<!-- Generated from crates/horizon-mod-sdk/wit/horizon.wit and the manifest's capabilities by crates/horizon-mods/tests/api_reference.rs; a test fails when this file is out of date. Update it with: HORIZON_WRITE_DOCS=1 cargo test -p horizon-mods --features pack --test api_reference -->
# Mod API reference

The contract between Horizon and its mods: the WIT package `horizon:mods@0.1.0`, every interface, type, function and hook in it, and the capabilities a manifest may list. The guide is [mods.md](../mods.md); stability and versions: [stability.md](stability.md). Manifest `api` lines this game serves: `0.1`, `0.2`.

## Capabilities

| In `capabilities` | Grants | The player reads "May …" |
| --- | --- | --- |
| `game-read` | [`game`](#game) | see your position, health and the time |
| `chat` | [`chat`](#chat) | show you chat messages |
| `hud` | [`hud`](#hud) | draw text on your screen |
| `storage` | [`storage`](#storage) | save its own settings to disk |
| `command` | [`command`](#command) | run commands as you, with your permissions |
| `registries` | data files (`assets/`, `data/`), no interface | add blocks and items to your worlds |
| `ui` | [`ui`](#ui) | show its own screens and a settings page |
| `render` | [`render`](#render) | draw on your screen, shake and zoom the camera |
| `input` | [`input`](#input) | see the keys you hold and your clicks, take over the use button on its own items and the mouse wheel, and add keys |

## World `horizon-mod`

- Imports (what the game offers): `log`, `game`, `chat`, `hud`, `storage`, `command`
- Exports (what the mod provides): `events`

## World `horizon-ui-mod`

A mod with widgets (capability `ui`): the SDK builds every mod against this world. Mods built for `horizon-mod` alone (no `ui-events` export) still load and simply hear nothing of their widgets.

- Imports (what the game offers): `ui`, `render`, `input`, `log`, `game`, `chat`, `hud`, `storage`, `command`
- Exports (what the mod provides): `ui-events`, `events`

## World `ui-host`

The game's side of `horizon-ui-mod` beyond `horizon-mod`.

- Imports (what the game offers): `ui`
- Exports (what the mod provides): `ui-events`

## World `render-host`

The game's side of capability `render` (imports only).

- Imports (what the game offers): `render`, `input`
- Exports (what the mod provides): 

## `log`

Access: always granted.

The mod's log, in the game's log under the mod's id. Always granted.

### enum `level`

| Case | |
| --- | --- |
| `debug` |  |
| `info` |  |
| `warn` |  |
| `error` |  |

| Function | Signature | What it does |
| --- | --- | --- |
| `write` | `func(level: level, message: string)` | One line in the game's log as `[mod <id>] message`, at that level. |

## `game`

Access: capability `game-read`.

Capability `game-read`: the local player and the world clock.

### record `player`

| Field | Type | |
| --- | --- | --- |
| `x` | `f64` | Feet position, in blocks. |
| `y` | `f64` |  |
| `z` | `f64` |  |
| `yaw` | `f32` | Degrees, as F3 shows them. |
| `pitch` | `f32` | Degrees: -90 looks straight up, 90 straight down. |
| `health` | `f32` | Hit points as vanilla counts them: 20 is full health without effects. |
| `food` | `u32` | 0 to 20, as the hunger bar. |
| `dimension` | `string` | `minecraft:overworld`, `minecraft:the_nether`, ... |

### record `clock`

| Field | Type | |
| --- | --- | --- |
| `game-time` | `u64` | Ticks since the world was created. |
| `day-time` | `u64` | 24000 ticks per day; 0 is 6:00 on the first day. |

| Function | Signature | What it does |
| --- | --- | --- |
| `get-player` | `func() -> option<player>` | None on the title screen. |
| `get-clock` | `func() -> option<clock>` | None on the title screen. |

## `chat`

Access: capability `chat`.

Capability `chat`: messages only the local player sees.

| Function | Signature | What it does |
| --- | --- | --- |
| `show` | `func(text: string)` | Shown in chat after the mod's name, like a system message. |

## `hud`

Access: capability `hud`.

Capability `hud`: one panel of text lines per mod, drawn by the game every frame until the mod changes or clears it (no call per frame).

### enum `corner`

| Case | |
| --- | --- |
| `top-left` |  |
| `top-right` |  |
| `bottom-left` |  |
| `bottom-right` |  |

### record `line`

| Field | Type | |
| --- | --- | --- |
| `text` | `string` | Plain text, up to 256 characters. |
| `color` | `u32` | 0xRRGGBB. |

| Function | Signature | What it does |
| --- | --- | --- |
| `set-panel` | `func(corner: corner, lines: list<line>)` | Replaces the mod's panel: up to 16 lines in that corner of the screen. |
| `clear` | `func()` | Removes the mod's panel. |

## `storage`

Access: capability `storage`.

