Inertia

An inertia as a stepper: a glide that carries a value on from its velocity and eases to a stop, as a flick or a drag's release does, springing back from min or max if it would pass one. resolve works out the glide once, init starts it, and step moves it a frame. For something you step yourself each frame; otherwise Machine.inertia runs one as a machine.

It rests at initial + power × velocity, or where modifyTarget moves that, easing out over timeConstant seconds. Where the glide would cross an edge, or starts past one, a spring takes it to the nearest edge, from the glide's speed there.

import * as Inertia from "@wardic/core/Inertia"

const glide = Inertia.resolve(x, velocity, { min: 0, max: 400 })
let state = Inertia.init(x, glide)
// each frame:
const frame = Inertia.step(state, dt, glide)
state = frame.state
x = frame.value // and `frame.done` once it rests

Its exports also name types the module doesn't export: its internal types.

Interfaces

Resolved

An inertia's options with every default filled in, and its glide worked out from where it starts.

Properties

amplitude

readonly amplitude: number;

How far the glide goes: target − initial.

bounce

readonly bounce: BounceSpring;

max

readonly max: number;

min

readonly min: number;

The edges, −∞ and +∞ where none is given.

restDelta

readonly restDelta: number;

target

readonly target: number;

Where the glide rests: initial + power × velocity, or where modifyTarget moved that.

timeConstant

readonly timeConstant: number;

The glide's time constant, in seconds.

State

An inertia's state: its time, and once it reached an edge, the bounce toward it.

Properties

bounce

readonly bounce: Bounce | undefined;

elapsed

readonly elapsed: number;

Functions

init()

function init(initial, inertia): State;

An inertia at its start: gliding, or, when initial is already past an edge, springing back to it from the glide's speed at 0 (Motion's checkCatchBoundary(0)).

Parameters

initial

number

inertia

Resolved

Returns

State

resolve()

function resolve(
   initial, 
   velocity, 
   options
): Resolved;

An inertia's defaults filled in, and its glide worked out from initial at velocity: once each run. A value no inertia can take, a negative timeConstant say, throws an OptionError naming it. Its options are checked once a bag, kept by its identity: one changed after it's handed over isn't seen, so pass a new one.

Parameters

initial

number

velocity

number

options

InertiaOptions

Returns

Resolved

restOf()

function restOf(
   initial, 
   velocity, 
   options
): number;

Where an inertia from initial at velocity comes to rest: its glide's end, held to its edges, or the edge nearest a start past one. Where one that's skipped lands at once. Throws as resolve does.

Parameters

initial

number

velocity

number

options

InertiaOptions

Returns

number

step()

function step(
   state, 
   dt, 
   inertia
): PhysicsStepResult<State>;

One frame of a resolved inertia: its next state, value, velocity, and whether it's done. While it glides, its value is the glide's at its time; the frame that crosses an edge shows the glide past it, and the bounce takes over from there, resting on the edge within restDelta and restSpeed.