Leap

An opinionated toolkit for creating 2D platformers on top of the Flame engine.

WARNING library under development

Be aware that this is still under development and is likely to change frequently, every release could introduce breaking changes up until a v1.0.0 release (which may never happen as this is a solo endeavour currently).

Features

Level creation via Tiled

Leap uses Tiled tile maps not just for visually rendering the level, but also for imbuing behavior and terrain in the level by creating corresponding Flame components automatically from the map's layers.

Physics

The crux of this physics engine is based on this post The guide to implementing 2D platformers: http://higherorderfun.com/blog/2012/05/20/the-guide-to-implementing-2d-platformers/

The "Type #2: Tile Based (Smooth)" section outlines the overall algorithm.

Note that Leap doesn't use Flame's collision detection system in favor of one that is more specialized and efficient for tile based platformers where every hitbox is an axis-aligned bounding box, and special handling can be done for tile grid aligned components (such as ground terrain).

Efficient collision detection

Essentially all physical objects (PhsyicalComponent) in the game have axis-aligned bounding boxes (AABBs) for hitboxes, determined by their size and position. The hitbox doesn't necessarily need to match the visual size of the component.

✅ Supported tile platformer features:

• Ground terrain
• One way platforms
• Slopes
• Moving platforms

🚧 Future tile platformer features:

• Ladders

Simple physics designed for 2D platformers

Long story short, physics engines like box2d are great for emulating realistic physics and terrible for implementing retro-style 2d platformers which are not remotely realistic. In order to get the snappy jumps and controls required for a responsive platformer a much more rudimentary physics engine is required.

In Leap, physical entities have a velocity attribute for storing the current x and y velocity, which will automatically update the entity's position. A moving entity colliding with the level terrain will automatically have its velocity set to 0 and position updated to be kept outside the terrain to prevent overlap. There is also a global gravity rate applied to the y velocity every game tick. Static entities will never be moved by velocity or gravity.

Getting started

Before using Leap, you should be familiar with the following Flame components:

• FlameGame
• Camera
• PositionComponent
• TiledComponent

Usage

LeapGame

To use Leap, your game instance must extend LeapGame (which in turn extends FlameGame). It's recommended to use game.loadWorldAndMap to initialize the game's world and map.

LeapWorld

Accessible via LeapGame.world, this component manages any global logic necessary for the physics engine.

LeapMap

Accessible via LeapGame.map, this component manages the Tiled map and automatically constructs the tiles with proper collision detection for the ground terrain. See [Tiled map integration](#Tiled map integration) below

Game code snippet

See the standard_platformer example for complete game code.

void main() {
  runApp(GameWidget(game: MyLeapGame()));
}

class MyLeapGame extends LeapGame with HasTappables, HasKeyboardHandlerComponents {
  late final Player player;

  @override
  Future<void> onLoad() async {
    await super.onLoad();

    // "map.tmx" should be a Tiled map the meets the Leap requirements defined below
    await loadWorldAndMap('map.tmx', 16);
    setFixedViewportInTiles(32, 16);

    player = Player();
    add(player);
    camera.followComponent(player);
  }
}

Tiled map integration

Leap automatically parses specific features out of specific Tiled layers.

Ground layer

Layer must be a Tile Layer named "Ground", by default all tiles placed in this layer are assumed to be ground terrain in the physics of the game. This means these tiles will be statically positioned and have a hitbox that matches the width and height of the tile.

Specialized ground tiles:

• Slopes for terrain the physical can walk up/down like a hill. These tiles must have their class property set to "Slope" and two custom int properties LeftTop and RightTop. For example, a 16x16 pixel tile with LeftTop = 0 and RightTop = 8 indicates slope that is ascending when moving from left-to-right.
• Platforms (name may change) for terrain the physical entities can move up (e.g. jump) through from below but still land on. These tiles must have their class property set to "Platform".