Capability `storage`: a per-mod folder of small key/value files, quota'd (docs/mods.md). Keys are `[a-z0-9_.-]{1,64}`, one path segment.

| Function | Signature | What it does |
| --- | --- | --- |
| `get` | `func(key: string) -> option<list<u8>>` | None if `key` has never been `set`, or the key is not well-formed. |
| `set` | `func(key: string, value: list<u8>) -> result<_, string>` | Errs on a malformed key or over quota (bytes for the whole mod). |
| `delete` | `func(key: string)` | No error if `key` was already absent. |

## `command`

Access: capability `command`.

Capability `command`: vanilla commands run as the player, with the player's permissions, after the hook call returns (8 per call). A mod's own commands are not run this way.

| Function | Signature | What it does |
| --- | --- | --- |
| `run` | `func(line: string)` | `line` without its slash; its feedback goes to the chat as if typed. |

## `ui`

Access: capability `ui`.

Capability `ui`: widgets the game draws with its own menus' widgets, for a screen the mod owns and a settings page in Options > Mods. Layout and drawing are the game's: a mod says what its widgets hold, never where they sit or how they look, and the game clips everything it is handed.

### record `text-widget`

A line of text.

| Field | Type | |
| --- | --- | --- |
| `id` | `u32` | The mod's own name for the widget; `ui-events` calls echo it. |
| `text` | `string` | Up to 256 characters. |
| `color` | `u32` | 0xRRGGBB; 0 is the menus' white. |

### record `item-widget`

An item's icon and a name beside it, as recipe viewers show them.

| Field | Type | |
| --- | --- | --- |
| `id` | `u32` | The mod's own name for the widget. |
| `item` | `string` | `minecraft:diamond`, `<mod>:<path>`; an unknown id draws a "?". |
| `name` | `string` | The name shown beside the icon (empty: the item's own). |

### record `list-widget`

Rows the player picks one of, as an options page lists choices: a pick is `ui-events::select`, `selected` is the row to mark now.

| Field | Type | |
| --- | --- | --- |
| `id` | `u32` | The mod's own name for the widget; `ui-events::select` echoes it. |
| `rows` | `list<string>` | The rows' texts, at most 512, each up to 256 characters. |
| `selected` | `option<u32>` | The row to mark as the current choice, if any. |

### record `button-widget`

A thing on a mod's screen or settings page. A button as the options pages have them (200 × 20): a click is `ui-events::select(id, 0)`. Mods cycle its label for a choice ("Corner: Top Left").

| Field | Type | |
| --- | --- | --- |
| `id` | `u32` | The mod's own name for the widget. |
| `label` | `string` | Up to 256 characters. |

### variant `widget`

| Case | Payload | |
| --- | --- | --- |
| `text` | `text-widget` | Text the player reads. |
| `button` | `button-widget` | A button. |
| `item` | `item-widget` | An item's icon and name. |
| `list` | `list-widget` | Rows the player picks from. |

### record `screen`

A screen the mod owns: a title over widgets stacked down the middle, the Done button the game adds, and Esc to leave it.

| Field | Type | |
| --- | --- | --- |
| `title` | `string` | Its title, up to 256 characters. |
| `widgets` | `list<widget>` | Its widgets, in order, at most 64. |

| Function | Signature | What it does |
| --- | --- | --- |
| `open-screen` | `func(screen: screen)` | Show `screen` over the game; a second call while it is open replaces it. Only the last call of one hook counts. |
| `close-screen` | `func()` | Close the mod's screen when it is open (the mod hears no `closed` for this: it was its own doing). |
| `set-settings` | `func(widgets: list<widget>)` | The mod's page in Options > Mods, reached from a settings button on its row: the widgets it shows, an empty list for no page. The player opening the page is `ui-events::settings-opened`: push fresh widgets then; later calls update the page while it shows. |

## `ui-events`

Access: exported by the mod: the game calls it.

What the game calls on a mod that shows widgets (capability `ui`): what the player did with them.

| Function | Signature | What it does |
| --- | --- | --- |
| `select` | `func(id: u32, row: u32)` | A row of one of the mod's `list` widgets was picked: the widget's `id` and the row, on the screen or the settings page that showed it. |
| `closed` | `func()` | The player left the mod's screen (Esc or Done); not for a screen the mod closed itself or one that went away with the mod. |
| `settings-opened` | `func()` | The player opened the mod's settings page in Options > Mods: call `ui::set-settings` now to fill it with fresh widgets. |

## `input`

Access: capability `input`.

