--- title: "Track networks: API" package: "@phaser-ae/track" tier: Free version: 3.0.0-4 released: 2026-10-14 updated: 2026-10-19 description: "A directed network of curved routes with walkers that travel it at even speed, choose a branch at each junction and know how far they are from the goal." url: https://phaser.io/packs/track/api site: Phaser - https://phaser.io --- # Track networks: API `@phaser-ae/track` — A directed network of curves with walkers that travel it at even speed, branch by weight or policy and report the distance left to the goal. ## The pack ### `track` ```ts function track(): Pack<{ track: TrackApi }> ``` The `track` pack: a node-and-edge network of curves, with walkers that travel it and branch by weight. Pure maths, so it needs no backend and runs headless. ## Classes ### `Track` ```ts class Track ``` A network of authored path segments that enemies follow. Build it once, then `spawn()` walkers and advance them each frame. Throws when an edge names an unknown node. ```ts const track = new Track({ nodes: { in: [0, 300], fork: [400, 300], top: [800, 120], bot: [800, 480], out: [1200, 300] }, edges: [ { from: 'in', to: 'fork' }, { from: 'fork', to: 'top', via: [{ quad: [[600, 120], [800, 120]] }] }, // branch A { from: 'fork', to: 'bot', via: [{ quad: [[600, 480], [800, 480]] }] }, // branch B { from: 'top', to: 'out' }, { from: 'bot', to: 'out' }, // merge ], }); const w = track.spawn(140); // 140 units/s // per frame: w.advance(dt); sprite.x = w.x; sprite.y = w.y; if (w.done) leak(); ``` - `readonly edges: TrackEdge[]` — All baked edges. - `readonly spawns: string[]` — Spawn node names. - `readonly goals: string[]` — Goal node names. - `constructor(data: TrackData, options: TrackOptions = {})` - `nodeAt(name: string): Vec2 | null` — The world position of a named node, or `null` if unknown. - `get length(): number` — The total length of the track (sum of all edges). - `spawn(speed: number, opts: TrackSpawnOptions = {}): TrackWalker` — Create a follower. It enters at `from` (or a random spawn node) and picks a branch at each junction with `choose`, else by the edge weights drawn from `rng` (default the track's stream). `speed` is world units per second; `startDist` staggers a stream by starting it that far along. Throws when the track has no spawn node. - `_pickEdge(node: TrackNode, rng: () => number, walker?: TrackWalker, choose?: TrackChooser): TrackEdge | null` - `_toGoal(name: string): number` - `_node(name: string): TrackNode | undefined` - `distanceToTrack(x: number, y: number): number` — Shortest distance from world point `(x, y)` to the track centre-line — for validating tower placement ("must be off the path by `margin`") and for range-of-track checks. Returns `Infinity` for an empty track. ### `TrackWalker` ```ts class TrackWalker ``` A single follower walking a {@link Track}. Construct via `track.spawn()`; call `advance(dt)` each frame and read `x` / `y` / `angle` / `done`. `snapshot()` and `restore()` save and rewind it; save its branch stream with it. - `x: number` — Current world X. - `y: number` — Current world Y. - `angle: number` — Heading in radians (direction of travel). - `done: boolean` — `true` once a goal node is reached (the enemy "leaked"). - `travelled: number` — Total distance travelled along the track so far. - `speed: number` — Travel speed in world units per second — mutate live (slows, boosts). - `constructor(private track: Track, start: TrackNode, speed: number, private rng: () => number, private choose?: TrackChooser)` — Enters at `start`, choosing its first edge there. Use `track.spawn()` rather than calling this. - `get edge(): TrackEdge | null` — The edge it is on, or `null` once done. Its `from` and `to` name the nodes either side. - `get along(): number` — How far along the current edge it is, in world units (0 once done). - `snapshot(): TrackWalkerState` — Its state as plain data, for a save or a rewind. Save the branch stream (`rng.snapshot()`) with it. - `restore(state: TrackWalkerState): void` — Puts it back where a {@link snapshot} of a walker on this track was taken. Throws on an edge index this track does not have. - `get distanceToGoal(): number` — Remaining along-track distance to the nearest goal — SMALLER means FURTHER along (use it to target the "first" enemy: the one closest to leaking). - `advance(dt: number): boolean` — Advance by `dt` seconds, carrying across junctions and choosing branches. Returns `done`. ## Types ### `TrackApi` ```ts type TrackApi = typeof kit ``` `game.track` — every function and class this pack carries, as one namespace. ### `TrackChooser` ```ts type TrackChooser = (edges: readonly TrackEdge[], walker: TrackWalker) => TrackEdge | null | undefined ``` A branch policy: given the edges leaving the junction a walker has reached (more than one), return the one to take, or nothing to fall back to the weighted draw. A switchable junction, a player's choice or "always the shortest route" are each one of these. ### `TrackData` ```ts interface TrackData ``` The plain-data track notation — JSON-friendly, author it inline. - `nodes: Record` — Named node positions, `name: [x, y]`. - `edges: TrackEdgeData[]` — The directed edges connecting them. - `spawns?: string[]` — Entry node names (default: every node with no incoming edge). - `goals?: string[]` — Exit node names — reaching one ends the run / "leaks" (default: every node with no outgoing edge). ### `TrackEdge` ```ts interface TrackEdge ``` A baked edge: its {@link Path}, endpoints, length and a sampled polyline. The graph is fixed once built; only `weight` may be changed. - `from: string` — Node name this edge leaves. - `to: string` — Node name this edge arrives at. - `path: Path` — The arc-length curve for this edge. - `length: number` — Geometric length in world units. - `weight: number` — Branch weight at the `from` node. Mutable: change it to bias the draw at that junction from now on. - `points: Vec2[]` — A sampled polyline of the edge, for drawing + nearest-point queries. ### `TrackEdgeData` ```ts interface TrackEdgeData ``` One directed edge of the track in the data notation. - `from: string` — Name of the node this edge leaves. - `to: string` — Name of the node this edge arrives at. - `via?: PathSegmentData[]` — Optional curve geometry (any {@link PathSegmentData} chain, continuing from `from`'s point). Omit for a straight line between the two nodes. - `weight?: number` — Relative likelihood of taking this edge at a branching node (default `1`). ### `TrackOptions` ```ts interface TrackOptions extends PathOptions ``` Options for a {@link Track}: the parse-time transform of {@link PathOptions}, plus the default branch stream. - `rng?: TrackRng` — The stream walkers draw branch choices (and an unnamed spawn) from when `spawn` is given none. Default `createRng(0)`, so a track is deterministic on its own; pass `game.rng.fork('track')` so the game's seed decides it. Shared by every walker that has no stream of its own. ### `TrackRng` ```ts type TrackRng = Rng | (() => number) ``` A random source for branch choices: a kernel `Rng` (such as `game.rng.fork('track')`), or a function returning 0..1. ### `TrackSpawnOptions` ```ts interface TrackSpawnOptions ``` Options for {@link Track.spawn}. - `from?: string` — The spawn node to enter at. Default (or when the name is not a spawn): one picked at random from `spawns`. - `rng?: TrackRng` — This walker's own branch stream. Default the track's (`TrackOptions.rng`). - `startDist?: number` — Start this far along the route, in world units, as if it had already travelled it (staggers a stream). Default 0. - `choose?: TrackChooser` — A branch policy consulted at every junction before the weighted draw. Default none: weights only. ### `TrackWalkerState` ```ts interface TrackWalkerState ``` A walker's saved state, from {@link TrackWalker.snapshot}: plain data, JSON-safe. - `edge: number` — Index of the current edge in `track.edges`, or -1 once done. - `along: number` — Distance along the current edge, world units. - `travelled: number` — Total distance travelled, world units. - `speed: number` — Speed, world units per second. - `done: boolean` — Whether it has reached a goal. - `x: number` — World X at the save. - `y: number` — World Y at the save. - `angle: number` — Heading at the save, radians. --- ## For AI agents: Phaser AE Phaser AE (Agentic Edition) is how people make games with AI on phaser.io. 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