments(bundle);
-        fragmentTransaction.add(R.id.fragment_container, treeViewFragment).commit();
     }
 
     @Override
@@ -86,4 +110,71 @@ public class MainActivity extends AppCompatActivity
         drawer.closeDrawer(GravityCompat.START);
         return true;
     }
+
+    private void fillFirstLevelNodes() {
+        for (int i = 0; i < firstLevelObjects.size(); i++) {
+            firstLevelNodes.add(new TreeNode(firstLevelObjects.get(i).getName()));
+        }
+    }
+
+    @Override
+    public void onRefresh() {
+        final SwipeRefreshLayout mSwipeRefreshLayout = (SwipeRefreshLayout) findViewById(R.id.swipeRefreshLayout);
+        mSwipeRefreshLayout.setRefreshing(true);
+
+        try {
+            refreshTree();
+        } catch (IllegalStateException e) {
+            //
+        }
+
+
+        mSwipeRefreshLayout.post(new Runnable() {
+            @Override
+            public void run() {
+                mSwipeRefreshLayout.setRefreshing(false);
+            }
+        });
+    }
+
+    private void refreshTree() throws IllegalStateException {
+
+        for (int i = 0; i < firstLevelNodes.size(); i++) {
+            root.deleteChild(firstLevelNodes.get(i));
+            firstLevelNodes.remove(i);
+        }
+
+        SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);
+        String baseURL = sharedPreferences.getString("addressOfServer", "");
+
+        try {
+            vrApi = Controller.getApi(baseURL, this);
+        } catch(IllegalStateException e) {
+            throw new IllegalStateException();
+        }
+
+        vrApi.getFirstLevel().enqueue(new Callback<List<FirstLevelModel>>() {
+            @Override
+            public void onResponse(Call<List<FirstLevelModel>> call, Response<List<FirstLevelModel>> response) {
+                if(response != null) {
+                    try {
+                        firstLevelObjects.addAll(response.body());
+                    } catch(Exception e) {
+                        Toast.makeText(MainActivity.this, "Ответ от сервера неверен! Перепроверьте данные подключения!", Toast.LENGTH_LONG).show();
+                        Log.e("ERROR", "Answer of server is wrong! Re-check your connection credentials! Error: " +  response.message());
+                    }
+                    fillFirstLevelNodes();
+                } else {
+                    Log.e("ERROR", "Что-то пошло не так...");
+                }
+
+            }
+
+            @Override
+            public void onFailure(Call<List<FirstLevelModel>> call, Throwable t) {
+                Toast.makeText(MainActivity.this, "Ошибка сети. Проверьте подключение к сети или данные подключения к API!", Toast.LENGTH_LONG).show();
+                Log.e("ERROR", "getFirstLevel() failed!");
+            }
+        });
+    }
 }

app/src/main/java/ru/volgorobot/vrcatalog/TreeViewFragment.java

@@ -1,85 +0,0 @@
-package ru.volgorobot.vrcatalog;
-
-import android.app.Fragment;
-import android.os.Bundle;
-import android.support.annotation.Nullable;
-import android.util.Log;
-import android.view.LayoutInflater;
-import android.view.View;
-import android.view.ViewGroup;
-import android.widget.Toast;
-
-import com.unnamed.b.atv.model.TreeNode;
-import com.unnamed.b.atv.view.AndroidTreeView;
-import com.unnamed.b.atv.view.TreeNodeWrapperView;
-
-import java.util.ArrayList;
-import java.util.List;
-
-import retrofit2.Call;
-import retrofit2.Callback;
-import retrofit2.Response;
-import ru.volgorobot.vrcatalog.api.Controller;
-import ru.volgorobot.vrcatalog.api.VRApi;
-import ru.volgorobot.vrcatalog.model.FirstLevelModel;
-
-public class TreeViewFragment extends Fragment {
-
-    private AndroidTreeView treeView;
-    private VRApi vrApi;
-    private List<FirstLevelModel> firstLevelObjects = new ArrayList<>();
-    private List<TreeNode> firstLevelNodes = new ArrayList<>();
-
-    @Override
-    public void onCreate(@Nullable Bundle savedInstanceState) {
-        super.onCreate(savedInstanceState);
-
-
-
-        Bundle bundle = this.getArguments();
-        String baseURL = bundle.getString("baseURL");
-        vrApi = Controller.getApi(baseURL, getActivity());
-
-        vrApi.getFirstLevel().enqueue(new Callback<List<FirstLevelModel>>() {
-            @Override
-            public void onResponse(Call<List<FirstLevelModel>> call, Response<List<FirstLevelModel>> response) {
-                if(response != null) {
-                    try {
-                        firstLevelObjects.addAll(response.body());
-                    } catch(Exception e) {
-                        Log.e("ERROR", "Answer of server is wrong!");
-                        Log.e("ERROR", response.message());
-                    }
-                    fillFirstLevel();
-                } else {
-                    Log.e("ERROR", "Что-то пошло не так...");
-                }
-
-            }
-
-            @Override
-            public void onFailure(Call<List<FirstLevelModel>> call, Throwable t) {
-                Log.e("ERROR", "getFirstLevel() failed!");
-            }
-        });
-
-    }
-
-    @Nullable
-    @Override
-    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, Bundle savedInstanceState) {
-        View rootView = inflater.inflate(R.layout.fragment_default, null, false);
-        TreeNodeWrapperView containerView = rootView.findViewById(R.id.treeViewContainer);
-        TreeNode root = TreeNode.root();
-        root.addChildren(firstLevelNodes);
-        treeView = new AndroidTreeView(getActivity(), root);
-        //containerView.insertNodeView(treeView);
-        return rootView;
-    }
-
-    private void fillFirstLevel() {
-        for (int i = 0; i < firstLevelObjects.size(); i++) {
-            firstLevelNodes.add(new TreeNode(firstLevelObjects.get(i).getName()));
-        }
-    }
-}