Capability `input` (experimental: the manifest needs `experimental = ["input"]`): the use button on items the mod claims, and the mod's own keys. Events wait for `poll`, at most 256.

### record `held-item`

The main hand's stack.

| Field | Type | |
| --- | --- | --- |
| `id` | `string` | `minecraft:spyglass`, `<mod>:<path>`. |
| `count` | `u32` |  |
| `slot` | `u32` | The hotbar slot, 0 to 8. |

### record `item-use`

| Field | Type | |
| --- | --- | --- |
| `item` | `held-item` |  |
| `ticks` | `u32` | Game ticks the button was held (0 on `use-start`). |

### variant `event`

| Case | Payload | |
| --- | --- | --- |
| `use-start` | `item-use` | The use button went down with a claimed item in the main hand. The game does nothing else with the click: no placing, no eating, no swing. |
| `use-stop` | `item-use` | It came up again, or the item left the hand, or a screen opened. |
| `key-down` | `string` | A key `register-key` named went down or up while playing (no screen open). The game's own mapping on that key still runs. A screen opening lets every held key up. |
| `key-up` | `string` |  |
| `wheel` | `f32` | Mouse wheel steps while the mod holds the wheel (`claim-wheel`): positive away from the player, as the hotbar reads them. |

### record `mapping-state`

A mapping as the player has bound it.

| Field | Type | |
| --- | --- | --- |
| `label` | `string` | The bound key's name as Controls shows it: `W`, `Space`, `LMB`. |
| `down` | `bool` | Held down now (as of this frame for a mod with a `frame` hook, else this tick). |

| Function | Signature | What it does |
| --- | --- | --- |
| `claim-items` | `func(ids: list<string>)` | The items whose use the mod handles instead of the game, by id (at most 64). Replaces the previous list. |
| `register-key` | `func(name: string, label: string, key: string) -> result<_, string>` | A key of the mod's: `name` comes back in `key-down` and `key-up`, `label` says what it does, `key` is the key as options.txt writes it (`key.keyboard.r`). At most 16; an unknown key is an error. |
| `held` | `func() -> option<held-item>` | The main hand's stack, as of this tick; none when it is empty. |
| `poll` | `func() -> list<event>` | The events since the last call, oldest first. |
| `claim-wheel` | `func(on: bool)` | Takes the mouse wheel while playing (true) or gives it back: while taken, its steps are the mod's `wheel` events and the hotbar doesn't move. The game gives it back when the mod stops. |
| `mapping` | `func(name: string) -> option<mapping-state>` | A movement or click mapping as the player has bound it: `key.forward`, `key.left`, `key.back`, `key.right`, `key.jump`, `key.sneak`, `key.sprint`, `key.attack` or `key.use`. None for another name. |
| `clicks-per-second` | `func() -> tuple<u32, u32>` | Clicks of the attack and use buttons in the last second. |

## `render`

Access: capability `render`.

Capability `render` (experimental: the manifest needs `experimental = ["render"]`): shapes the game draws for the mod with its own renderer, kept until the mod replaces them (no call per frame). A mod never supplies shader code.

### enum `corner`

A corner of the screen, as `hud`'s panels use them.

| Case | |
| --- | --- |
| `top-left` |  |
| `top-right` |  |
| `bottom-left` |  |
| `bottom-right` |  |

### record `rect`

A rectangle in GUI pixels: the top left corner and the size.

| Field | Type | |
| --- | --- | --- |
| `x` | `f32` |  |
| `y` | `f32` |  |
| `w` | `f32` |  |
| `h` | `f32` |  |

### record `point`

A point in GUI pixels from the screen's top left.

| Field | Type | |
| --- | --- | --- |
| `x` | `f32` |  |
| `y` | `f32` |  |

### record `fill-shape`

| Field | Type | |
| --- | --- | --- |
| `rect` | `rect` |  |
| `color` | `u32` | 0xAARRGGBB. |

### record `quad-shape`

Any four-cornered shape (a wheel's segment, an arrow): the corners in order round it.

| Field | Type | |
| --- | --- | --- |
| `corners` | `tuple<point, point, point, point>` |  |
| `color` | `u32` | 0xAARRGGBB. |

### record `sprite-shape`

| Field | Type | |
| --- | --- | --- |
| `sprite` | `string` | One of the game's GUI sprites, as `gui/sprites/` names them (`hud/heart/full`, `widget/button`), the resource packs' versions included. An unknown one draws nothing. |
| `rect` | `rect` |  |
| `color` | `u32` | A tint, 0xAARRGGBB: 0xFFFFFFFF draws it as it is. |

