How to add live code reload to your game
Adding live code reloading is one of the best things I did when working on my current game, Sol Trader.
Live code reload reduces our debug loop down to milliseconds. Now when I recompile the game, it notices I’ve done so and loads new code whilst the game is still running. Here’s a demo:
Thanks to Casey Muratori (again) and his excellent Handmade Hero series for teaching me the basics, and to this github project for demonstrating how to do it in OSX, my development platform of choice.
There are a few architecture changes that we’ll need in order to put this feature in our code. Here’s how I did it.
Splitting our game code from our platform
Sol Trader now has a clearly defined platform layer, separate from the game. The game code itself knows nothing about the platform at all and it passed in everything it needs in order to calculate what’s next. Here’s the method that we call to run a game loop:
bool gameUpdateAndRender(Memory* memory, v2i screenDim, Input* input);The update and render call takes some Memory, the screen dimensions and the input that we received this frame from the player. It is the job of this method to render the next frame to the screen, in under 16ms.
Memory is just a big chunk of raw memory allocated by the platform layer at the start of the game. The gameUpdateAndRender function is free to do what it likes with this space. It’s important to note that it’s persistent across live reloads which means that all state should be saved here. The game is not allowed to allocate any memory itself, it has to use the memory given to it.
gameUpdateAndRender actually is implemented as a call into a shared library (a DLL on windows, or a dylib on Linux/OSX) using a #define trick I learnt from Handmade Hero:
// Platform.h
#define GAME_UPDATE_AND_RENDER(name) bool name(Memory* memory, v2i screenDim, Input* input)
typedef GAME_UPDATE_AND_RENDER(GameUpdateAndRender);
// Game.cpp
extern "C" GAME_UPDATE_AND_RENDER(GAME_updateAndRender) {
// game code here
}(We need the extern "C" here to stop the compiler from mangling the name, so we can find it in the shared library.)
Running the game code
This is a cut down OSX version of the platform layer I’m using. Similar code exists for other platforms:
// Platform.cpp
void OSX_loadGameCode(GameCode* game, char const* path) {
if ((game->code = dlopen(path, RTLD_LAZY | RTLD_GLOBAL))) {
game->updateAndRender = (sol::GameUpdateAndRender*)dlsym(game->code, "gameUpdateAndRender");
game->lastWriteTime = OSX_lastWriteTime(gameCodePath);
}
}
void main() {
Memory memory;
allocateMemory(&memory);
initOpenGL();
v2i screenDim = createGameWindow();
GameCode game;
char const* gameCodePath = "soltrader.dylib";
OSX_loadGameCode(game, gameCodePath);
// Main loop
while (!quit) {
Input newInput;
getInput(&newInput);
// Check if the game has been recompiled
time_t newWriteTime = OSX_lastWriteTime(gameCodePath);
if (newWriteTime != game->lastWriteTime) {
OSX_unloadGameCode(&game);
OSX_loadGameCode(&game, gameCodePath);
}
if (game.updateAndRender) {
game.updateAndRender(&memory, screenDim, &newInput);
}
}
}At the heart of it, it’s a standard game loop. We first allocate enough memory using one big alloc call at the beginning. This is all platform specific code, so it’s ok to use OSX, Linux or Windows specific calls here. We figure out our screen dimensions from the platform, create a window, and initialise OpenGL or whatever graphics library we’re using.
Then we load the code using dlopen and if that succeeds, we find the gameUpdateAndRender function and save the location. In the main loop, assuming that all worked, we call the saved function with the info it needs to render the frame.
Building the shared library
Here’s how the build.sh script looks:
// Build the shared library
$CC $CFLAGS -dynamiclib ../src/Unity.cpp -o ../libsol.dylib $LIBS
// Build the executable
$CC $CFLAGS -o ../$EXE ../src/platforms/sdl2/Application.cpp $LIBSWe build the shared library containing only the game code, not the platform code. We then use the platform code to load and run the shared game library.
