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Hands-On Game Development with Rust

Data-driven Spawning

Explore how to implement data-driven spawning of entities in a Rust game using ECS patterns and command buffers. Learn to filter, weight, and place entities like monsters and items dynamically based on level data. Understand how to optimize spawning by batching commands and use flexible effect lists to customize gameplay without additional programming.

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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:

Rust 1.40.0
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:

Rust 1.40.0
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 ...