Hands-On Game Development with Rust/

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Data-driven Spawning

Data-driven Spawning

Learn about data-driven spawning.

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Spawning data-driven entities is the process of reading the game definition data. Outputting an in-game object like a health potion begins as a data template and awaits discovery on the map:

Let’s create a new function, implemented as part of the Template. First, start with the function signature:

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impl Templates {
    pub fn load() -> Self {
        let file = File::open("resources/template.ron")
            .expect("Failed opening file");
        from_reader(file).expect("Unable to load templates")
    }
    pub fn spawn_entities(
        &self,
        ecs: &mut World,
        rng: &mut RandomNumberGenerator,
        level: usize,
        spawn_points: &[Point]
    ) {

The parameters for this function align closely with the spawning functions we already have in mod.rs. We take a borrowed reference to the ECS world and the RandomNumberGenerator. Note which game level we’re creating and store a list of locations in which we can spawn entities.

The next section builds a spawn table for the current level. Add the following code to our new function:

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let mut available_entities = Vec::new();
   self.entities
    .iter()
    .filter(|e| e.levels.contains(&level))
    .for_each(|t| {
        for _ in 0 .. t.frequency {
            available_entities.push(t);
        }
    }
);

  • Line 1: Create a mutable vector in which we’ll store a list of entities that may be spawned on this level.

  • Line 3: Iterate the list of entity templates we loaded in the constructor.

  • Line 4: Use filter to include only entities whose levels list includes the current level.

  • Line 6: Here’s the for loop to add each available entity a number of times equal to the entry’s frequency.

The available_entities list contains references to members of the entities collection. Rather than duplicating each entry, we store a pointer to the original, saving a lot of memory. Each possible entity appears a number of times in the ...