This is an experiment in making a high level DSL for specifying the sequence of events that should happen in a game level.

Here's an example from the example:

LevelPlan::<ExampleLevelContext>::new(
    Sequence::default()
        .push(ForDistance::new(
            level_length - 1800.0,
            Cycle::new(
                Sequence::default()
                    .push(ForDistance::new(
                        500.0,
                        SetComponent::new(DiverSpawner::default()),
                    ))
                    .push(ForDistance::new(
                        500.0,
                        SetComponent::new(SwooperSpawner::default()),
                    )),
            ),
        ))
        .push(ForDistance::new(
            1000.0,
            Nop
        ))
        .push(Conditional::<ExampleLevelContext>::new(
            move |context| context.player_health < 4,
            SpawnPowerups,
        ))
        .push(While::<ExampleLevelContext>::new(
            |context| context.boss_spawned,
            SpawnBoss,
        ))
        .push(YouWin),
)

The LevelPlan is a component which can be spawned into the world and then referenced from systems. This example, using a mix of generic LevelPlanElement types and some specific to the structure of the example game, says to repeat a cycle two different enemy types each lasting for 500px of vertical travel. Once the player nears the end of the level the plan leaves a dead space where nothing happens for 1000px then if the player is damaged spawns some heals. After that there's a boss encounter which lasts until the boss is no longer spawned. Then a win condition trigger.

The different steps interact with the world by conditioning sub steps, like ForDistance, While and Condition; adding a component to the LevelPlan entity, like SetComponent does, which can be used to trigger custom, state specific systems; or take custom actions on activation, deactivation or each tick while active as SpawnBoss and SpawnPowerups do. LevelPlan is generic over a context type, ExampleLevelContext in this example, which can be used to carry information about the world into plan elements, for example the condition closures on Condition and While.

Generally the plan is executed by the generic level_plan_system.