168 lines
4.3 KiB
Markdown
168 lines
4.3 KiB
Markdown
# shoal
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<img src="assets/logo.png" alt="shoal logo" width="200" height="200">
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my silly little ecs engine for odin and raylib :3
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## contents
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- [install](#install)
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- [usage](#usage)
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## install
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clone this repo in your project directory
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```git clone https://git.egfk.net/harper/shoal```
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and import the package
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```odin
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import ecs "shoal/src"
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```
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## usage
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first create a new app struct. this is where your systems and plugins live.
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```odin
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import ecs "shoal/src"
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app: ecs.App
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```
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you can run your app with `run()`. run takes a reference to your app to run it's systems.
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```odin
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main :: proc() {
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ecs.run(&app)
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}
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```
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### systems
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systems are procedures that run on set schedules
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a schedule just determines when a producure runs\
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schedules are:
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- STARTUP
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- UPDATE
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- DRAW
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`STARTUP` runs once at the start of an app\
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`UPDATE` runs every frame\
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`DRAW` runs every frame *after* update, and makes draw calls to raylib\
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```odin
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main :: proc() {
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ecs.add_system(&app, ecs.STARTUP, example_startup)
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ecs.add_system(&app, ecs.UPDATE, example_update)
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}
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example_startup :: proc() {
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// system that runs once
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// ...
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}
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example_update :: proc() {
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// system that runs every frame
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// ...
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}
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```
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### components
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components are just structs with data, and kept with other components of that type in `Storage`.\
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create a new component
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```odin
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ExampleComponent :: struct {}
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```
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and create it's storage
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```odin
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Example: ecs.Storage(ExampleComponent)
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```
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### spawn entities
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create a new entity with `spawn_entity()`
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```odin
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main :: proc() {
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entity := ecs.spawn_entity(&app)
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}
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```
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and give it new components with `add_component()`
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```odin
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main :: proc() {
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ecs.add_component(entity, &Example) // make sure to pass a reference to the storage, not the component type
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}
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```
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### query
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since components are saved in storage next to their entity, you can loop through a Storage and pick out what you like
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```odin
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main :: proc() {
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components := ecs.slice(&Example)
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entities := ecs.slice_entity(&Example)
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for v, i in components {
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e := entities[i] // get entity that owns the component
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}
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}
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```
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or if you know your storage only has one entry use `single()`
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```odin
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main :: proc() {
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component, entity_id := ecs.single(&Example)
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}
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```
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### plugins
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plugins are just a procedure that takes `App` as a parameter, meaning it can make changes to `app`, just like main()
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```odin
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ExamplePlugin :: proc(app: ^App) {
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ecs.add_system(app, ecs.STARTUP, some_plugin_system)
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}
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```
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```odin
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main :: proc() {
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ecs.add_plugin(&app, ecs.DefaultPlugin)
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ecs.add_plugin(&app, ExamplePlugin)
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}
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```
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plugins make your scripts very reuseable. in fact, much of shoal's default behaviors come from plugins via `DefaultPlugin()`
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### signals
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signals are structs your systems can talk to each other with. one system writes a signal with data, another system reads it later in the same frame — as long as the reader runs before `signal_system` clears the buses at POST_UPDATE (see the caveat at the end of this section)
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```odin
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DamageEvent :: struct { amount: int }
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```
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`write()` queues a signal on a per-type bus. `read()` gives you back every signal of that type written since the last POST_UPDATE (an empty slice if there were none), so you can iterate it. reads don't consume, so any number of systems can read the same signal and its data
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```odin
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attack :: proc() {
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ecs.write(DamageEvent{amount = 5})
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}
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// registered after attack in UPDATE, so it sees the event the same frame
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check_damage :: proc() {
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for e in ecs.read(DamageEvent) {
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fmt.println("took damage!", e.amount)
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}
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}
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```
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you can also write an empty payload with the bare type: `ecs.write(DamageEvent)`
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scheduling caveat: every system sees exactly the signals written since the last `signal_system()` run, and that runs at POST_UPDATE — *before* PRE_DRAW/DRAW. so:
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- systems running before POST_UPDATE (STARTUP/PRE_UPDATE/UPDATE/early POST_UPDATE) see same-frame writes
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- a DRAW system sees none of them — POST_UPDATE already cleared the bus. it only sees signals written after POST_UPDATE by an earlier system in the same frame (say, one DRAW system reacting to another's write)
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- writes made after POST_UPDATE survive the rest of the frame and are picked up by the next frame's PRE_UPDATE/UPDATE systems, then cleared at the next POST_UPDATE — so the next frame's DRAW never sees them either
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