HOWTO-Engines

Introduction

This page is meant as a mini-HOWTO which roughly outlines the steps needed to add a new engine to ScummVM. It does not tell you how to create an engine for a given game; rather it is meant to tell a developer how to properly "hook" into ScummVM.

I will assume that you are at least roughly familiar with ScummVM, and have a fresh checkout of our Subversion repository. Note that it's strongly adviced to base your work on the current development version of ScummVM, and not on a release version. This will ease integration of your work.

Overview

Essentially, you will have to implement a subclass of the Engine class. Our Doxygen documentation is your friend and should hopefully explain enough about this: Engine class .

You also must hook yourself into the regular ScummVM main build system. Actually, some ports use custom build system, but their maintainers will usually add your new engine once it has been added to ScummVM.

Finally, you need to make ScummVM aware of your new engine by updating a couple source files (see below).

Steps

In the following I assume your engine is called "quux".

1. Add a new directory engines/quux/
2. Add engines/quux/module.mk (take a look at module.mk files of 2-3 existing engines to understand the content).
3. Add engines/quux/quux.h and engines/quux/quux.cpp; this will contain your Engine subclass (or at least parts of it).
4. Add engines/quux/detection.cpp; It will contain the plugin interface code (more on that in the next section).
5. Modify engines/engines.mk by adding your engine. It should be clear what to do by looking at what is done for the other engines there.
6. Modify configure by adding a new add_engine line. Again, just check out what is done for the existing engines.
7. Modify base/plugins.cpp; in particular, you have to add your engine to the list in PluginManager::loadPlugins.

That's it. The difficult part is of course writing the Engine subclass. More on that in the next section!

Important note: Use a C++ namespace for all your work, e.g. "namespace Quux" in this case.

File name conventions

Since of the course of its existence, many people will have to deal with the source code of a given engine (be it to fix bugs in it, modify it to still compile after changes made to tbe backend code, or to simply add new functionality), it is useful to adhere to various conventions used throughout all engines. Besides source code conventions (see Code Formatting Conventions), this affects filenames. We suggest you use the following names for specific parts of your engine:

ENGINENAME.h	Contains your (primary) Engine subclass.
ENGINENAME.cpp	Contains at least the constructor and destructor of your (primary) Engine subclass, as well as the implementations of the mandatory (i.e. pure virtual) Engine methods.
detection.cpp	Code related to game detection. Also contains the implementation of the plugin interface, as described in base/plugins.h.
saveload.cpp	Code related to savegames
debug.cpp, debugger.cpp	(console) debugger
gfx.cpp (alt: graphics.cpp)	Graphics code
sound.cpp	Sound code
music.cpp	Music code
inter.cpp, logic.cpp, script.cpp	Game logic, resp. script/bytecode interpreter

Subclassing MetaEngine

Let's implement the plugin interface: You'll have to create a custom MetaEngine subclass which provides the information and functionality related to the engine that can be used without running any game. That includes detecting games, listing savegames and instancing the engine. You have to specify your MetaEngine class to the REGISTER_PLUGIN_* macros. (See the example below)

You can alternatively subclass AdvancedMetaEngine from common/advancedDetector.h to reuse some common functionality like MD5 based game detection.

Subclassing Engine

TODO: We should probably give some sample code, maybe even provide a full (empty) Engine demo class. Maybe even provide a real mini engine project somewhere on our site which demonstrates using events, drawing, etc. ? Not sure whether this would be worth the effort, though.

Important: If you're using the ScummVM GUI (g_gui and stuff) you have always to call g_gui.handleScreenChanged() if you received a OSystem::EVENT_SCREEN_CHANGED event, else it could be that your gui looks strange or even crashes ScummVM.

Infrastructure services

Header file common/scummsys.h provides services needed by virtually any source file:

• defines platform endianness
• defines portable types
• defines common constants and macros

Moreover, it deals with providing suitable building environment for different platforms:

• provides common names for non-standard library functions
• disables bogus and/or annoying warnings
• provides a lean environment to build win32 executables/libraries

TODO: give descriptions for other commonly used header files.

Common portability issues

There are wrapper around number of non-portable functions. These are:

 max() -> MAX()
 min() -> MIN()
 rand() -> use Common::RandomSource class
 stricmp() -> scumm_stricmp()
 strnicmp() -> scumm_strnicmp()
 strrev() -> scumm_strrev()

Also we have predefined common integer types. Please, use them instead of rolling your own:

 byte
 int8
 uint8
 int16
 uint16
 int32
 uint32

Additionally ScummVM offers way of recording all events and then playing them back on request. That could be used for "demoplay" mode. But to ensure that it will work for your engine, you have to register your RandomSource class instance. See example engine below.

Example

Example: engines/quux/quux.h

#ifndef QUUX_H
#define QUUX_H

#include "engines/engine.h"
#include "gui/debugger.h"

namespace Quux {

class Console;

// our engine debug levels
enum {
	kQuuxDebugExample = 1 << 0,
	kQuuxDebugExample2 = 1 << 1
	// next new level must be 1 << 2 (4)
	// the current limitation is 32 debug levels (1 << 31 is the last one)
};

class QuuxEngine : public Engine {
public:
	QuuxEngine(OSystem *syst);
	~QuuxEngine();

	virtual int init();
	virtual int go();

private:
	Console *_console;

	// We need random numbers
	Common::RandomSource _rnd;
};

// Example console class
class Console : public GUI::Debugger {
public:
	Console(QuuxEngine *vm);
	virtual ~Console(void);
};

} // End of namespace Quux

#endif

Example: engines/quux/quux.cpp

#include "common/scummsys.h"

#include "common/events.h" // for getEventManager()
#include "common/config-manager.h"
#include "common/file.h"
#include "common/fs.h"

#include "quux/quux.h"

namespace Quux {

QuuxEngine::QuuxEngine(OSystem *syst) 
 : Engine(syst) {
	// Put your engine in a sane state, but do nothing big yet;
	// in particular, do not load data from files; rather, if you
	// need to do such things, do them from init().