Summary: everyone should have live code reload
This is an amazing way to develop games. For too long in the past I have sat watching a long compile, then ploughed through the game from the main menu, to find the bug I’m trying to fix, only to find that I’ve made a stupid error and have to start again. We need to find fun as fast as possible - anything we can do to reduce the debug loop is a good thing.
Live code reload also does away with much of the need to use a scripting language (fast feedback). I don’t have any designers who don’t write C (it’s just me!) so I haven’t implemented one for this game. I also don’t need any GUI configuration files for layout, it’s all just implemented in C with live code reload for positioning tweaks.
Trust me: once you’ve tried it, you’ll never go back.
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The sixth of June is a significant day for me personally. In 2004, I spent the entire of the day in hospital. I remember the 60th anniversary commemorations of D-day on the TV in the background, as I sat beside my wife, in labour with our first child. I became a father an hour after midnight on the 7th June; my son becomes a teenager tomorrow.
Twelve years later, in 2016, I spent the entire of 6th June glued to Steam watching and waiting whilst my first game Sol Trader was released to the world. This was a career dream come true: since I started programming at six years old I’d always wanted to create and ship my own games. Sol Trader’s release was ultimately a painfully formative experience for me, which I wrote about at the time and was interviewed about recently in GamesIndustry.biz.
Over the last year, I’ve been keeping busy doing two things. One is to support Sol Trader as much as I can with countless updates and patches. I’ve also been very busy working on a new game, Ealdorlight, a medieval RPG-style take on Sol Trader’s mechanics, with turn-based combat, realistic damage and great graphics. I announced Ealdorlight in March and demonstrated it at Rezzed, strengthening my hope that the idea was a good one.
I decided fairly early on that I wanted to take Ealdorlight to Kickstarter. Sol Trader’s successful Kickstarter was a brilliant experience. The Kickstarter community is one of the kindest, most positive on the Internet. I also needed funding for this game: Sol Trader was self-funded through many long evenings and contracting work, and for Ealdorlight I need a bigger team to realise the vision. It’s built in Unreal Engine 4, which simultaneously saves me loads of development time and means I need a bigger team to pull off the realistic art style I’ve gone for.
As time came near to launch, the first anniversary of Sol Trader’s released seemed an appropriate day to launch the campaign. So today, 6th June 2017, I will spend the entire day glued to Kickstarter as my campaign goes live at 4pm today.
Visit Ealdorlight’s Kickstarter Campaign
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The trick is layering a great story on top of a generated world with random characters. Building empathy with the main character and his family when all characters are generated is hard, and hinges around being able to hook the story in at the right moments.
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Ealdorlight: backstory teaser (updated)
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Rezzed was fantastic: it was great to see lots and lots of people wearing our Ealdorlight crowns. We ran out of crowns on both days, with some creative head displays on offer:
View post on imgur.com
Ealdorlight Kickstarter launch date: 6th June
Yes, I know I said May :) I’ve decided to go for a 31-day campaign, starting on the 6th June, for a few reasons:
- It doesn’t clash with any major US holidays, like Memorial Day. The 48-hour reminder email should go out on the day after 4th July.
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Sol Trader 1.3 released!
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Here are the highlights:
- You can now chat to characters directly on the right if they’re in the same location as you
- Pirate Chief and criminals are now more likely to try to destroy you
- Fix a bug where you’re not paid enough for a mission
- Dignitaries now fly around a little less than before to make it easier to pin them down
- Inter-faction missions now pay slightly less
- Business trips now pay slightly less
- Taxi missions now pay slightly more
- Talking to your criminal parents will no longer cause them to forget who you are
- Fix crash where a character develops an opinion of the player mid-conversation
- Fix crash when showing GUI for a ship the AI is driving
- Fix crash where a character would attempt to sell a good on a ship they’ve lent to the player
- Can now initiate conversations when paused - will restart the game but at realtime speed
- Fixed the Tiger II achievement
Your steam copy should automatically update. I’ll be releasing an updated downloadable version to SendOwl in the next few days.
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