Resources

You may find yourself struggling to represent some game state as an entity, or you may be integrating a third-party library like Three.js or Rapier. Javelin provides resources as an alternative to one-off entities or singleton components.

Resources are useful for:

• Run criteria dependencies, e.g. a promise that must resolve before a system can run
• Plugin configuration values
• Third-party library objects like a Three.js scene or Rapier physics world
• Global state that applies to all entities, like a game clock
• Abstractions over external dependencies like device input or Web Audio

Resource Registration

Resources are identifiers for arbitrary values. A resource identifier is created with the resource function. resource accepts a single type parameter that defines the resource’s value.

let Scene = j.resource<Three.Scene>()

Resources are added to an app with the addResource method.

app.addResource(Scene, new Three.Scene())

Resource Retrieval

The value of a given resource can be read using an app’s getResource method.

let scene = app.getResource(Scene)

getResource will throw an error if the resource had not been previously added.

Resources are stored in an app’s world so that systems can request resources.

app.addSystem(world => {
  let scene = world.getResource(Scene)
})

Systems can overwrite resources using a world’s setResource method.

app.addSystem(world => {
  world.setResource(Now, performance.now())
})