	// Do not initialize graphics here

	// However this is the place to specify all default directories
	Common::File::addDefaultDirectory(_gameDataPath + "sound/");

	// Here is the right place to set up the engine specific debug levels
	Common::addSpecialDebugLevel(kQuuxDebugExample, "example", "this is just an example for a engine specific debug level");
	Common::addSpecialDebugLevel(kQuuxDebugExample2, "example2", "also an example");

	// Don't forget to register your random source
	syst->getEventManager()->registerRandomSource(_rnd, "quux");

	printf("QuuxEngine::QuuxEngine\n");
}

QuuxEngine::~QuuxEngine() {
	// Dispose your resources here
	printf("QuuxEngine::~QuuxEngine\n");

	// Remove all of our debug levels here
	Common::clearAllSpecialDebugLevels();
}

int QuuxEngine::init() {
	// Initialize graphics using following template
	// You have to use transactions
	_system->beginGFXTransaction();
		// This is handled by base Engine class and processes command
		// line parameters
		initCommonGFX(true);

		// Specify dimensions of game graphics window.
		// In this example: 320x200
		_system->initSize(320, 200);
	_system->endGFXTransaction();
	
	// Create debugger console. It requires GFX to be initialized
	_console = new Console(this);

	// Additional setup.
	printf("QuuxEngine::init\n");
	return 0;
}

int QuuxEngine::go() {
	// Your main even loop should be (invoked from) here.
	printf("QuuxEngine::go: Hello, World!\n");

	// This test will show up if -d1 and --debugflags=example are specified on the commandline
	debugC(1, kQuuxDebugExample, "Example debug call");

	// This test will show up if --debugflags=example or --debugflags=example2 or both of them and -d3 are specified on the commandline
	debugC(3, kQuuxDebugExample | kQuuxDebugExample2, "Example debug call two");

	return 0;
}

} // End of namespace Quux

Example: engines/quux/detection.cpp

The following example implements a custom MetaEngine instead of using the AdvancedMetaEngine.

#include "quux/quux.h"

#include "base/game.h"
#include "base/plugins.h"
#include "engines/metaengine.h"

static const PlainGameDescriptor quux_setting[] = {
	{ "quux", "Quux the Example Module" },
	{ "quuxcd", "Quux the Example Module (CD version)" },
	{ 0, 0 }
};

class QuuxMetaEngine : public MetaEngine {
public:
	virtual const char *getName() const {
		return "Quux the Example Module";
	}

	virtual const char *getCopyright() const {
		return "Copyright (C) Quux Entertainment Ltd.";
	}

	virtual GameList getSupportedGames() const {
		GameList games;
		const PlainGameDescriptor *g = quux_setting;
		while (g->gameid) {
			games.push_back(*g);
			g++;
		}
	
		return games;
	}

	virtual GameDescriptor findGame(const char *gameid) const {
		const PlainGameDescriptor *g = quux_setting;
		while (g->gameid) {
			if (0 == scumm_stricmp(gameid, g->gameid))
				break;
			g++;
		}
		return GameDescriptor(g->gameid, g->description);
	}

	virtual GameList detectGames(const FSList &fslist) const {
		GameList detectedGames;
	
		// Iterate over all files in the given directory
		for (FSList::const_iterator file = fslist.begin(); file != fslist.end(); ++file) {
			if (!file->isDirectory()) {
				const char *gameName = file->getName().c_str();
	
				if (0 == scumm_stricmp("README", gameName)) {
					// You could check the contents of the file now if you need to.
					detectedGames.push_back(quux_setting[0]);
					break;
				}
			}
		}
		return detectedGames;
	}

	virtual PluginError createInstance(OSystem *syst, Engine **engine) const {
		assert(syst);
		assert(engine);

		// Scan the target directory for files (error out if it does not exist)
		FSList fslist;
		FilesystemNode dir(ConfMan.get("path"));
		if (!dir.getChildren(fslist, FilesystemNode::kListAll)) {
			return kInvalidPathError;
		}

		// Invoke the detector
		Common::String gameid = ConfMan.get("gameid");
		GameList detectedGames = detectGames(fslist);

		for (uint i = 0; i < detectedGames.size(); i++) {
			if (detectedGames[i].gameid() == gameid) {
				// At this point you may want to perform additional sanity checks.
				*engine = new Quux::QuuxEngine(syst);
				return kNoError;
			}
		}

		// Failed to find any game data
		return kNoGameDataFoundError;
	}
};

#if PLUGIN_ENABLED_DYNAMIC(QUUX)
	REGISTER_PLUGIN_DYNAMIC(QUUX, PLUGIN_TYPE_ENGINE, QuuxMetaEngine);
#else
	REGISTER_PLUGIN_STATIC(QUUX, PLUGIN_TYPE_ENGINE, QuuxMetaEngine);
#endif

Example: engines/quux/module.mk

MODULE := engines/quux

MODULE_OBJS := \
	quux.o

MODULE_DIRS += \
	engines/quux

# This module can be built as a plugin
ifeq ($(ENABLE_QUUX), DYNAMIC_PLUGIN)
PLUGIN := 1
endif

# Include common rules 
include $(srcdir)/common.rules