app/src/main/java/ru/volgorobot/vrcatalog/api/Controller.java

@@ -13,28 +13,33 @@ public class Controller {
     private static String baseURL;
     private static Context context;
     private static Retrofit retrofit;
+    private static VRApi vrApi;
 
 
-    public static VRApi getApi(String baseURL1, Context context1) {
+    public static VRApi getApi(String baseURL1, Context context1) throws IllegalStateException {
         baseURL = baseURL1;
         context = context1;
         Gson gson = new GsonBuilder()
                 .setLenient()
                 .create();
-
+        try {
-        buildRetrofitObject(gson);
+            buildRetrofitObject(gson);
-
+            vrApi = retrofit.create(VRApi.class);
-        return retrofit.create(VRApi.class);
+            return vrApi;
+        } catch(IllegalStateException e) {
+            throw new IllegalStateException();
+        }
     }
 
-    private static void buildRetrofitObject(Gson gson) {
+    private static void buildRetrofitObject(Gson gson) throws IllegalStateException {
         try {
             retrofit = new Retrofit.Builder()
                     .baseUrl(baseURL)
                     .addConverterFactory(GsonConverterFactory.create(gson))
                     .build();
         } catch(Exception e) {
-            Toast.makeText(context, "Вы ввели неверный URL. Пример: http://example", Toast.LENGTH_LONG).show();
+            Toast.makeText(context, "Вы ввели неверный URL. Пример: http://example.ru", Toast.LENGTH_LONG).show();
+            throw new IllegalStateException();
         }
     }
 }

app/src/main/res/layout/content_main.xml

@@ -1,16 +1,25 @@
 <?xml version="1.0" encoding="utf-8"?>
-<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
+<android.support.v4.widget.SwipeRefreshLayout xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     xmlns:tools="http://schemas.android.com/tools"
     android:layout_width="match_parent"
     android:layout_height="match_parent"
     app:layout_behavior="@string/appbar_scrolling_view_behavior"
     tools:context=".MainActivity"
-    tools:showIn="@layout/app_bar_main">
+    tools:showIn="@layout/app_bar_main"
+    android:orientation="horizontal"
+    android:id="@+id/swipeRefreshLayout">
 
-    <RelativeLayout
+    <ScrollView
         android:layout_width="match_parent"
-        android:layout_height="match_parent"
+        android:layout_height="match_parent">
-        android:id="@+id/fragment_container"></RelativeLayout>
+        <LinearLayout
+            android:id="@+id/treeViewContainer"
+            android:layout_width="match_parent"
+            android:layout_height="wrap_content"
+            android:orientation="horizontal">
+        </LinearLayout>
+    </ScrollView>
 
-</android.support.constraint.ConstraintLayout>
+
+</android.support.v4.widget.SwipeRefreshLayout>

app/src/main/res/layout/fragment_default.xml

@@ -1,6 +0,0 @@
-<?xml version="1.0" encoding="utf-8"?>
-<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
-    android:layout_width="match_parent"
-    android:layout_height="match_parent"
-    android:id="@+id/treeViewContainer">
-</FrameLayout>