# Grapple and swing (movement.grapple)

## What it is for

Adds anchor-and-rope traversal with slack and taut ropes, steering, moving anchors, bounded reeling and a release that keeps momentum, from one shared Rust core.

It needs `game.engine`; installing it brings them in.

## Turn it on

Install the kit in a project, then change a setting (the values shown are its defaults):

```text
gripforge_gamekit_install   { project_id: "gkp_…", id: "movement.grapple" }
gripforge_gamekit_configure { project_id: "gkp_…", id: "movement.grapple",
                              config: { "minLength": 1, "maxLength": 60 } }
```

## First example

<!-- test: kits/movement_grapple/tests/movement_grapple.rs::attach_records_the_rope_length_from_the_distance -->
This is the body of the passing test `attach_records_the_rope_length_from_the_distance`; the helper functions it calls are defined at the top of the same test file.

```rust
#[test]
fn attach_records_the_rope_length_from_the_distance() {
    let mut s = rope(json!({}));
    assert_eq!(
        req(&mut s, KIT, json!({"op": "status"})),
        json!({"attached": false, "rope": null})
    );
    let st = attach(&mut s);
    assert_eq!(st["attached"], true);
    assert!(near(&st["rope"]["length"], 10.0));
    assert_eq!(st["rope"]["anchor"], json!([0.0, 10.0, 0.0]));
}
```

## Settings and defaults

Descriptions come from the kit manifest and stay in English.

| Setting | Type | Default | Range or values | Meaning |
|---|---|---|---|---|
| `minLength` | number | `1` | 0.01–1000 | Minimum rope length; must not exceed maxLength. |
| `maxLength` | number | `60` | 0.01–10000 | Maximum attachment distance and rope length. |
| `reelSpeed` | number | `8` | 0–100 | Rate of rope shortening or extension. |
| `steeringAcceleration` | number | `12` | 0–200 | Full tangential steering acceleration. |

## Services, events, variables

- Service `movement.grapple`: Named instances: create(id, config), request(id, command), remove(id), ids(). Optional move(id, entity, command) uses PhysicsPort. Commands: attach, step, release; status, config, preview, save, load. Default instance uses installed config.
- Events emitted:
  - `movement.grapple.created` (`instance`, `preset`) — An instance was created.
  - `movement.grapple.removed` (`instance`) — An instance was removed.
  - `movement.grapple.changed` (`instance`, `op`, `result`) — A decided command applied to an instance (never a read or a fixed step).
  - `movement.grapple.refused` (`instance`, `op`, `by`, `reason`) — A tap of movement.grapple.request cancelled a command.
- Verbs of the scripting layer:
  - `movement_grapple_request` (action) — Send a command to an instance of the kit (through the request hook).
  - `movement_grapple_exists` (condition) — The instance exists (default: default).

## Hooks and modding

- Hooks (a game or a mod can intercept them):
  - `movement.grapple.create` (fields a tap may change: `config`; cancellable) — An instance is about to be created (create(id, config)).
  - `movement.grapple.request` (fields a tap may change: `command`; cancellable) — A decided command (every op but the reads and the fixed steps) is about to apply to an instance.
- Moddable settings: `minLength`, `maxLength`, `reelSpeed`, `steeringAcceleration`
- Moddable collections: `movement_grapple_presets`

## Engines

Declared targets: `rust`, `web`. `gripforge kit api movement.grapple` lists the exact settings and services of your version.

## Versions

Kit version 1.1.0.