Metadata layer

Layer must be an Object Group named "Metadata", used to place any objects to be used in your game like level start/end, enemy spawn points, anything you want.

Object Group layers

Any Object Group layer (including the Metadata layer) can include arbitrary objects in them, if you wish to automatically create Flame Components for some of those objects you can do so based on the Class string in Tiled. All that is required is implementing the TiledObjectFactory interface and mapping each Class string you care about to a factory instance when loading the LeapMap, for example...

In your LeapGame:

await loadWorldAndMap(
  camera: camera,
  tiledMapPath: 'map.tmx',
  tiledObjectHandlers: {
    'Coin': await CoinFactory.createFactory(),
  },
);

Your custom factory:

class CoinFactory implements TiledObjectFactory<Coin> {
  late final SpriteAnimation spriteAnimation;

  CoinFactory(this.spriteAnimation);

  @override
  void handleObject(TiledObject object, Layer layer, LeapMap map) {
    final coin = Coin(object, spriteAnimation);
    map.add(coin);
  }

  static Future<CoinFactory> createFactory() async {
    final tileset = await Flame.images.load('my_animated_coin.png');
    final spriteAnimation = SpriteAnimation.fromFrameData(
      tileset,
      SpriteAnimationData.sequenced(...),
    );
    return CoinFactory(spriteAnimation);
  }
}

class Coin extends PhysicalEntity {
  Coin(TiledObject object, this.animation)
      : super(static: true, collisionType: CollisionType.standard) {
    anchor = Anchor.center;
    
    // Use the position from your Tiled map
    position = Vector2(object.x, object.y);
    
    // Use custom properties from your Tiled object
    value = tiledObject.properties.getValue<int>('CoinValue');
  }
  
  ...
}

Other layers

Any other layers will be rendered visually but have no impact on the game automatically. You can add additional custom behavior by accessing the layers via LeapGame.map.tiledMap and integrating your own special behavior for tiles or objects.

Moving platforms

To create a moving platform, you need to implement your own component which extends MovingPlatform and provides a Sprite (or some other rendering).

If you choose to implement this component with a Tiled object (recommended), many of the fields can be directly read from the object's custom properties:

• MoveSpeedX (double), speed in tiles per second on the X axis
• MoveSpeedY (double), speed in tiles per second on the y axis
• LoopMode (string), one of resetAndLoop, reverseAndLoop, none
• TilePath (string), a list of grid offsets to define the platforms path of movement. For example, 0,-3;2,0 means the platform will move up 3 tiles and then move right 2 tiles.

Customizing layer names and classes

Even though the structure explained above should always be followed, the developer can ask Leap to use different classes, types, names.

In order to do so, a custom LeapConfiguration can be passed to the game.

Example:

class MyLeapGame extend Leap {
  MyLeapGame() : super(
    configuration: LeapConfiguration(
      tiled: const TiledOptions(
        groundLayerName: 'Ground',
        metadataLayerName: 'Metadata',
        playerSpawnClass: 'PlayerSpawn',
        hazardClass: 'Hazard',
        damageProperty: 'Damage',
        platformClass: 'Platform',
        slopeType: 'Slope',
        slopeRightTopProperty: 'RightTop',
        slopeLeftTopProperty: 'LeftTop',
      ),
    ),
  );
}

Roadmap 🚧

• Improved collision detection API.
  • The current API is fairly awkward, see CollisionInfo.
  • There is no great way to detect collision start or collision end.
• Add support for moving platforms and ladders.
• Add more robust and reusable base class for players/enemies/etc. (Character class).
  • Integrated with sprite animations based on character state
• Improved API for PhysicalEntity, addImpulse etc.
• Lots of code clean-up to make usage of Leap more ergonomic and configurable.

Contributing

1. Ensure any changes pass:
   • melos format
   • melos analyze
   • melos test
2. Start your PR title with a conventional commit type (feat:, fix: etc).