Resolve "Login screen setup (without backend)"

Closes #19 (closed)

frontend/core/src/com/game/tankwars/ResourceManager.java

+package com.game.tankwars;
+
+import com.badlogic.gdx.assets.AssetDescriptor;
+import com.badlogic.gdx.assets.AssetManager;
+import com.badlogic.gdx.assets.loaders.FileHandleResolver;
+import com.badlogic.gdx.assets.loaders.resolvers.InternalFileHandleResolver;
+import com.badlogic.gdx.graphics.g2d.BitmapFont;
+import com.badlogic.gdx.graphics.g2d.TextureAtlas;
+import com.badlogic.gdx.graphics.g2d.freetype.FreeTypeFontGenerator;
+import com.badlogic.gdx.graphics.g2d.freetype.FreeTypeFontGeneratorLoader;
+import com.badlogic.gdx.graphics.g2d.freetype.FreetypeFontLoader;
+import com.badlogic.gdx.scenes.scene2d.ui.Skin;
+import com.badlogic.gdx.scenes.scene2d.utils.Drawable;
+import com.badlogic.gdx.utils.GdxRuntimeException;
+import com.github.acanthite.gdx.graphics.g2d.FreeTypeSkinLoader;
+
+/**
+ * A Singleton that manages all assets in application.
+ * It uses the LibGDX AssetManager, but encapsulates it to only expose necessary methods.
+ *
+ * The application uses scene2d.ui and loads Skin objects, each with a
+ * TextureAtlas and a JSON file.
+ */
+public class ResourceManager {
+
+    private static ResourceManager instance = null;
+    private final AssetManager manager;
+    private final AssetDescriptor<TextureAtlas> MENU_ATLAS =
+            new AssetDescriptor<>("menu-textures.atlas", TextureAtlas.class);
+    private final AssetDescriptor<Skin> MENU_SKIN =
+            new AssetDescriptor<>("menu-textures.json", Skin.class);
+
+    public ResourceManager() {
+        manager = new AssetManager();
+
+        manager.setLoader(Skin.class, new FreeTypeSkinLoader(manager.getFileHandleResolver()));
+    }
+
+    /**
+     * Singleton getter
+     * @return The ResourceManager instance
+     */
+    public static ResourceManager getInstance() {
+        if (instance == null) instance = new ResourceManager();
+        return instance;
+    }
+
+    /**
+     * Loads the TextureAtlas and the Skin for the menus,
+     * and attaches the TextureAtlas to the Skin.
+     *
+     * @return loaded Skin object with JSON file and TextureRegions
+     *         null on loading error
+     */
+    public Skin loadAndGetMenuAssets() {
+        if (!manager.isLoaded(MENU_ATLAS)) manager.load(MENU_ATLAS);
+        if (!manager.isLoaded(MENU_SKIN)) manager.load(MENU_SKIN);
+
+        try {
+            manager.finishLoading();
+            manager.get(MENU_SKIN).addRegions(manager.get(MENU_ATLAS));
+            return manager.get(MENU_SKIN);
+        } catch(GdxRuntimeException error) {
+            System.out.println(error.getMessage());
+        }
+        return null;
+    }
+
+    /**
+     * Block until all currently loaded assets are finished loading
+     */
+    public void finishLoading() {
+        manager.finishLoading();
+    }
+
+    /**
+     * Unload all currently loaded assets
+     */
+    public void clear() {
+        manager.clear();
+    }
+
+    /**
+     * Dispose of the manager
+     */
+    public void dispose() {
+        manager.dispose();
+    }
+
+}