#2 Allow starting the emulator without a game rom

Previously a game would have to be passed on the command line, since it
was a required argument. This change enables to first open the program
and then select a game rom to run.
Passing roms on the command line is still supported.

src/game-boy-emulator/window.cpp

@@ -100,23 +100,26 @@ void Window::draw_frame() {
     draw_menubar();
 
     auto& options = m_emulator.get_options();
-    if (options.draw_debug_background) {
-        draw_background();
-    }
-    if (options.draw_debug_window) {
-        draw_window();
-    }
-    if (options.draw_debug_sprites) {
-        draw_sprites();
-    }
-    if (options.draw_info_window) {
-        draw_info();
-    }
-    if (options.draw_debug_tiles) {
-        draw_vram();
-    }
+    const auto& state = m_emulator.get_state();
+    if (state.rom_file_path.has_value()) {
 
-    draw_game();
+        if (options.draw_debug_background) {
+            draw_background();
+        }
+        if (options.draw_debug_window) {
+            draw_window();
+        }
+        if (options.draw_debug_sprites) {
+            draw_sprites();
+        }
+        if (options.draw_info_window) {
+            draw_info();
+        }
+        if (options.draw_debug_tiles) {
+            draw_vram();
+        }
+        draw_game();
+    }
 
     // Rendering
     ImVec4 clear_color = ImVec4(0.45f, 0.55f, 0.60f, 1.00f);

src/main.cpp

@@ -14,7 +14,7 @@
 
 int main(int argc, char** argv) { // NOLINT
     argparse::ArgumentParser program("game boy emulator");
-    program.add_argument("rom").help("Path to game ROM file to run.");
+    program.add_argument("--game").help("Path to game ROM file to run.");
     program.add_argument("--boot").default_value("").help("Path to boot ROM file.");
 
     spdlog::set_level(spdlog::level::info);
@@ -28,31 +28,46 @@ int main(int argc, char** argv) { // NOLINT
     }
 
     std::optional<std::filesystem::path> boot_rom_path;
+    std::optional<std::filesystem::path> game_rom_path;
     if (program.is_used("boot")) {
         boot_rom_path = std::filesystem::absolute(program.get("boot"));
     }
-    auto rom_path = std::filesystem::absolute(program.get("rom"));
+    if (program.is_used("game")) {
+        game_rom_path = std::filesystem::absolute(program.get("game"));
+    }
 
     Emulator emulator({});
-    if (boot_rom_path.has_value()) {
-        emulator.load_boot_game(boot_rom_path.value(), rom_path);
+    if (boot_rom_path.has_value() and game_rom_path.has_value()) {
+        emulator.load_boot_game(boot_rom_path.value(), game_rom_path.value());
+    } else if (game_rom_path.has_value()) {
+        emulator.load_game(game_rom_path.value());
     } else {
-        emulator.load_game(rom_path);
+        // This means only a boot rom without a game was given
+        spdlog::error("Running boot rom without a game rom not supported.");
+        std::exit(1);
     }
 
     Window window(emulator);
     emulator.set_draw_function([&]() { window.vblank_callback(); });
 
     Audio audio(emulator);
     if (audio.is_working()) {
-        emulator.set_audio_function([&](SampleFrame sample){audio.callback(sample);});
+        emulator.set_audio_function([&](SampleFrame sample) { audio.callback(sample); });
     }
 
     // Main loop
     while (!window.is_done()) {
-        auto sucess = emulator.step();
-        if (!sucess) {
-            return EXIT_FAILURE;
+        const auto& state = emulator.get_state();
+        if (state.rom_file_path.has_value()) {
+            auto sucess = emulator.step();
+            if (!sucess) {
+                return EXIT_FAILURE;
+            }
+        } else {
+            // Normally the vblank interrupt would trigger this callback. But since we are in a
+            // state where no game is loaded and the emulator is not running, we have to call it
+            // manually for the UI to render
+            window.vblank_callback();
         }
         if (emulator.get_state().new_rom_file_path.has_value()) {
             auto path = emulator.get_state().new_rom_file_path.value();