### record `item-shape`

| Field | Type | |
| --- | --- | --- |
| `item` | `string` | `minecraft:diamond`, `<mod>:<path>`; an unknown one draws nothing. |
| `at` | `point` | The icon's top left. |
| `scale` | `f32` | 1.0 is the hotbar's 16 × 16. |

### record `text-shape`

| Field | Type | |
| --- | --- | --- |
| `text` | `string` | Up to 256 characters, one line. |
| `at` | `point` |  |
| `color` | `u32` | 0xRRGGBB, as `hud`'s lines. |
| `centered` | `bool` | Centred on `at.x` instead of starting there. |
| `shadow` | `bool` |  |

### variant `shape`

| Case | Payload | |
| --- | --- | --- |
| `fill` | `fill-shape` |  |
| `quad` | `quad-shape` |  |
| `sprite` | `sprite-shape` |  |
| `item` | `item-shape` |  |
| `text` | `text-shape` |  |

### record `frame-rate`

| Field | Type | |
| --- | --- | --- |
| `fps` | `u32` | Frames drawn in the last second. |
| `min` | `u32` | The slowest frame's rate, and the 1% low (the 99th percentile frame time's rate), over the last 5 seconds. |
| `low` | `u32` |  |

### record `zoom`

| Field | Type | |
| --- | --- | --- |
| `factor` | `f32` | The field of view is divided by it: 1 is none, at most 10. |
| `smooth` | `bool` | Ease in and out through the game's field-of-view easing, as the spyglass does, instead of at once. |
| `turn` | `f32` | Mouse turning is multiplied by it while zoomed: 1 keeps the player's sensitivity, at least 0.05. |

| Function | Signature | What it does |
| --- | --- | --- |
| `set-hud` | `func(shapes: list<shape>)` | Replaces the mod's HUD shapes: up to 1024, drawn every frame in order over the HUD (after the hotbar, under chat and screens), hidden with the HUD (F1). An empty list removes them. |
| `gui-size` | `func() -> tuple<f32, f32>` | The screen's size in GUI pixels (width, height), as of this tick. |
| `shake` | `func(strength: f32, seconds: f32)` | Shakes the camera: a jolt of `strength` (0 to 1, 1 the hardest) that fades out over `seconds` (at most 10). Jolts add up to 1. The view turns, the player doesn't, and the player's Distortion Effects option scales it (0 turns it off). |
| `set-corner` | `func(corner: corner, width: f32, height: f32, shapes: list<shape>)` | Replaces the mod's corner panel: shapes in GUI pixels from the top left of a `width` × `height` box that the game places in `corner`, stacked with the game's overlays and the other mods' panels (under the status effects at the top right), as `hud`'s panels are. |
| `clear-corner` | `func()` | Removes the mod's corner panel. |
| `text-width` | `func(text: string) -> f32` | The width of one line of text in GUI pixels, in the HUD's font: exact for Latin letters (U+0000 to U+024F), 6 per other character. |
| `frame-stats` | `func() -> frame-rate` | The game's frame counter, recomputed twice a second. |
| `set-zoom` | `func(zoom: zoom)` | Zooms the camera until the next call (`factor` 1 ends it), only while playing: a screen opening or the spyglass's own view (which wins) leave the camera vanilla's. With several mods, the strongest zoom counts. |

## `events`

Access: exported by the mod: the game calls it.

What the game calls. Only the hooks `init` asks for are ever called.

### record `registration`

| Field | Type | |
| --- | --- | --- |
| `commands` | `list<string>` | Chat commands, without the slash. A name vanilla already uses is refused (vanilla's command runs). |
| `tick` | `bool` | Call `tick` every game tick (20 per second while playing). |
| `frame` | `bool` | Call `frame` every rendered frame. Budget: 0.1 ms. |
| `chat` | `bool` | Call `chat` for each chat line the player receives. |

| Function | Signature | What it does |
| --- | --- | --- |
| `init` | `func() -> result<registration, string>` | Once, after loading. An error keeps the mod off and is shown. |
| `tick` | `func(tick: u64)` | Each game tick while in a world, with the game's tick counter. |
| `frame` | `func(seconds: f32)` | Each rendered frame, with the seconds since the last one. |
| `command` | `func(name: string, args: string) -> result<option<string>, string>` | `/name args`: ok(some(text)) is shown as feedback, err(text) in red. |
| `chat` | `func(text: string)` | Each chat line the player receives, as plain text. |