39 lines
1.3 KiB
Markdown
39 lines
1.3 KiB
Markdown
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---
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title: "Engine"
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date: 2019-05-22T13:01:28-07:00
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weight: 15
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---
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An Engine is the Encompass notion of an ECS System. Much like the engine on a train, your Engines make the simulation move along.
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{{% notice notice %}}
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I never liked the term System. It is typically used to mean structures in game design and I found this confusing when discussing code implementation vs design.
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{{% /notice %}}
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Engines are responsible for reading the game state, reading messages, emitting messages, and creating or mutating Entities and Components.
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An Engine which Reads a particular message is guaranteed to run *after* all Engines which Emit that particular message.
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To define an Engine, extend the Engine class.
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Here is an example Engine:
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```ts
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import { Engine, Mutates, Reads } from "encompass-ecs";
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import { LogoUIComponent } from "../../components/ui/logo";
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import { ShowUIMessage } from "../../messages/show_ui";
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@Reads(ShowUIMessage)
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@Mutates(LogoUIComponent)
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export class LogoDisplayEngine extends Engine {
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public update() {
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const logo_ui_component = this.read_component_mutable(LogoUIComponent);
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if (logo_ui_component && this.some(ShowUIMessage)) {
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logo_ui_component.image.isVisible = true;
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}
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}
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}
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```
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If a LogoUIComponent exists, and a ShowUIMessage is received, it will make the logo image on the LogoUIComponent visible. Simple!
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