silphius/app/particles/emitter.js

import K from 'kefir';

export default class Emitter {
  constructor(ecs) {
    this.ecs = ecs;
    this.scheduled = [];
  }
  async allocate({entity, shape}) {
    const allocated = this.ecs.get(await this.ecs.create(entity));
    if (shape) {
      switch (shape.type) {
        case 'filledRect': {
          allocated.Position.x += Math.random() * shape.payload.width - (shape.payload.width / 2);
          allocated.Position.y += Math.random() * shape.payload.height - (shape.payload.height / 2);
          break;
        }
      }
    }
    return allocated;
  }
  emit(particle) {
    particle = {
      ...particle,
      entity: {
        Position: {},
        Sprite: {},
        VisibleAabb: {},
        ...particle.entity,
      },
    }
    let {count = 1} = particle;
    const {frequency = 0} = particle;
    const stream = K.stream((emitter) => {
      if (0 === frequency) {
        const promises = [];
        for (let i = 0; i < count; ++i) {
          promises.push(
            this.allocate(particle)
              .then((entity) => {
                emitter.emit(entity);
              }),
          );
        }
        Promise.all(promises)
          .then(() => {
            emitter.end();
          });
        return;
      }
      const promise = this.allocate(particle)
        .then((entity) => {
          emitter.emit(entity);
        });
      count -= 1;
      if (0 === count) {
        promise.then(() => {
          emitter.end();
        });
        return;
      }
      const promises = [promise];
      let accumulated = 0;
      const scheduled = (elapsed) => {
        accumulated += elapsed;
        while (accumulated > frequency && count > 0) {
          promises.push(
            this.allocate(particle)
              .then((entity) => {
                emitter.emit(entity);
              }),
          );
          accumulated -= frequency;
          count -= 1;
        }
        if (0 === count) {
          this.scheduled.splice(this.scheduled.indexOf(scheduled), 1);
          Promise.all(promises).then(() => {
            emitter.end();
          });
        }
      };
      this.scheduled.push(scheduled);
    });
    return stream;
  }

  tick(elapsed) {
    for (const ticker of this.scheduled) {
      ticker(elapsed);
    }
  }

}